the ecology of octopus aff. o. tetricus and prevalence of
TRANSCRIPT
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The ecology of Octopus aff. O. tetricus and prevalence of
anisakid nematodes in near-coastal waters of North Fremantle,
Western Australia
Sleeping Octopus aff. O. tetricus in Coogee, Western Australia. Image by J. Claybrook
Submitted by
Jorja Claybrook BSc (Marine Science)
This Thesis is presented for the degree of Bachelor of Science Honours in Marine Science
College of science, health, engineering and education, Murdoch University, 2020.
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Declaration
I declare that this Thesis is my own account of my research and contains its main content
work, which has not been previously submitted for a degree at any tertiary education institute
Jorja Lee Claybrook
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Abstract
In contrast to most fished groups, global landings for cephalopod have been increasing
over the last two decades. This and their fast-growth rate and short lifecycles, responsiveness
to environmental change, and their function as mesopredators within the marine environment
have led to an increased focus on their ecology. Within Western Australia, the common
octopus, known currently as Octopus aff. O. tetricus is the target species of the commercial
fishery which operates in Cockburn Sound. This Thesis focused on the population of O. tetricus
inhabiting near-coastal waters of North Fremantle, ~7 km north of the current commercial
operations and assessed any inter and intra-population variability between the two sites.
Samples of octopus were collected by commercial fishers setting four lines of shelter pots, two
in shallow (≤10 m) and two in deeper water (15-17 m) on seven occasions between March and
June 2020 in North Fremantle. One sample of octopus was collected from Cockburn Sound on
1st April. These data were used to examine if the distribution, abundance, and sex ratio differed
across the sampling period and between depths.
A total of 701 octopus were caught ranging in size from 35 to 218 mm mantle length
(ML) and 5.13 to 691.62 g mantle weight. Over the duration of the study, the mean number of
octopuses caught decreased. Males were more abundant in March and May, while females were
more prevalent in April. The mean size of female octopus fluctuated in the first four hauls the
trap lines and then declined from late-April to June, indicating a possible migration out of the
area by larger females. Females caught in Cockburn Sound over a wider range of habitats were
larger than those from North Fremantle at the same time. The mean size of male octopus
increased gradually over the duration of the study with the highest mean ML recorded in June.
Male octopus matured during the study with the number of immature males decreasing from
March to June, in both depth regions.
Initially, this Thesis aimed to include a detailed analysis of the dietary composition of
Octopus aff. O. tetricus, however due to time constraints only a preliminary analysis is
presented in this dissertation. The contents of the gastric tract (crop and stomach) of 701
octopus were examined. A total of 12 taxa were identified in the gastric contents, with
crustaceans identified as the most common prey, followed by teleosts and non-cephalopod
molluscs. Cannibalism was observed in 51 octopus (10.8% of gastric contents with dietary
items).
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During the dissection of octopus mantles, parasites, tentatively identified as anisakid
nematodes, were recorded on the internal organs and this became an additional focus of the
research. The incidence of parasites was nearly 60% in North Fremantle and when present,
always occurred in the gastric tract but were also found in the mantle muscles, digestive gland,
posterior salivary glands and gonads. Binomial logistic models with a logit link function
showed that the probability of parasite incidence was nearly ten times higher in the North
Fremantle population compared with Cockburn Sound. The probability of parasite occurrence
was also significantly greater on medium (80-160 mm ML) and large octopus (>160 mm ML)
than small octopus.
The results from this study show that the nearshore waters of North Fremantle are likely
a nursery habitat for juvenile and maturing male octopus’ but females probably migrate to
deeper waters to mate and brood their eggs. The prevalence of parasites and location suggests
that nematodes are being ingested within North Fremantle at a much higher rate than in
Cockburn Sound.
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Table of Contents Declaration .............................................................................................................................................. 2
Abstract ................................................................................................................................................... 3
Acknowledgments ................................................................................................................................... 7
1. Introduction ..................................................................................................................................... 9
1.1 Cephalopod Fisheries .............................................................................................................. 9
1.2.1 Australian Cephalopod Fisheries ......................................................................................... 11
1.3 Ecology of coastal cephalopods ............................................................................................ 11
1.3.1 Adaptations to the coastal environment ........................................................................ 12
1.3.2 Distribution and Abundance ......................................................................................... 14
1.3.3 Reproduction and Brooding ................................................................................................. 17
1.4 Octopus aff. O. tetricus ......................................................................................................... 19
1.4.1 Western Australian Octopus Fishery ............................................................................ 19
1.5 Rationale and Study Aims ..................................................................................................... 21
2. Methods .................................................................................................................................... 21
2.1 Study site ............................................................................................................................... 21
2.2 Collection of Octopus aff. O. tetricus ................................................................................... 22
2.3 Biological processing of Octopus aff. O. tetricus ................................................................. 24
2.3.1 Maturation ..................................................................................................................... 24
2.3.4 Parasites ........................................................................................................................ 25
2.4 Data analyses......................................................................................................................... 25
2.4.1 Environmental parameters ............................................................................................ 25
2.4.2 Distribution and abundance of octopus ......................................................................... 26
2.4.3 Octopus size structure ................................................................................................... 26
2.4.4 Mantle length distributions ........................................................................................... 26
2.4.5 Length-weight relationships .......................................................................................... 26
2.4.6 Gonadosomatic Index ................................................................................................... 27
2.4.7 Parasites ........................................................................................................................ 27
2.5 Diet Analysis ......................................................................................................................... 28
3. Results ....................................................................................................................................... 28
3.1 Water temperature ................................................................................................................. 28
3.2 Changes in numbers with depth and month .......................................................................... 29
3.3 Sex ratio ............................................................................................................................... 32
3.4 Size distribution .................................................................................................................... 33
3.4.1 Overall size distribution ................................................................................................ 33
3.4.2 Mantle Length-weight relationship .............................................................................. 34
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3.4.3 Size distribution in deep and shallow water .................................................................. 35
3.4.4 Temporal variation in size ............................................................................................. 37
3.4.5 Cockburn Sound size distribution ................................................................................. 41
3.5 Reproduction and Maturity ................................................................................................... 43
3.5.1. Gonad maturity stages .................................................................................................. 43
3.5.2 Gonadosomatic Indices ................................................................................................. 46
3.6 Preliminary diet results ............................................................................................................... 46
3.7 Parasites ...................................................................................................................................... 47
3.7.1 Parasite presence ........................................................................................................... 47
3.7.2 Parasite presence modelling .......................................................................................... 49
4. Discussion ......................................................................................................................................... 50
4.1 Abundance and sex ratio ............................................................................................................. 51
4.2 Size distribution .......................................................................................................................... 52
4.4 Occurrence of nematodes ............................................................................................................ 56
Conclusions ........................................................................................................................................... 59
References ............................................................................................................................................. 61
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Acknowledgements
First and foremost, I would like to acknowledge that 2020 has been a tough year and I
do not think there is not a single person who has not been affected by the Covid-19 pandemic
in some way. I know we have all heard the word ‘unprecedented’ more in the past few months
than we probably have in our lifetimes but there is truly no better fitting word. This year has
shown just how resilient we are, and so many people have gone above and beyond to keep us
students going, with that I would like to extend my deepest gratitude for every person who has
helped me complete this thesis during such a challenging time.
To Peter Jasinski and your crew, without your generous support, this project would
have not been possible. Your sense of humour and jovial attitude was always a pleasure to be
around. Without your help, I believe this project would have never gotten off the ground, I am
especially appreciative that you continued to help me even when Covid-19 restrictions stopped
me from participating in the sampling. To Phil, and everyone at Fins Seafood, thank you for
allowing me to use your facilities to freeze and process all my samples. It is no exaggeration
to say that your generosity saved me so much stress during these challenging times, and
everyone was so kind and welcoming every day I was there.
To my supervisors, Neil Loneragan and Anthony Hart, thank you deeply for your
ongoing encouragement and support. Anthony, thank you so much for bringing this project to
life, all your hard work behind the scenes and expertise has been invaluable. I thank you so
much for sharing your knowledge of the octopus’ ecology with me. Neil, where do I even
begin? It seems like so long ago I came to you wanting to do an honours project on octopus,
and despite what felt like a million setbacks, here we are today with a complete thesis! I have
nothing but appreciation and admiration for you Neil. Your unwavering support and kindness
have kept me going even through some of the trickiest times, and without you reminding me
to take a break every now and then I think I would have burnt myself out long ago. I know that
I am so lucky to have had you in my corner throughout this journey, and there are no words to
truly describe how much you have helped this project come to fruition. I thank you from the
bottom of my heart for everything.
Thank you to Claire Greenwell, without you I do not even know if I would have pursued
an Honours at all. From the very get-go, you encouraged me to push for a project I wanted, and
I am truly fortunate to have you as not only a mentor but also a friend. You are incredibly
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inspirational, with your passion and creativity, if I can become half the scientist you are, I will
consider myself very lucky.
To Charlie Maus, Kurt Krispyn, Thea Linke, James Tweedley and all of the beautiful
people from the fish and fisheries research group, I thank you for your friendship, all of the
laughs and especially all of the cakes. Thank you, Dave Murphy, for all your advice and for
keeping me on track. Thanks to Mikki and Shelly Smith, and especially your Nanna who leant
me her chest freezer and saved me in my moment of need. I would like to especially
acknowledge the beautiful staff members working in the Science Store, on more than one
occasion I had difficulties with accessing my account and I would not have been able to resolve
it without your help and patience.
To all my non-sciencey friends, your support and love have never gone unnoticed.
Thank you for not forgetting about me, putting up with me going MIA and being a much-
needed respite when I am feeling overwhelmed.
A massive thank you to my parents Bob and Jill, who have supported me through each
and every one of my crazy plans throughout the years. There are no words to describe just how
lucky I am to have you. To all of my family and Jake’s family, I love and cherish you all. A
special shoutout to Steve Cossington, as I do not think I would be a marine scientist without
having you as an inspiration.
And to Jake, safe to say that neither of us could have predicted that we would be here
together, all those years ago. Honestly, I do not think I can explain just how much you have
done for me not just throughout this Honours but also through all my years at university. Every
hurdle this year threw at us, your positive, can-do attitude kept me going. Thank you for putting
up with the 200 hundred octopuses in the freezer in our house and all my general clutter when
I had to try and make a home laboratory. For all your efforts, including learning how to dissect
octopus with me, your uncanny knack of being able to remove stylets and just generally keeping
me sane over the past year. Thank you for all the late nights you stayed up with me, for bringing
me dinners and even not complaining all the times you were covered in octopus’ ink and goop.
You did this all while you completed the final year of your degree, you truly are an incredible
person.
Finally, I would like to acknowledge the support afforded by the Myrtle AB Lamb
scholarship and the scholarship provided by the Harry Butler Institute.
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1. Introduction
Interest in cephalopod ecology has increased exponentially over the past few decades,
with a spotlight being focused on their importance in not only within the marine ecosystem but
also as valuable food resource, especially with growing concerns of the sustainability of
traditionally fished species (O'Brien et al., 2018). Coeloid cephalopods (i.e. those lacking an
external shell) are considered keystone species within many marine ecosystems. In many food
webs, cephalopods are mesopredators which establish the link between apex predators and
lower trophic levels (Boyle & Rodhouse, 2005; Heery et al., 2018; Yick et al., 2012). Being
mesopredators, cephalopods have the potential to influence the structure and dynamics of their
ecosystem (Yick et al., 2012). Due to their sensitivity to environmental change, cephalopods
can act as ecological indicators and exhibit top-down control over their prey (Boyle &
Rodhouse, 2005; Rodhouse et al., 1997). Furthermore, their short-lived, fast-growing life
history means they can rapidly respond to fluctuations within their environment, which can
cause detrimental consequences for upper and lower trophic levels within their ecosystem
(Tian, 2009).
In recent years, considerable focus has been directed to the increasing economic
importance of cephalopods as traditionally targeted finfish species continue to decline due to
anthropogenic stresses such overfishing and climate change (Caddy & Rodhouse, 1998; Rosa
et al., 2019; Watson & Pauly, 2001). This thesis focuses on examining the distribution and
abundance of the target species of the Western Australian Octopus fishery, Octopus aff. O.
tetricus, in near-coastal waters of Western Australia. It aims to build upon current knowledge
of cephalopods and their importance in commercial and ecological settings.
1.1.1 Cephalopod Fisheries
Scientific advances throughout the past 50 years have led to a massive increase in the
comprehension of marine ecosystem functions (FAO, 2020). Commercial fish production from
capture fisheries and aquaculture are estimated to have reached 179 million tonnes in 2018
worldwide, with an estimated value of 401 billion USD (FAO, 2020). Consumption of seafood
has increased by 3.1% annually between 1961 to 2017, which is almost twice the rate of
population increase (1.6%), and higher than other protein foods (2.1% annual increase) (FAO,
2020). Fisheries can, directly and indirectly, affect marine ecosystem predator-prey
relationships, which if overexploited may lead to the shift in trophic relationships and
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potentially loss biodiversity or collapse of fish stocks (Coleman & Williams, 2002; Thrush et
al., 2016).
Cephalopods have been an increasingly important resource in fisheries worldwide and
contribute 2‒5% of global landings from capture fisheries. Unlike finfish fisheries, which have
plateaued or declined, cephalopod fisheries have gradually increased from 0.5 million tonnes
in 1950 to a peak of 4.85 million tonnes in 2014, this number decreased to 3.5 million in 2018
but has continued to stay at relatively high levels (FAO, 2020).
Worldwide, octopus’ are fished using passive shelter pots, which are low cost, easy to
operate and cause minimal impact on the surrounding environment. However, the pots often
cannot be deployed in deep waters or areas of high energy (Hart et al., 2016; Leporati et al.,
2015; Sauer et al., 2019). Alternative trapping methods used by commercial octopus fisheries
include the frame traps, which utilise a netting that allows the octopus to push in but not back
out (Hernández-Garcı́a et al., 1998). More recently, the use of octopus traps with an active
closing mechanism, triggered by a tripwire, and bait have been trialled in commercial fisheries
(Carreira & Gonçalves, 2009; Sauer et al., 2019). The utilisation of active traps is prohibited
in many octopus' fisheries worldwide. However, the Department of Primary Industries and
Resource Development in Western Australia has recently conducted a trigger trap trial and
allows recreational fisherman to use up to six active traps (Hart et al., 2019; Leporati et al.,
2015; Sauer et al., 2019).
The management and assessment of commercial cephalopod fisheries are challenging
(FAO, 2020) because of the short life-cycle, fast growth rates and the variability in recruitment
and catch rates of cephalopods with environmental fluctuations (Boyle & Boletzky, 1996).
These challenges are highlighted by recent declines in three of the main commercial squid
species; jumbo flying squid (Dosidicus gigas), Argentine shortfin squid (Illex argentinus) and
Japanese flying squid (Todarodes pacificus) (FAO, 2020). In recent years, there has been a
decline in commercial octopus’ suppliers due to an increasing scarcity of octopus, as some
important fisheries have become less productive, requiring urgent and strict management
strategies (FAO, 2020).
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1.2.1 Australian Cephalopod Fisheries
Commercial cephalopod fisheries are a relatively new development in Australia.
Initially, international chartered vessels discovered significant resources of arrow squid
Nototodarus goudi off the southern coast of Australia in the 1970s (Kailola et al., 1993).
However, it was not until the mid-1980s that cephalopod stocks were exploited commercially
(Kailola et al., 1993; Nottage et al., 2007). Most cephalopod landings in Australia are taken as
a by-product from other fisheries, particularly lobster fisheries. Australian cephalopod
commercial landings primarily consist of squid, followed by octopus. Cuttlefish are caught
only by-catch of other fishing efforts (Kailola et al., 1993; Nottage et al., 2007).
The southern squid jig fishery (SSJF) operates between Fraser Island in Queensland to
the South Australian and Western Australian border and includes waters around Tasmania
(Nowara & Walker, 1998; Trinder & McKinley, 2003). The target species of the fishery is
arrow squid (Nototodarus gouldi) and has incidences of Sepioteuthis australis (southern
calamari), Todarodes filippovae (Southern Ocean arrow squid) and Omnastrephes bartrami
(red ocean squid) as bycatch. Incidental catches of N. gouldi are also caught in the Southern
and Eastern Scalefish and Shark Fishery. SSJF uses up to 10 automatic jig machines on each
vessel, each with 2 spools of heavy line, with ~ 25 jigs attached to each line (Furlani et al.,
2007).
At present, Australia has two dedicated octopus’ fisheries in the Eastern Indian Ocean,
the Octopus aff. O. tetricus fishery in Western Australia (for further detail see 1.4.1 below) and
the O. pallidus fishery in Tasmania (Sauer et al., 2019). The Tasmanian octopus’ fishery was
established in 1980, is operated by a single company with two full-time vessels which fish in
the Bass Strait (Leporati et al., 2009). The fishery uses passive shelter pots on demersal
longlines ~4 km in length, each line contains up to 1000 pots per line and the lines are deployed
in depths from18–85 m (Leporati et al., 2009).
1.3 Ecology of coastal cephalopods
Coleoid cephalopods (i.e. those without an external shell) are present in practically all
types of coastal habitats worldwide (Boyle & Rodhouse, 2005). They are not known to
inhabit marine environments with low salinity, and no cephalopods are endemic to estuarine
environments. Coastal species exhibit demersal lifestyles (i.e. strong association to the
seafloor) (Boyle & Rodhouse, 2005).
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1.3.1 Adaptations to the coastal environment
Demersal cephalopods utilise sophisticated organs to solve problems they encounter
within the natural environment (Hanlon & Messenger, 2018). Cephalopods have practical
locomotive adaptations (i.e. strong mantle, funnel, and fin muscles) and highly evolved sensory
organs. The sensory organs of cephalopods are the most evolved of all invertebrates and rival
the sophistication of vertebrate systems (Boyle & Rodhouse, 2005). Coleoid cephalopods
possess a ‘camera-like’ eye similar to vertebrates, containing a pupil, lens and rhabdomeric
photoreceptors which form the retina (Serb & Eernisse, 2008). Cephalopods are colour-blind
(Marshall & Messenger, 1996) but are sensitive to polarisation (Moody & Parriss, 1960;
Pignatelli et al., 2011; Shashar et al., 2000).
Most coleoid cephalopods have a complex nervous system, with a multi-lobed brain.
The brain to body ratio of cephalopods is akin to those of lower vertebrates (Mather & Kuba,
2013). The complexity of the nervous systems allows cephalopods to adapt to a wide range of
learning (Mather & Kuba, 2013) including; associative (Cole & Adamo, 2005), spatial (Alves
et al., 2006; Scatà et al., 2016), social (Tricarico et al., 2011) and problem-solving (Richter et
al., 2016). Cephalopods exhibit remarkable behavioural plasticity (Alves et al., 2006; Brown
& Piscopo, 2013; Hanlon & Messenger, 2018).
Coastal species actively use the benthic environment. They are predominantly bottom
feeders, feeding on the rich diversity of invertebrates and small fish available (Boyle &
Rodhouse, 2005). Octopuses have adapted sophisticated hunting and prey-handling (Boyle &
Rodhouse, 2005; Hanlon & Messenger, 2018). Demersal cephalopod eggs are encapsulated in
tough protective sheaths and fixed to the hard substrate, kelp plants and artificial surfaces such
as buoys or fishing traps. Moreover, octopuses are highly dependent on crevices, rock, and
reefs within the coastal environment for protection from predation (Anderson, 1997; Boyle &
Rodhouse, 2005; Mather, 1994; Mather & O'Dor, 1991).
Feeding ecology
Feeding and foraging of coleoid cephalopods have been comprehensively studied and
reviewed (Boyle & Rodhouse, 2005; Villanueva et al., 2017). Coleoid cephalopods have a
voracious, exclusively carnivorous feeding regime, functioning as mesopredators within the
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marine environment (Boyle & Rodhouse, 2005; Greenwell et al., 2019a; Hanlon & Messenger,
2018). Cephalopods possess a range of physiological, morphological, and behavioural
adaptations that help them be highly successful predators within the marine environment,
which can exhibit significant predatory pressure on benthic communities.
Cephalopods use versatile feeding apparatus (arms and tentacles) to feed on a diverse
group of prey (Boyle & Rodhouse, 2005). All cephalopods exclusively feed by dismembering
prey with their beak, consequently, diet analysis can be exceptionally challenging. Demersal
cephalopods primarily feed on crustaceans, fish and other cephalopods (Boyle & Rodhouse,
2005; Greenwell et al., 2019a; Greenwell et al., 2019b; Villanueva et al., 2017)As cephalopods
are opportunistic, their prey selection varies with prey availability which varies with depth and
habitat type (Smith, 2003). Variations in diets of octopus are also related to size and maturation
stage (Smith, 2003).
The main predators of coastal cephalopods are seabirds, marine mammals, and larger
fishes. Animals which predate on fish will also take cephalopods (Boyle & Rodhouse, 2005).
Except for a few elasmobranch species, no animals are known to exclusively predate on
cephalopods (Boyle & Rodhouse, 2005).
Habitat utilisation
Coeloid cephalopods have lost the external shell to increase mobility, and as a
consequence, they are very vulnerable to predation (Cigliano, 1993). Demersal species often
use dens or naturally occurring crevices for protection (Aronson, 1986; Hanlon & Messenger,
2018). The characteristics of the substrate where benthic species reside are thought to be a key
factor in influencing their distribution (Hanlon & Messenger, 2018). Studies investigating
habitat preference of octopus species found den availability and preference for the reef edges
were key factors affecting octopus’ distribution (Anderson, 1997; Mather, 1994).
Den availability has also been described as a limiting factor for octopus’ density
(Hartwick et al., 1984; Katsanevakis & Verriopoulos, 2004; Mather, 1994). Demersal
cephalopods routinely compete for shelters; with size and sex influencing the success of shelter
occupancy (Mather, 1982; Narvarte et al., 2013).
Sandy substrates often have limited shelter available for cephalopods which can impact
the density of cephalopods and the capacity for females to brood their eggs in these areas.
Molluscan shells are often used by an octopus to build suitable dens on sandy substrate.
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Narvarte et al. (2013) found that juvenile Patagonian octopus used barnacle shells and adults
utilised larger oyster and clamshells to create dens.
Camouflage
Crypsis and cryptic behaviour is the primary defence mechanism of coastal
cephalopods (Boyle & Rodhouse, 2005; Hanlon et al., 2010; Marshall & Messenger, 1996).
Cephalopod camouflage abilities exploit surrounding environmental features and allow them
to ‘blend in’ to their surroundings (Boyle & Rodhouse, 2005). Hanlon and Messenger (2018)
summarised the different camouflage techniques of cephalopods, including background
resemblance, countershading, disruptive colouration, and deceptive resemblance (Doubleday
et al., 2016; Villanueva et al., 2016).
1.3.2 Distribution and abundance
Cephalopod distribution varies considerably depending on species (Doubleday et al.,
2016). Demersal cephalopods exhibit the lowest dispersal capacity in shelf waters (within 10's
km). Pelagic cephalopods inhabit open oceanic waters and have the highest capacity for
dispersal (thousands of km), due to having a paralarvae stage (duration ~5-115 d) and a highly
motile adult stage
Octopus and cuttlefish tend to be concentrated in demersal habitats in coastal
environments (Boyle & Rodhouse, 2005; Quetglas et al., 2000), while the squids from the order
Myopsida are restricted to the neritic zone, likely due to their dependence on the benthos for
egg-laying and feeding opportunities (Pierce et al., 2008). Within shallow coastal habitats,
benthic octopuses dominate rocky reef habitats whereas, cuttlefish and squid species are more
commonly found in soft substrate habitats. True squids are predominantly pelagic, and their
distribution and depth of occurrence vary widely among species. Most studies examining
distribution and abundance of coastal cephalopods focus on north Atlantic and Mediterranean
populations (Arechavala-Lopez et al., 2019; Belcari et al., 2002; Quetglas et al., 2000; Vargas-
Yáñez et al., 2009) with far less published data on those in Southern Hemisphere (Ramos et
al., 2014).
Impact of urbanisation
Understanding the impact of urbanisation on cephalopod distribution and abundance is
still in its infancy. As marine urbanisation changes natural habitats, this is likely to alter
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cephalopod interactions with these environments. Artificial structures are known to support
distinct assemblages of marine fauna, especially mobile species such as octopus (Mather,
1994). At present, there is one published study investigating the impact of urbanisation on
distribution and abundance of octopus. Heery et al. (2018) observed urbanisation-related
patterns of Entereoctopus dofleini. Unlike terrestrial animals in urban environments which
benefit directly or indirectly from enhanced food resources from humans, there is no evidence
to suggest that octopus’ diets are subsidised by urbanisation (Heery et al., 2018). However,
octopuses do frequently utilise structures as dens and were more likely to occur in areas with
large amounts of anthropogenic debris (Heery et al., 2018). The use of artificial structures for
dens has been established in several studies (Katsanevakis & Verriopoulos, 2004; Mather,
1994). Urbanisation is likely to affect octopus abundance more in populations which occur on
sandy environments, where dens are scarce (Katsanevakis & Verriopoulos, 2004).
Historically, octopuses were identified as habitually solitary animals, only interacting
with one another in mating periods, with other encounters being hostile. Recently, a few studies
have identified more social behaviours (i.e. aggregation and interaction) in certain species
including O. tetricus (Scheel et al., 2017) and the undescribed Larger Pacific Striped Octopus
(Caldwell et al., 2015). Scheel et al. (2017) identified O. tetricus residing in high densities in
two locations in Jervis Bay, on the east coast of Australia. Typically, O. tetricus has been
identified as a solitary species but within these locations exhibit complex social behaviours. It
has been hypothesised that these large aggregations form in response to limited den availability
but an abundant food supply (Scheel et al., 2017). It has been suggested that clustered shelters
heighten social interactions of octopus (Scheel et al., 2017).
Environmental influences on distribution and abundance
Environmental conditions and the strength of recruitment have a significant impact on
abundance (Sobrino et al., 2002). The size of cephalopod populations is variable at annual
timescales with rapid expansion occurring when conditions are favourable and rapid decline
when conditions become unfavourable (Rodhouse, 2010). In recent years there has been
growing speculation that cephalopod populations may be proliferating in response to changing
conditions, as evidenced by the increasing trends in cephalopod catch rates (Doubleday et al.,
2016). It is well established that cephalopods are highly sensitive to shifts in environmental
conditions. Cephalopods respond to changes in ecological conditions either actively (i.e.
moving with changing conditions to areas more favourable for spawning or feeding) or
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passively (i.e. effects on growth and paralarvae survival) (Pierce et al., 2008). Due to their
sensitivity to changing conditions, cephalopods could potentially act as environmental
indicators (Pierce et al., 2008).
Octopus distribution shows a significant relationship between size and depth, with an
ontogenetic shift to deeper waters with increasing size (Leite et al., 2009). The ontogenetic
distribution also appears to be linked with environmental conditions such as temperature.
Smaller individuals of O. insularis are known to occupy warmer waters than larger individuals,
likely due to the high productivity attributed to warm waters, which promotes faster growth
and therefore, shortens the period in which they are most vulnerable to predation (Leite et al.,
2009). Migration to brooding habitats has been documented for several species of octopus
including Octopus. aff. O. tetricus (Leporati et al., 2015), O. magnificus (Smith et al., 2006),
O. tetricus (Anderson, 1997), O. insularis (Leite et al., 2009; Lima et al., 2014) and, O. vulgaris
(Belcari et al., 2002; Katsanevakis & Verriopoulos, 2004).
Many coastal octopus species exhibit interannual variability and cyclical patterns of
abundance (Regueira et al., 2014). The relationship of the abundance of O. vulgaris and the
variability of SST were investigated in the Alboran Sea (Vargas-Yáñez et al., 2009). Results
showed that O. vulgaris abundance was adversely affected by warm water anomalies occurring
in the previous year (Vargas-Yáñez et al., 2009). These findings indicated that O. vulgaris
landings could be negatively affected by long-term warming of the Western Mediterranean.
Similarly, Sobrino et al. (2002) identified a beneficial influence from low winter temperature
for the next years landings. They found that cephalopod landings in the Gulf of Cadiz reached
a maximum in winter before decreasing to their minimum in September. Other studies in the
Mediterranean Sea found similar results, with minimum abundance occurring between August
and September (Vargas-Yáñez et al., 2009). The influence of temperature is intrinsically linked
to other environmental and biological variables (i.e. upwelling intensity, enhanced primary
production, embryonic and paralarvae growth) (Sobrino et al., 2002; Vargas-Yáñez et al.,
2009).
Abundance patterns of tropical cephalopod species do not exhibit the same seasonal
variations as those seen in temperate and sub-tropical areas (i.e. O. vulgaris, Katsanevakis and
Verriopoulos, 2004; Entereoctopus dofleini, Hartwick et al., 1984). The abundance of tropical
species appears to be more influenced by habitat characteristics such as depth and substrate
than temperature (Leite et al., 2009).
17
There are growing concerns about the impact of long-term warming and climate change
on cephalopod populations. Warming temperatures are known to increase growth rates which
subsequently accelerates their life cycles (Doubleday et al., 2016) and could cause a faster
population turnover. Warmer temperatures are also known to cause premature hatchings,
leading to decreased chances of survival during the crucial first few days (Uriarte et al., 2016).
The rapid life cycle of cephalopods will likely aid in their adaptations to changing oceanic
conditions, and some cephalopod species may benefit from changing conditions (Doubleday et
al., 2016).
1.3.3 Reproduction and Brooding
Nearly all cephalopods are semelparous (i.e. one reproductive episode) (Boyle &
Boletzky, 1996; Boyle & Rodhouse, 2005). Unlike other molluscs, the sexes are separate, and
there is no evidence of hermaphroditism or sex change within cephalopods.
Assessment of maturity stages is an important aspect of both ecological studies and
fishery management. The temporal and spatial distributions of breeding aggregations, the size
at maturity and the sex ratio all require an assessment to ensure population levels are sustained.
The mating strategy of cephalopods is uncommon among other molluscs, reproduction
always takes place through individual mating. Octopus males possess a hectocotylus (i.e.
modified arm) which is used to transfer spermatophores to females during copulation.
Spermatophores can be stored within the mantle cavity, which means that a significant time
may pass between mating and fertilisation. Males are likely to mate with several females.
Unlike other cephalopods that may spawn in batches over an extended period, octopuses tend
to only lay eggs over a few days (Boyle & Rodhouse, 2005)
Locating a suitable egg-laying habitat is important to demersal cephalopod
reproductive success (Pratasik et al., 2017). Habitat selection for other demersal cephalopods,
especially for mature females, is influenced by suitable environments to brood their eggs
(Guerra et al., 2015; Narvarte et al., 2013; Pratasik et al., 2017). The quality of cephalopod
shelters is known to affect reproductive output (Iribarne, 1990; Narvarte et al., 2013).
18
All female octopuses studied are known to protect and clean their eggs throughout the
entire duration of embryonic development (Robin et al., 2014). Female benthic octopuses brood
their eggs in rocky crevices, coralline overhangs, or confined spaces (Boyle & Rodhouse,
2005). Eggs are deposited into the den using two methods: (i) the eggs are individually
encrusted into the hard substrate or; (ii) cemented into the hard substrate in clusters where
individual eggs are intertwined with one another (Hanlon & Messenger, 2018). Some genera
of octopus that occupy habitats without suitable brooding shelters carry their developing eggs
within the webbing of their arms, e.g. Hapaloclaena (Norman et al., 2000) and Amphioctopus
(Guerrero-Kommritz et al., 2016; Guerrero-Kommritz & Rodriguez-Bermudez, 2018).
Narvarte et al. (2013) investigated the effect of egg predation and competition on
brooding shelter selection of O. tehuelchus in San Antonio Bay, Patagonia Argentina. They
found that shelter availability was a constraint on brooding and affected the level of parental
care for the fertilised eggs. Furthermore, Iribarne (1990) found that the fecundity of
O. tehuelchus was higher in large-volume shelters than smaller shelters. If octopuses of both
sexes at a similar size compete for a single shelter, the female will win occupancy of the shelter
(Narvarte et al., 2013). Contest over shelters between sexes has been observed for several
species including O. tehuelchus (Narvarte et al. 2013), O. dofleini (Hartwick et al. 1984) and
O. briareus (Aronson, 1986). Competition for shelters is more apparent during brooding
periods and a female octopus may inhabit a single shelter for more than two months (Narvarte
et al., 2013).
The final life stage for octopuses is a period of senescence (Anderson et al., 2002).
Usually, males undergo senescence after mating, while females senesce while brooding their
eggs. Typically, symptoms of senescence include lack of feeding, retraction of skin around the
eyes, decreased physical abilities and lesions on the skin (Anderson et al., 2002; Boyle &
Rodhouse, 2005; Wodinsky, 1977). Many aspects of octopus senescence are still poorly
understood (Anderson et al., 2002).
After spawning, female octopus' decrease and then stop feeding during the incubation
of the eggs, focusing all energy entirely on egg care until the hatchlings emerge (Boyle &
Rodhouse, 2005; Hanlon & Messenger, 2018; Robin et al., 2014; Roumbedakis et al., 2018b).
Maternal care from female octopuses usually involves protecting eggs from predators,
increasing ventilation by flushing water over the eggs, cleaning eggs and removing any dead
embryos (Roumbedakis et al., 2018b). The physiological condition of female brooding octopus
declines over time. The starvation behaviour of female octopuses during the brooding stage
19
is well established throughout literature. In a study of O. hummelincki, Wodinsky (1977)
suggested that the biological trigger for this behaviour could be linked to secretions of the
optical glands. Experiments involving the removal of the optical gland after spawning
revealed that O. hummelincki females returned to feeding, gained weight and the lifespan was
extended by four months or more (Wodinsky, 1977).
1.4 Octopus aff. O. tetricus
Octopus aff. O. tetricus is a merobenthic (i.e. has a paralarval stage) species endemic
to temperate coastal waters (≤ 70 m deep) of Western Australia. This species is relatively short-
lived (≤ 18 months), medium-sized (< 4 kg) and is highly fecund (125 000 – 150 000) (Joll,
1976). Once considered synonymous with the Common Sydney Octopus, Octopus tetricus,
taxonomic studies suggest that Octopus aff. O. tetricus (not yet formally described) is a
separate species, with a geographically distinct population that extends from Shark Bay to the
South Australian Border (Amor et al., 2014). Octopus aff. O. tetricus and O. tetricus are closely
related to the cosmopolitan species O. vulgaris.
Similar to many other octopus’ species, Octopus aff. O. tetricus (hereafter O. tetricus)
is semelparous. Both sexes of O. tetricus are known to have similar life expectancies, with
males reaching maturity at an earlier age than females (Leporati & Hart, 2015). The maximum
size for females (~4 kg) is considerably larger than males (~2.5 kg). This species inhabits rocky
reefs, sandy substrates and seagrass meadows Recent studies have provided evidence of
mature females migrating to deep waters to find appropriate dens to brood their eggs,
followed by mature males seeking mates (Leporati et al., 2015).
1.4.1 Western Australian Octopus Fishery
The first commercial fishing operation for octopus in Western Australia was instigated
by Japanese scientists during 1979 – 1981 as a mitigation strategy for the extensive octopus’
predation and continual bycatch within the Western Australian rock lobster (Panulirus cygnus)
fishery (Joll, 1977). At present, commercial octopus fishing yields approximately 204 tonnes
from three primary fisheries; the Western Australian Developmental Octopus Fishery, the
West Coastal Rock Lobster Fishery and the Cockburn Sound Pot and Line Fishery (Hart et
al., 2016; Sauer et al., 2019) Since the establishment of the Western Australian Octopus
Fishery in 2001, the primary trap used has been passive shelter pots; a light, open-ended
plastic pot which is attached to a
20
demersal long line and deployed in water < 30 m. Due to the tendency for the shelter pots to
be buried or lost in high energy areas, the fishery operations have been limited to shallow areas
around the Perth coastline. The Perth fishery consistently yielded 30 tonnes per year while
exclusively using passive shelter pots (Sauer et al., 2019). In 2010, active trigger traps, consisting
of a rectangular trap baited with a plastic crab attached to a tripwire that activates a trap door when
seized by an octopus (Figure 1.1), were introduced into the fishery (Leporati et al., 2015). These
traps are considerably heavier and require less soak time than passive pots and can be used in
deeper waters around Perth. The introduction of trigger traps in Perth saw the catches increase
substantially to 170 tonnes in the first year of their use. Research conducted by Leporati et al. (2015)
suggested that the catch composition of each trap type also differed significantly, with passive traps
predominantly catching smaller, maturing individuals of both sexes and trigger traps catching
predominantly large (>1 kg), mature males with very few mature females. However, the authors
state that there was no overlap of depths in which each trap type was deployed, therefore
differences in catch compositions between traps cannot be determined conclusively.
The target species in the Western Australian Octopus Fishery is Octopus aff. O. tetricus
although other species are also caught occasionally, such as O. cyanea off the northern coast
and Macroctopus maorum in southern waters (Hart et al., 2016).
Figure 1.1 Cradle of the trigger traps used in the Western Australian Octopus Fishery.
The volume of trap ~15 L, opening area ~12.8 cm2 (Leporati et al., 2015).
21
1.5 Rationale and study aims
The overall aim of this Honours project is to investigate the spatial and temporal
distribution patterns of Octopus aff. O. tetricus in the nearshore, coastal waters in North
Fremantle, Western Australia (Figure 2.1). This region is located ~7 km north of the
traditional commercial fishing grounds. The North Fremantle site was selected following
discussions with commercial octopus fisher (P. Jasinski) about catches of small juvenile
octopuses in the area, indicating that this area is potentially a spawning or nursery area for O.
tetricus. The specific aims of the thesis are to:
(i) Investigate how octopus abundance varies with depth and sex;
(ii) Determine the size distribution and growth of octopus within the area; and
(iii) Compare the size distribution of octopus from this area with that on the fishing
grounds in Cockburn Sound, about 7 km south of the North Fremantle study site.
Detailed studies of octopus diet in shallow and deep waters of the area north of
Fremantle were also planned. Although gastric tracts were removed during dissection and all
dietary items in the gastric mill and stomach identified, it was not possible to analyse these
data comprehensively within the time constraints of Honours. However, preliminary findings
are presented from this component of the research. During the dissection, high numbers of
parasites were identified, and an additional aim was introduced:
(iv) Provide a preliminary description of the diet of octopus in coastal waters, and
(v) Determine the incidence, distribution, and abundance of parasites in octopus in the
coastal waters north of Fremantle and compare this (a) size classes of octopus, (b)
between shallow and deep water and (c) with that in Cockburn Sound.
2. Methods
2.1 Study site
Octopus’ were collected within the near coastal waters between North Fremantle and
Cottesloe, Perth, Western Australia during contracted commercial fishing in the area (31°58 S
115°49 E) (Fig. 2.1). This area is north of the main fishing grounds of the Octopus Fishery,
which is conducted within the Cockburn Sound (DoF, 2015), The study site is located
approximately 1.5 km from the nearest land, where depths range from ~8 to 22 m and the
22
benthic habitat consists of a sandy substrate with seagrass and macroalgal beds dispersed
irregularly throughout (P. Jasinski, Octopus fisher, pers. comm.).
2.2 Collection of Octopus aff. O. tetricus
Octopus were collected using open-ended, weighted, passive shelter pots (Figure. 2.2),
that rely on the octopuses entering the pots for shelter. Shelter pots have a 16.9 cm2 opening,
are 38 cm long and have a volume of 6 L. A commercial fishing vessel, chartered by the
Department of Primary Industries and Regional Development (DPIRD), deployed four
demersal long lines (1 – 1.4 km in length), each comprising 150 pots placed < 10 m apart, and
left in the water for soak times of ~7 to 22 days on each sampling occasions. A total of seven
separate sampling trips were conducted between the first haul on 23rd of March and ending
with the final retrieval of the lines on 15th of June 2020. Two lines (A and B) were set in shallow
water depths of 8 to 10 m and the other two lines (C and D) were set in water > 10 m deep
(Figure. 2.1, see Appendix 1.1 for coordinates of each line). The lines were reset in the same
location each time after being hauled following the normal commercial octopus fishing
operation (Figure. 2.1). The lines were separated by distances of ~500–1500 m and covered
an area of ~7.8 km2. One sample of octopus mantles was obtained from the commercial
octopus’ fishery which operates within Cockburn Sound and, in surrounding waters of Carnac
and Garden Island on 1st of April 2020. Data on size distribution, sex ratios, gonad condition
and parasites from this sample were compared with those from the main study area at the same
time.
After collection, octopus' mantles were severed from their arms, following the
standard commercial practice, and the mantles placed immediately placed on ice. The arms of
the octopus of commercial size i.e. > ~300 g wet weight, were kept by the commercial
fisherman for sale. Whole small octopus’ i.e. < ~300 g were retained on the sampling trips
between 14th April and 15th June and placed on ice. Samples were frozen within 2 h of capture.
23
Figure 2.1 Map showing the location of the four octopus trap lines set between March
and June 2020 in the coastal waters north of Fremantle Harbour, Western
Australia. Each line had 150 traps and was ~1 to 1.5 km in length.
Figure 2.2 Passive shelter pots used in the Western Australian developmental octopus’ fishery (from Leporati et al. 2015). Trap dimensions are 38 x 14 x 17 cm.
24
2.3 Biological processing of Octopus aff. O. tetricus
After defrosting the mantles, the mantle weight (MW; to 0.1g) and mantle length (ML;
mm) were recorded for each octopus. The small octopuses were weighed whole (TW).
The total weight (TW) was estimated from ML using the empirical relationships in Hart et al.
(2019):
Females: TW = 0.00054 ML 2.94 (r2 = 0.90, n = 345)
Males: TW = 0.0005 ML 2.86 (r2 = 0.90, n = 372)
The mantle weight of small juvenile octopus collected was estimated using the empirical
relationships in Hart et al. (2019):
Females: MW = 0.0004 ML 2.94 (r2 = 0.90, n = 10)
Males: MW = 0.0005 ML 2.86 (r2 = 0.90, n = 2)
2.3.1 Maturation
The mantles were then dissected, and the sex of the octopus determined from a
macroscopic inspection of the gonads. The gonads were removed and weighed to the nearest
0.01 g, and the maturity stage was determined by visual examination using the classification
scheme outlined in Leporati et al. (2015) and summarised in Table 2.1. The gastric tract,
including the gastric mill and stomach, was also removed from the mantle for dietary studies.
Although all samples have been analysed to identify and quantify dietary items, these data are
not presented as part of the Thesis due to time constraints. However, details of the processing
of samples are provided below.
25
Table 2.1 Summary of female and male gonad maturation stages of Octopus aff. O. tetricus from
Leporati et al. (2015). Note, Stage IV males were not described by Leporati et al.
Gonad stage Female Male
I Immature Ovary whitish, very small, no signs of granulation. Spermatophoric organ transparent and whitish.
II Developing Ovary yellowish with signs of granulation. Spermatophoric organ with white streaks of sperm
III Mature Ovary very large, yellow/ orange in colour, or clear if in the process of laying eggs. Oviducts and oviducal glands enlarged
Needhams sack full of spermatophores
IV Spent Ovary purple in colour and flaccid, with few to no eggs
2.3.4 Parasites
During preliminary stages of dissecting octopus’ mantles, several parasites were
observed within the viscera. Consequently, the internal organs of the mantle i.e. the crop,
digestive gland, alimentary canal, gonads and tissues, were examined for parasites under a
dissecting microscope. Parasites were removed with tweezers, counted, with their location
recorded and preserved in 70% ethanol.
The parasites found in the mantle and its associated organs were tentatively identified
as anisakid nematodes by Professor Alan Lymbery (Murdoch University). The identification
of nematodes to greater resolutions requires the use of molecular methods which was not
possible within the scope of this Thesis. Therefore, their identification was only possible to
family level (Anisakidae).
2.4 Data analyses
All data exploration and analyses were carried out using R version 4.0.2 (R Core Team,
2020) in the integrated development RStudio, version 1.3.1093 (RStudio Team 2020). All
figures were created using Microsoft ExcelTM.
Prior to all parametric statistical analysis, the assumptions of normality and
homogeneous variance were assessed by using Q-Q plot and Levene’s test, respectively. When
violations of assumptions were detected, the data were transformed.
2.4.1 Environmental parameters
Data on the monthly mean minimum and maximum sea surface temperatures for the
period from July 2019 to June 2020 were obtained from seatemperature.org that summarises
26
NOAA daily satellite readings (http://www. seatemperature.org, 30th October 2020). This covers
the sampling period and the months before sampling commenced.
2.4.2 Distribution and abundance of octopus
A three-way analysis of variance (ANOVA) was used to test whether the numbers of
octopus differed between sexes (Sex), water depths (Depth) and sampling times (Haul). A post
hoc Tukey’s Honest Significance Difference (HSD) was used to investigate any significant
difference between levels of factors. Abundances were compared without adjusting for soak
times.
2.4.3 Octopus size structure
The overall mean (± 1 SE) of the mantle weight (MW) and mantle length (ML) of
Octopus aff. O. tetricus males and females were calculated. Preliminary diagnostic plots
established that no variables violated the assumptions of the T-test and so the data were not
transformed. A two-sample independent T-test was used to test whether mean values for MW
and ML differed significantly between male and female octopuses.
A two-way analysis of variance (ANOVA) was performed to test whether ML differed
significantly between the North Fremantle and Cockburn Sound sites and between sexes.
2.4.4 Mantle length distributions
Octopuses were grouped into 10 mm mantle length (ML) classes and length percentage
frequency histograms were constructed for each sex, both depth regions and each sampling
trip, including octopus collected from the Cockburn Sound site. Descriptive statistics were
used to summarise the size distribution of the mantle length (ML).
The mean size of octopus’ from North Fremantle and Cockburn Sound on 1 April was
compared using an independent T-test.
2.4.5 Length-weight relationships
The relationships between mantle length (ML) and mantle weight (MW) for Octopus
aff. O. tetricus were estimated using a power function in Microsoft Excel for each sex. The
equation for the length-weight relationship was MW = aMLb where a and b are constant.
In R, the ML and MW data were transformed (Log10) and a linear equation was fitted to the
data:
27
𝐿𝑜𝑔10(𝑀𝑊) = 𝑎𝐿𝑜𝑔10(𝑀𝐿) + 𝑏
Analyses of covariance (ANCOVA) was used to determine if the relationship between MW
and ML differed between the sexes (α = 0.05) using ML as the covariate.
2.4.6 Gonadosomatic Index
The gonadosomatic index (GSI) was calculated as a percentage for each octopus as the
gonad weight (GW) divided by the mantle weight (MW) minus the gonad weight (GW):
𝐺𝑊 𝐺𝑆𝐼 = (
𝑀𝑊 − 𝐺𝑊) ∗ 100
The mean GSI (± 1 SE) was calculated for each month for males and females separately, for
both North Fremantle and Cockburn Sound samples. The monthly proportion of O. tetricus at
each stage of gonad development was plotted separately for males and females in North
Fremantle.
2.4.7 Parasites
The intensity (i.e. the number of parasites per octopus) and prevalence (i.e. the number
of octopuses with at least one parasite) was calculated following the definitions given in Bush
et al. (1997). Generalised linear models (GLMs) were used to test whether the presence of
parasites was influenced by depth, month, sex and size of an octopus. Octopuses were assigned
to three size groups based on mantle length classes; i.e., small = < 80 mm, medium = 80-150
mm and large = ≥160 mm, in an attempt to measure the influence of octopus size on parasite
infection.
Three binomial GLMs were used to test the presence of at least one parasite per octopus.
For each GLM the binomial response variable was the presence of nematodes (presence = 1,
absence = 0), which was realised through the logit link function. The binomial model was:
𝑝 𝐺(𝑦) = 𝑙𝑜𝑔𝑖𝑡 (
(1 − 𝑝)
) = 𝑥𝑇𝛽
where, p is the probability of at least one parasite being present, and 𝑥𝑇 denotes the vector of
predictor variables and 𝛽 is the vector of coefficients.
The first model aimed to assess the predictor variables of time (month), size and sex for
the North Fremantle O. tetricus population. The second model used size and sex as predictor
variables for the population from Cockburn Sound and the third model, tested for differences
28
between North Fremantle and Cockburn Sound, using only samples from North Fremantle
collected on the 1st April 2020
2.5 Diet Analysis
During dissection, the gastric tract was removed, with the crop being clamped to avoid
stomach and crop contents mixing. The stomach fullness was estimated by eye on a scale 0 to
10, with 10 being the equivalent to 100% full. The contents from the gastric tract were rinsed
under water through a 500 µm sieve to remove silt to ease in the identification of prey
components. Contents were examined under a dissecting microscope and identified to the
lowest taxonomic level possible, based mainly on an examination of hard parts such as
crustacean exoskeletons, teleost vertebrae, radulae of molluscs and the byssus strands of
bivalves. Cephalopods could be identified by flesh with distinct appearance of chromophores
and suckers, remnants of beaks and presence of black ink throughout the gastric tract. The
contribution of each prey item was estimated by eye and expressed as a percentage. Any prey
items which were heavily masticated or in an advanced stage of digestion was recorded as
‘unidentified’. The percentage frequency of occurrence was calculated and the %volume of
each item was estimated. Only preliminary findings from the dietary studies are presented in
the results.
2.6 Permits and approval of ethics committees
All necessary permits were obtained for this study. Octopus were collected by the
commercial fisher for this research under Murdoch Animal Ethics – Cadaver and/or Tissue
Notification, Permit No. 744 “Cadaver application for octopus’ ecology”.
3. Results
3.1 Water temperature and storms
The mean maximum water temperature derived from NOAA data on SST during the
period from March to June 2020 ranged from 25.1 °C in March to 23 °C in June (Figure 3.1).
In the 12 months before this, the lowest mean maximum water temperature was in September
2019 (19.9 °C) and the highest mean (25.3 °C) was in February 2020.
29
During the soaking period of the fourth and fifth haul, considerable storm activity in
Perth caused the loss of several octopus’ pots and moved large amounts of sand into the traps,
resulting in the suffocation of at least one octopus.
27
25
23
21
19 Minimum
17 Maximum
15
Month Study period
Figure 3.1 Mean sea surface temperature in North Fremantle, Western Australia from daily
satellite readings extracted from http://www.seatemperature.org (30th October
2020). Arrow on the X-axis shows the duration of sampling octopus in the region
3.2 Changes in numbers with depth and month
A total of 713 octopuses were collected during the seven sampling trips between 23rd
March and 16th June 2020. These comprised 344 females, 358 males and 12 individuals whose
sex could not be determined. The highest number of octopus were caught from Line B (n =
194), followed by Line A (n = 172), both in waters < 10 m deep. A similar number were caught
on Line C (n = 163) and the lowest numbers from Line D (n =140) in the deepest water (> 10
m). A total of 49 octopuses (30 females, 19 males) were collected from the Cockburn Sound
on 1st April 2020. This compares with the second haul, caught in the waters of North Fremantle
on 1st April 2020 of 90 octopus, with 43 males and 47 females.
The third haul on 14th April 2020 yielded the highest number of octopuses (135) when
54 males and 78 females were caught (Figure 3.2). The last haul yielded the lowest number of
octopus (70), with equal numbers of males and females. The lowest number of males (6) per
line was collected on 2nd June and the lowest number of females (5) per line on 11th May, both
Tem
per
atu
re °
C
July
Au
gust
Sep
tem
ber
Oct
ob
er
No
vem
ber
Dec
emb
er
Jan
uar
y
Feb
ruar
y
Mar
ch
Ap
ril
May
Jun
e
30
on Line D. The biggest discrepancies between male and female catches were recorded was on
the 23rd March on Line B (9 females, and 23 males) and on 14th April on Line D (21 females,
and 7 males).
A two-way analysis of variance (ANOVA) showed that the mean number of octopuses
collected per haul differed significantly (P = 0.01) among hauls but not between depths or sexes
and that none of the interaction terms were significant (P >0.05, Table 3.1). The Tukey’s post
hoc test showed that only the third and seventh haul differed significantly (P = <0.05), with a
significantly higher mean catch on the third haul (14th April, 16.5±0.49) than on the 7th haul
(15th June, 8.4±0.20).
31
Nu
mb
er o
f O
cto
pu
ses
Nu
mb
er o
f O
cto
pu
ses
a) c)
25 25
20 Females 20 Males
15 15
10 10
5 5
0 0
Haul Haul
b) d)
25 25
20 20
15 15
10 10
5 5
0 0
Haul Haul
Figure 3.2 The number of female (black) and male (grey) Octopus aff, O. tetricus collected on each line from North Fremantle, Western Australia for each sampling trip between 23rd March and 15th June 2020. a) Line A, b) Line B, c) Line C and d) Line D. Mean depths of lines were: A = 9.29 m; B = 9.64 m; C = 16.79 m; D = 15.34 m.
Nu
mb
er o
f O
cto
pu
ses
Nu
mb
er o
f O
cto
pu
ses
23
-Mar
2
3-M
ar
1-A
pr
1-A
pr
14
-Ap
r 1
4-A
pr
24
-Ap
r 2
4-A
pr
11
-May
1
1-M
ay
2-J
un
2
-Ju
n
15
-Ju
n
15
-Ju
n
23
-Mar
2
3-M
ar
1-A
pr
1-A
pr
14
-Ap
r 1
4-A
pr
24
-Ap
r 2
4-A
pr
11
-May
1
1-M
ay
2-J
un
2
-Ju
n
15
-Ju
n
15
-Ju
n
32
1
1
Table 3.1 Summary of results for the three-way analysis of variance (ANOVA) to test differences in the abundance of Octopus aff. O. tetricus between depth regions, sex, and sampling haul. Significant factors are in bold. df = degrees of freedom, MS = mean squares.
Source of variation df MS % MS P-value
Haul 6 49.03 26.94 0.01
Depth 1 27.16 14.92 0.18
Sex 1 17.16 9.43 0.29
Haul*Depth 6 27.41 15.06 0.12
Haul*Sex 6 32.99 18.13 0.07
Depth*Sex 1 0.16 0.09 0.92
Haul*Depth*Sex 6 13.49 7.41 0.49
Residuals 28 14.59 8.02
3.3 Sex ratio
Overall, the sex ratio for female and male Octopus aff. O. tetricus sampled from the
North Fremantle site was 1.08:1 (339 males, 313 females), which does not differ significantly
from 1:1 (χ21 = 0.959, P = 0.328) (Table 3.2). Neither did the overall sex ratio differ
significantly between shallow and deep locations (χ21 = 0.041, P = 0.8). Comparisons of the
sex ratios across the study region (i.e. pooled for shallow and deep water) for each haul showed
a significant dominance of males in the catches in May (1.89:1) (χ21 = 7.440, P <0.01) but that
sex ratios did not differ significantly from 1:1 for any other haul. (Table 3.2).
Within the shallow region, the only haul in which the sex ratio differed significantly
from 1:1 was the first haul (23 March, χ21 = 4.063, P = 0.04), with a male to female ratio of
1.73:1. In the deeper region, sex ratios differed from 1:1 in two hauls: on the third haul (14
April) significantly more females than males were caught (0.58:1, χ2 = 3.750, P = 0.05), and
on the fifth haul, significantly more males than females were present (1.89:1, χ2 = 5.357,
P = 0.02) (Table 3.2).
The sex ratio for octopuses collected from Cockburn Sound (0.60:1) did not differ significantly
from that for octopuses sampled from North Fremantle on 1st April 2020 (0.91:1) (χ21 = 0.956,
P = 0.328).
33
Table 3.2 The observed sex ratio (male: female), sample sizes (n), P-values (P) and chi-squared values (χ2), presented for overall ratio, by month and the two depth regions for Octopus aff. O. tetricus collected from North Fremantle near-coastal waters between March and June 2020. Bold values = Significant differences from 1:1
Haul <10 m n χ2 P >10 m n χ2 P Overall n χ2 P
23-Mar 1.73:1 63 4.063 0.04 1.05:1 37 ~0 ~1 1.43:1 100 2.890 0.09
1-Apr 0.72:1 55 1.163 0.28 1.34:1 35 0.457 0.50 0.91:1 90 0.100 0.75
14-Apr 0.80:1 72 0.681 0.41 0.58:1 60 3.750 0.05 0.69:1 132 4.008 0.05
24-Apr 1.35:1 56 0.446 0.50 0.89:1 34 0.029 0.86 1.09:1 90 0.100 0.75
11-May 1.63:1 42 1.929 0.17 2.23:1 42 5.357 0.02 1.89:1 84 7.440 <0.01
2-Jun 1.05:1 39 ~0 ~1 1.32:1 51 0.706 0.40 1.20:1 90 0.544 0.45
15-Jun 1.12:1 36 0.028 0.868 31 0.129 0.72 0.97:1 67 ~0 ~1
Overall 1.10:1 362 0.798 0.371 1.06:1 290 0.169 0.681 1.08:1 652 0.959 0.327
3.4 Size distribution
3.4.1 Overall size distribution
A total of 709 octopuses were measured and weighed. The smallest female was
measured at 35 mm ML, 31.94 g total weight (TW) and as the octopus was taken whole the
mantle weight was estimated at 5.13 g. The largest female was 211 mm ML, 687.72 MW and
an estimated TW of 2397.96 g (Table 3.3). The smallest male was 14 mm ML larger than the
smallest female (mm), but the largest male was a similar size to the largest female (Table
3.3). The mean estimated TW for females was
882.38 g which was significantly lighter than that of males (1253.01 g) (T = -7.926, df =
633.56, P <0.001). The mean ML and MW did not differ significantly between females (141.49
mm, 262.52 g) and males (144.83 mm, 264.44 g) (P > 0.5, Table 3.3).
Eleven of the small juvenile O. tetricus were too small to determine their sex. The
smallest of which was 14 mm ML and 2.75 g TW. The average TW for undetermined
individuals was 3.70±1.70 g and average ML was 35.3±6.82 mm.
Females collected from the Cockburn Sound site ranged from 102 to 217 mm ML and
71.3 to 588.8 g MW, with an estimated TW range of 299.9 to 2598.2 g. Males collected from
Cockburn Sound ranged from 99 to 191 mm ML, 80.5 to 441.74 g MW and an estimated total
weight range of 368.2 to 2542.2 g.
34
Table 3.3: Summary of the mean sizes (± 1 SE) and range of mantle length (ML), mantle weight (MW) and total weight (TW), for Octopus aff. O. tetricus caught in waters north of Fremantle, Western Australia. The t-statistic (T) and significance (P) of the Welch two-sample t-test between sexes also shown. df = degrees of freedom
Sex n Mean±SE Range T P
Mantle length (mm) (df = 611.05)
F 313 141.49±1.95 35-211
M 339 144.83±1.57 49-218 -1.333 0.18
Mantle weight (g) (df = 604.99)
F 313 262.52±8.35 5.13-687.72
M 339 266.86±6.59 15.22-691.62 -1.333 0.18
Estimated total weight (g) (df = 633.56)
F 313 882.38±28.59 31.94-2598.18
M 329 1253.01±34.75 29.95-3750.01 -7.926 <0.001
The total weight (TW) estimated from the equation in Hart et al. 2019
The mean MLs in the shallow and deep waters was almost identical (< 10: 143.21 mm,
> 10: 143.25 mm, P ~ 1). The mean MW for the shallower region was 268.52 g, slightly, but
not differ significantly greater than that for the deeper region of 260.11 g.
3.4.2 Mantle Length-weight relationship
An analysis of covariance (ANCOVA) found that the slopes of the relationship between
Log mantle length (ML) and the Log mantle weight (MW) (Figure 3.3) did not differ
significantly between males and females, therefore the data were pooled. The equations for the
relationship was:
Log(MW) = 2.483Log(ML) – 6.849 (n = 696, r2 = 0.92)
Table 3.4 Summary of results from the analysis of covariance (ANCOVA) to test whether the
relationship between mantle weight (MW) and mantle length (ML) differed
significantly between sexes in Octopus aff. O. tetricus. Both MW and ML were log-
transformed.
Variable df MS F P
LogML 1 259.38 5137.46 < 0.001
Sex 1 0.002 0.03 0.853
LogML*Sex 1 0.184 3.65 0.057
Residuals 646 0.050
35
a)
b)
Figure. 3.3 The relationship between mantle length (ML) and mantle weight (MW) and mantle
length (ML) for Octopus aff. O. tetricus for (a) untransformed and (b) log-
transformed (Log) data sampled from North Fremantle, Western Australia collected
between March and June 2020. (n = 655).
3.4.3 Size distribution in deep and shallow water
The mantle length (ML) size distribution pooled over all sampling times and depths
showed that most male and female octopus were between 120 and 170 mm long with modal
sizes of 150 mm ML for both males and females (Figure 3.4a). The number of females in each
size class declined more sharply than the number of males between 160 and 190 mm and few
male or female octopus >200 mm ML were caught.
The modal ML class for females in the < 10 m depth region was 160 mm and 150 mm
for the >10 m region. The modal ML class for males was the same for both depth regions (150
mm). The largest individuals for both sexes were collected from the shallow region. The
smallest males were found in the shallow waters (Figure 3.4b) in comparison, the smallest
females were found in the deeper region (Figure 3.4c). There is a sharp decline in the number
of females after the 150 to 160 mm ML classes for both shallow and deep regions.
36
a)
20 Females (n = 313)
15 Males (n = 340)
10
5
0
Mantle Length Classes (mm)
b)
20
15
Females (n = 191)
Males (n = 172)
10
5
0
Mantle Length Class (mm)
c) 20
18 16 14 12 10
8 6 4 2 0
Females (n = 141)
Mantle Length Class (mm)
Figure 3.4 Mantle length-frequency distributions for female and male Octopus aff. O. tetricus for pooled (a), ≤10 m (b) and > 10 m (c) depth areas in North Fremantle, Western Australia. Collected between March and June 2020
Fre
qu
ency
%
Fre
qu
ency
%
Fre
qu
ency
%
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
30
40
50
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70
80
90
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110
120
130
140
150
160
170
180
190
200
210
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
37
3.4.4 Temporal variation in size
One small juvenile male was collected during the first sampling trip on 23rd March (0-
9.9 mm ML class). Except for this individual, small octopus (< 80 mm ML class) were not seen
until the third sampling trip, hauled on 1st April. Individuals in the smallest length classes (<
80 ML mm) were the least common across the sampling period, with small females (1.84%)
being more common than small males (0.61%). Small juveniles of indeterminate sex were
collected on the 14th April (n = 6, mean ML 48.6 mm), 11th May (n = 2, mean ML 22.5 mm)
and 2nd June (n = 3, mean ML 18.34) trip but are not shown on the ML frequency figure
(Figure 3.5).
Female length distribution varied across the sampling period but tended to decrease
from March to June, which contrasts with males, who increased in size across the sampling
periods (Figure 3.5). Throughout the majority of the sampling period, the modal ML classes
for females stayed between 150 to 160 mm but was more variable in the third sampling trip
(Figure 3.5a). From the fifth sampling trip, a second modal ML class can be identified for
females at 100 mm and remains consistent between 100 to 130 mm for the until the conclusion
of the hauls. The ML size distribution for males varied over the four months of sampling.
Initially, the ML modal length for males was 130 mm, over the duration of the sampling period
the modal length for males increased to 160 – 170 mm. From 14th April onwards most males
were above 140 mm.
The mean ML for females above 80 mm ML was highest in March (158.3 mm) and
lowest in May (133.7 mm). The mean ML for males gradually increased during the study, with
the lowest mean recorded on 1st April (140.7 mm) and the highest mean on 15th June (153.6
mm) (Figure 3.6). The mean ML of male and female octopus >80 mm ML was almost identical
on April 1 and 24 (~140‒142 mm, Figure 3.6).
A two-way analysis of variance (ANOVA) showed that the mean ML differed
significantly among hauls (P < 0.001) and that the haul x sex interaction was significant (P =
0.01) (Table 3.5). The factor Haul accounted for 46.1% of the total mean squares and the
interaction for 30.5%. A Tukey’s post hoc test showed that the mean ML for females collected
on the 23rd March (156.44 mm) was significantly longer than that on 14th April (131.95 mm)
(P = 0.003). The mean ML for female octopuses collected on 11th May (125.72 mm) were
significantly smaller than those collected on March 23rd (Tukey post hoc P = 0.004). In contrast
38
to females, the mean ML of males did not differ significantly between any of the sampling trips
(P >0.05), which explains the significant sex x haul interaction.
210
Frequency % Frequency % Frequency %
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
30
40
50
60
70
80
90
100
110
120
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140
150
160
170
180
190
200
210
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
Frequency % Frequency % Frequency %
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
a) d
)
30
3
0
Females (n
= 43
)
Males (n
= 47
)
Females (n
= 41
)
Males (n
= 59
) 2
5
25
20
2
0
15
1
5
10
1
0
5
5
0
0
Man
tle Length
Class (m
m)
Man
tle Length
Class (m
m)
b)
e)
30
3
0
Females (n
= 47
)
Males (n
= 43
) 2
5
25
Fem
ales (n = 2
9)
Males (n
= 55
) 2
0
20
15
1
5
10
1
0
5
5
0
0
Man
tle Length
Class (m
m)
Man
tle Length
Class (m
m)
c) f)
30
3
0
Females (n
= 78
)
Males (n
= 54
) 2
5
25
Fem
ales (n = 4
1)
Males (n
= 49
) 2
0
20
15
1
5
10
1
0
5
5
0
0
Man
te Length
Class (m
m)
Man
tle Length
Class (m
m)
39
40
g) 30
25
20
15
10
5
0
Mantle Length Class (mm)
Figure 3.5 Mantle length frequencies for female and male Octopus aff. O. tetricus for each
sampling trip collected from North Fremantle, Western Australia between March
and June 2020. a) = 23rd March, b) = 1st April, c) = 14th April, d) = 24th April, e) = 11th
May, f) = 2nd June, g) = 15th June.
180
160
Females
Males
140
120
100
80
Haul
Figure 3.6 Mean (± 1SE) mantle length (ML) of Octopus aff. O. tetricus larger than 80 mm for each haul in North Fremantle, collected between March and June 2020.
Females (n = 34)
Males (n = 33)
Man
tle
Len
gth
(m
m)
Freq
uen
cy %
23
-Mar
3
0
40
50
60
70
80
90
10
0
11
0
12
0
13
0
14
0
15
0
16
0
17
0
18
0
19
0
20
0
21
0
1-A
pr
14
-Ap
r
24
-Ap
r
11
-May
2-J
un
15
-Ju
n
41
Table 3.5 Summary of the results from the two-way analysis of variance (ANOVA) to test if the
mean mantle length (ML) of Octopus aff. O. tetricus differed significantly between
sexes and among each haul from March to June, in North Fremantle, Western
Australia. Bold = significant terms
Variable df MS MS% P
Haul 6 4014 46.1 < 0.001
Sex 1 1067 12.3 0.293
Haul*Sex 6 2654 30.5 0.01
Residuals 636 965 11.1
3.4.5 Cockburn Sound size distribution
Octopus collected from the Cockburn Sound in April 2020 had a modal ML of 120 mm
for males and 160 mm for females (Figure 3.7a). This is smaller than the modal sizes for males
(140 to 150 mm) and larger for females (150 mm) caught in North Fremantle (Figure 3.7b).
A two-way analysis of variance (ANOVA) showed the mean ML for octopuses
collected on April 1 differed significantly between North Fremantle and Cockburn Sound
(P = 0.003) and sexes (P = 0.02) and that the site and sex interaction was significant (P = 0.03)
(Table 3.6). The mean ML for male (139.78 mm) and females (162.27 mm) collected from the
Cockburn Sound differed significantly (P = 0.02). The mean ML of female octopus sampled
from Cockburn Sound was 162.27 mm which was significantly longer than that from North
Fremantle (140.36 mm). Moreover, the males collected from North Fremantle were
significantly smaller than the females collected from Cockburn Sound on the same date (Tukey
post hoc test P = < 0.05). The mean ML for males did not differ significantly between the sites
(Table 3.6).
42
a)
30
25 Females (n = 30)
Males (n = 18) 20
15
10
5
0
Mantle Length Class (mm)
b)
30
25
20
15
10
5
0
Females (n = 47)
Males (n = 43)
Mantle Length Class (mm)
Figure 3.7 Mantle length frequencies for male and female Octopus aff. O. tetricus for samples
collected from the Cockburn Sound (a) and North Fremantle (b), collected from 1st
April 2020.
Table 3.6 Summary of results for the two-way analysis of variance (ANOVA) to test differences
in mean mantle length (ML) of Octopus aff. O. tetricus collected between the two sites (North Fremantle and Cockburn Sound) and sex on 1st April 2020, df = degrees of freedom, MS = mean squares.
Source of variation df MS % MS P-value
Sex 1 2788 21.22 0.04
Site 1 6683 50.86 0.002
Sex*Site 1 3029 23.01 0.03
Residuals 134 640 4.87
Fre
qu
ency
%
Fre
qu
ency
%
30
40
50
60
70
80
90
10
0
11
0
12
0
13
0
14
0
15
0
16
0
17
0
18
0
19
0
20
0
21
0
30
40
50
60
70
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90
10
0
11
0
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0
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0
14
0
15
0
16
0
17
0
18
0
19
0
20
0
21
0
43
3.5 Reproduction and Maturity
3.5.1. Gonad maturity stages
The majority (74.4%) of females sampled from North Fremantle between March and
June 2020 were immature, 23.72% were maturing, 1.60% mature and 0.32% spent (Figure 3.8).
Catches of immature females were lowest in March and peaked in May before declining again
in June in both depth regions. Mature females were only caught during April in both depth
regions. The only spent female was collected from the deep region in April (Figure 3.8).
Maturing males dominated catches at 46.38% overall, with immature males at 26.25%
and mature 25.39%. All male maturity stages were found each month. The rate of immature
males being caught decreased from March to June, with a more notable decline in shallower
waters. The lowest proportion of mature males was recorded in March in shallow waters in
April for the deep.
More immature females (232) were caught between March and June than males (89).
Most males (165) collected from North Fremantle were in the second maturity stage (maturing)
(Table 3.5). The mean mantle length and mantle weight for females were larger than males in
each maturity stage except for Stage III (mature) ML, in which the mean ML was ~171 mm
for both sexes. The mean gonad weight (GW) was considerably heavier for mature female
octopus (41.1 g) than that for mature males (17.8 g) (Table 3.5). One spent (Stage IV) female
was collected during this study on 14th April, an individual with a ML of 132 mm, weighing
233.4 g MW and 23 g GW (GSI = 10.92%) (not included in Table 3.5).
On 1st April, most females were classed as immature in samples from both North
Fremantle and Cockburn Sound (Figure 3.9). Cockburn Sound had considerably more maturing
females (36.67%) than North Fremantle (17.02%), but no mature females were recorded in the
Sound at this time. Males collected from both sites on the 1st April were predominantly at the
maturing stage, with 48.4% maturing males at North Fremantle and 72.2% from Cockburn
Sound. North Fremantle had a higher frequency for immature and mature males when
compared with Cockburn Sound (Figure 3.9).
44
Freq
uen
cy %
Fr
equ
ency
%
a) b)
n = 172 n = 141 100%
80%
60%
40%
20%
0%
March April May June
Month
Stage III
Stage II
Stage I
100%
80%
60%
40%
20%
0%
March April May June
Month
Stage IV
Stage III
Stage II
Stage I
c) d)
n = 191 n = 149
100%
80%
100%
80%
60%
40%
20%
0%
March April May June
Month
Stage III
Stage II
Stage I
60%
40%
20%
0%
March April May June
Month
Stage III
Stage II
Stage I
Figure 3.8 Percentage frequency of gonad stages for females (n = 318) (a) and males (n = 331) (b) Octopus aff. O. tetricus collected between 23rd March and 15th June 2020 from North Fremantle, Western Australia.
Freq
uen
cy %
Fr
equ
ency
%
45
Table 3.7 Summary of the mean sizes (± 1SE) for mantle length (ML), mantle weight (MW) and gonad weight (GW) for each maturity stage of Octopus aff. O. tetricus sampled from North Fremantle, collected between March to June 2020.
Sex n ML (mm) MW (g) GW (g)
Stage I (Immature)
F 232 131.23(±2.09) 209.59(±7.62) 2.58(±0.16)
M 89 110.24(±2.11) 124.60(±6.31) 3.23(±0.28)
Stage II (Maturing)
F 74 171.49(±2.56) 415.45(±13.4) 11.81(±0.85)
M 165 149.54(±1.45) 275.99(±5.82) 10.96(±0.26)
Stage III (Mature)
F 5 171.6(±13.3) 460.17(±86.1) 49.14(±9.49)
M 86 171.71(±1.72) 396.68(±8.20) 17.80(±0.31)
a)
100
80
60
North Fremantle (n = 48)
Cockburn Sound (n = 30)
40
20
0
b) 100
Immature Maturing Mature
Maturity Stage
North Fremantle (n = 51) 80
Cockburn Sound (n = 18)
60
40
20
0
Immature Maturing Mature
Maturity Stage
Figure 3.8 Percentage frequency of gonad stages for Octopus aff. O. tetricus collected from North Fremantle and Cockburn Sound on 1st April 2020, a) Females, b) Males.
Freq
uen
cy %
Fr
equ
ency
%
46
1
% ex d n I c i at m so Female o ad Male n Go
0
0 0 0 0
Month
1
% ex d n I c i at m so
Female o ad Male n Go
0
0 0 0 0
Month
3.5.2 Gonadosomatic Indices
The maximum GSI recorded for females from North Fremantle was 20.33% and 7.64%
for males. The monthly mean gonadosomatic index (GSI) for males increased from 3.35 % in
March to 4.17 % in June (Figure 3.7). In contrast, the mean GSI for females, which was always
lower than that for males, ranged from only 2.08% in March to 1.48% in June and did not show
an increasing trend (Figure 3.7).
The mean GSI for female octopuses collected from Cockburn Sound was marginally
smaller (1.30%) than those collected from North Fremantle on 1st April (1.4%). Similarly,
males collected from Cockburn Sound had a smaller mean GSI (3.40%) than those from North
Fremantle (3.78%) at this time.
Figure 3.9 Monthly mean gonadosomatic index (±1SE) for male (n = 312) and female (n = 291)
Octopus aff. O. tetricus for specimens collected in North Fremantle, Western
Australia between March and June 2020.
3.6 Preliminary diet results
In total, 701 octopus’ gastric tracts were examined and 67% contained dietary items.
Preliminary analysis of the dietary contents found that the dominant categories in both male
and female gastric tracts by frequency of occurrence were brachyurans (>60), teleosts (30 and
56) and non-cephalopod molluscs (~20) (Figure 3.10). The frequency of brachyurans and
5
4
3 Female
2 Male
1
0
March April May June
Month
Go
nad
oso
mat
ic In
dex
%
47
teleosts was higher for males than females. A total of 50 octopus contained cephalopods in
their stomachs (Figure 3.10).
80
Females (n = 205)
60 Males (n = 264)
40
20
0
Prey Species
Figure 3.10 The frequency of each prey item of Octopus aff. O. tetricus collected from North Fremantle and Cockburn Sound between March and June 2020.
3.7 Parasites
3.7.1 Parasite presence
Nearly 60% of all octopus in North Fremantle (58.19%) had anisakid nematodes
(hereafter nematodes) present in at least one internal organ. Only one parasite taxon was found
during the dissection of all octopus. In octopus with at least one parasite, it was always found
in the crop (100 % prevalence), 58.5% in the mantle muscles, 9.56% in the digestive gland,
2.82% in posterior salivary glands, 0.25 % in the gonads (Figure 3.11). Across all samples, 7
octopuses (1.53 %) had eggs of parasites present in the crop.
The mean intensity (± 1 SE) of nematode infection in North Fremantle was 18.31 ± 0.68
per octopus and ranged from 1 to 75 nematodes per octopus. Of the infected octopus, 59.9%
had nematodes in one internal organ, 32.2% in two and 8.0% in three or more organs. The
prevalence of nematodes was markedly higher in octopus from North Fremantle on April 1
(75.6%) than those from Cockburn Sound (25%) on the same date (Figure 3.11). The GLM of
nematode presence found that the odds of octopus collected from North Fremantle having at
Occ
urr
ence
Bra
chyu
ran
Her
mit
Cra
b
Iso
po
d
Po
lych
aete
Ost
raco
d
Dec
apo
d
Tele
ost
Cep
hal
op
od
Biv
alve
Am
ph
ipo
d
No
n-c
eph
alo
po
d M
ollu
sc
Gas
tro
po
d
48
least one nematode present were 9.27 (= e2.227) times greater (P < 0.001) than those from
Cockburn Sound (Table 3.8c).
Figure 3.11 Examples of Anisakidae infestation in Octopus aff. O. tetricus in crop (a) and
digestive gland (b) from samples collected in Perth, Western Australia between
March and June 2020 (Photo: Jorja Claybrook).
49
80
60
40
20
0
North Fremantle Cockburn Sound
Site
Figure 3.11 The prevalence of infections of anisakid nematode in Octopus aff. O. tetricus collected from North Fremantle and Cockburn Sound on 1st April 2020.
3.7.2 Parasite presence modelling
Binomial logistic regression with a logit link function was used to examining the
relationship between on the presence of at least one nematode and depth, haul and size of
octopus, using March and the size group 'Small' as the intercept. The odds of nematode
infestation for octopus were significantly greater for both larger classes of octopus than small
octopus: by 2.35 times (= e0.855) for medium (80-150 ML), and 2.69 times (= e0.991) for large
octopus (≥160 mm ML) than the small individuals (<80 mm ML) (Table 3.8a). The odds of
an octopus having a nematode present were significantly lower in June (0.58) than March (1.0)
(= e-0.549; Table 3.8a) The presence of nematodes did not differ significantly between sexes (P
= 0.9).
Nematode presence also differed significantly with octopus size in Cockburn Sound,
with the odds of nematode infestation being significantly higher for the medium (2.09, = e0.738)
and large (2.16, = e0.771) than small octopus (Table 3.6b). Like North Fremantle, the presence
of nematodes did not differ significantly between sexes in Cockburn Sound (P = 0.65, Table
3.8b).
n = 90
n = 49
Freq
uen
cy %
50
Table 3.8 Coefficients of binomial Generalised Linear Models (GLMs) analysis with logit link function for the presence of anisakid nematodes in Octopus aff. O. tetricus caught in North Fremantle and Cockburn Sound and the influence of octopus size, month and sex.
Variable Estimate SE Z -value P-value
a) North Fremantle (size + month + sex)
Intercept (Size: Small, March, Females)
-0.253
0.077
-0.776
0.438
Size: Medium 0.855 0.065 3.314 0.002 Size: Large 0.991 0.068 3.426 <0.001 April 0.066 0.243 -0.271 0.787 May -0.505 0.306 -1.650 0.099 June -0.549 0.265 -2.070 0.038 Males 0.022 0.163 0.135 0.893
b) Cockburn Sound (size + sex)
Intercept (Size: Small, Females) -0.471 0.255 -1.847 0.064 Medium 0.738 0.267 2.767 0.006 Large 0.771 0.280 2.753 0.006 Males 0.070 0.154 0.453 0.650
c) North Fremantle vs. Cockburn Sound
Intercept (Site: Cockburn Sound) -1.099 0.333 -3.296 <0.001 Site: North Fremantle 2.227 0.412 5.381 <0.001
4. Discussion
This study evaluated the spatial and temporal variability of the North Fremantle population of
Octopus aff. O. tetricus. The data used for these analyses were collected by commercial fishing
operation setting four lines of traps, two in shallow (≤ 10 m) and two in deeper water (>15-
17 m) on seven occasions between March and June 2020. Differences in size distribution,
gonad stage and body size were examined to test whether they differed between the sexes,
depths of water and sampling time. Preliminary investigations of octopus’ diet in this region
have also been made. During the study, an anisakid parasite was found infesting octopus’ and
data on their incidence and intensity of infection were collected to investigate the site of
infection. Comparisons of the biology and parasite infestation have been made between
octopus in North Fremantle and those collected at one time from octopus fishing grounds in
Cockburn Sound, ~7 km south of the main study.
51
4.1 Abundance and sex ratio
The abundance of octopus varied between sampling times but not between depths or
sexes of octopus. The major difference between sampling times was the significantly greater
numbers caught in early April compared with those in mid-June. The overall decrease in the
abundance of octopus in the North Fremantle site in June coincided with the decrease in SST.
Octopuses exhibit large fluctuations in abundance, mainly due to their susceptibility to
environmental factors during their paralarval stage (Pierce et al., 2008).
Significant storm activity in the region during the fifth and sixth hauls in late April and
May could have contributed to the lower catches at this time. The increased wave action
associated with the storms resulted in several octopus’ pots becoming submerged in sand and
at least one octopus was suffocated by sand (P. Jasinski, pers. comm.). Potentially, any small
juvenile octopus’, which are at mercy to the currents, may have swept out to other areas during
these weather events. Decreases in octopuses per haul from within the North Fremantle study
may also be the result of the targeted removal of octopuses during the study period. Decreases
in abundance due to targeted fishing pressure were examined in San Matas Gulf, northern
Patagonia. The relative abundance of O. tehuelchus decreased during the fishing season on
commercial fishing grounds compared with no change in abundance within a Marine Protected
Area (Narvarte et al., 2013)
The abundance of octopus was not significantly affected by depth (<10 m and ≥10 m)
within the waters of North Fremantle. While there was not any statistically significant
difference, Line C and D continually yielded the least number of octopus per haul. The soak
time of shelter pots was considered an important variable for the number of octopuses collected
per haul; it was expected that catches would increase with increasing soak time. However, the
number of octopus did not appear to be affected by the soak time of traps, as the hauls that
were submerged for the shortest amount of time and the haul which was submerged for longest
had very similar catches.
Sex ratio
The sex ratio did not differ significantly from 1:1, except on one occasion in the shallow
waters and twice in the deeper waters. On the 23rd March, nearly twice as many males were
caught in the shallow lines than females, while in the deep, females were significantly more
prevalent than males on the 14th April, but this was reversed in early May when more than
double the number of males to females were caught. The overall sex ratio did not differ for
52
either depth region or the Cockburn Sound population and did not differ between Cockburn
Sound and North Fremantle in early April.
The sex ratio for April is consistent with the known composition of O. tetricus in the
Western Australian fishery, where males tend to dominate the catches but have the lowest
frequency of capture in April and the highest in October (Hart et al., 2016; Hart et al., 2019;
Leporati et al., 2015). Until senescence, males will actively hunt and move around their
environment seeking potential mates, which explains why males typically dominate catches
(Leporati et al., 2015). Aside from the octopus’ collected in April, males consistently
dominated the catches over females. This is quite unusual to other octopus’ population in
which fluctuation between male and female abundance occur throughout the year, usually
coinciding with peaks in the GSI (Alonso-Fernández et al., 2017; Guard & Mgaya, 2002).
Similar patterns of male dominance have been observed in the Mexican O. maya fishery
(Duarte et al., 2018). Currently, female coastal octopuses are hypothesised to cease feeding
around the time they lay their eggs, seeking refuge in dens to care for their eggs until they
hatch (Hart et al., 2019). During this time the females are unlikely to be actively hunting and
would be less likely to enter traps (Leporati et al., 2015). It is not known precisely why the
Western Australian octopus’ fishery has mostly continuous male dominance, but it is likely
attributed to the use of active trigger trap use in the deep regions of the fishery (>30 m) which
is considerably more likely to capture actively hunting octopus’ (Hart et al., 2019; Leporati et
al., 2015).
4.2 Size distribution and growth
The mean mantle length (ML) of octopus from North Fremantle did not differ
significantly between the sexes. Although the overall modal ML over the four months was the
same for both males and females, the frequency of males in the larger size classes (>160 mm)
was higher than females. The largest individual collected over the sampling period was a male
caught in June, with a ML of 218 mm and MW of 691.62 g from the shallow region. This
octopus was larger than those in previous studies of O. tetricus from within the commercial
fishery where the largest males was 210 mm (Larsen, 2008). The maximum ML recorded for
females in Larson (2008) was 250 mm, substantially longer than at the maximum of 211 mm
recorded in the current study.
Small juveniles were first observed in the third haul in mid-April, with the smallest
individual (sex undetermined) being observed at the end of April. As Octopus aff. O. tetricus
has a two-month paralarval stage, these individuals were potentially hatched between January
53
and February. Octopus aff. O. tetricus have been identified as recruitment pulses every six
months, with a peak spawning event occurring in September (Joll, 1978; Larsen, 2008; Leporati
et al., 2015) and a secondary one in the summer months (Larsen, 2008; Leporati et al., 2015).
The two modal length classes that become prominent from the mid-April to the end of June
indicate that two cohorts are present in the population.
The monthly mean ML of male octopus increased steadily between March and June,
with the largest mean ML (152.27 mm) recorded in June. In contrast to males, the mean ML
of Females fluctuated between months, with octopus captured in March being significantly
larger than those in April and May. This coincides with the influx of juveniles on the third haul
in mid-April. Male and female octopuses typically exhibit similar growth rates until the onset
of maturation, however males reach maturity earlier, and females reach a significantly larger
maximum size (Leporati et al., 2015). Leporati et al. (2015) estimated the ML at 50% maturity
for male Octopus aff. O. terticus in the WA commercial fishery was 128 mm and reached at
243 d of age, 150 d faster and 54 mm smaller than the size at maturity for females (182 mm
ML at 379 d), (Leporati et al., 2015). These findings are consistent with octopus’ collected in
North Fremantle and demonstrate that two cohorts are likely to be present, one that spawned
the previous year in August or September of 2019, and the second from spawning in January
or February 2020.
During the current study, growth from the North Fremantle region did not appear to be
very rapid for either sex. This may be due to two main reasons: (1) larger individuals migrate
from the shallow waters and move outside the study region where the deepest line samples
were 16.79 m, and move into waters deeper than 20 m, as was found previously for octopus in
the WA fishery (Larsen, 2008; Leporati et al., 2015), and (2) growth slowed with the decreasing
SST from March to June, which is consistent with the Larson (2008), who examined samples
from the developing octopus fishery in the Perth region between June and August in water
<20 m deep and in deeper water (30‒40 m) from May to September. She found slower growth
between April and August. Temperature is a major influence on the growth of octopus,
especially in the early life stages. Modelled projections of temperature-dependent growth of O.
ocellatus and O. pallidus suggests that an increase of 1 °C (from 22 to 23°C) could lead to a
62.6% greater octopus body mass after 100 days at the higher temperature (André et al., 2009).
Temperature related growth is also intrinsically linked with an increase in metabolism, food
intake and food availability (Rigby & Sakurai, 2004). The food intake of O. briareus doubled
when water temperatures increased from 20 to 30 °C (Borer & Lane, 1971).
54
4.3 Maturity and reproduction
Most females collected during the sampling period were immature. The percentage of
immature females increased over the sampling period reaching a maximum in May, with more
than 90% of the females being immature in both depth regions. This is consistent with the
findings of Larson (2008), who found that virtually all females in waters < 20 m deep were
immature.
The data on gonad stages and gonadosomatic indices (GSIs) collected during this study
are not sufficient to define the spawning season for octopus in this region. However, in a
longer-term study of octopus based on samples from the commercial fishery in Cockburn
Sound, Larsen (2008) suggested that some spawning may occur in late autumn (April-May)
and early spring (September-October). Moreover, based on laboratory studies, Joll (1978)
proposed that the peak spawning for O. tetricus wild populations may occur in spring.
The maturity stages recorded for O. tetricus in Cockburn Sound were very similar to
those collected in North Fremantle on the same day in early April. North Fremantle had a higher
frequency of immature females than Cockburn Sound, however, North Fremantle also recorded
at least one female in each maturity stage, and no mature females were collected from the
Cockburn Sound site. Both sites had a high proportion of maturing males and recorded at least
one male in each maturity stage. Despite Cockburn Sound females having significantly larger
body sizes, the mean GSI was slightly lower than that for North Fremantle. The mean GSI
differed significantly for males and females in both North Fremantle and Cockburn Sound sites,
with males having a higher GSI for both areas. Results from this study provide strong evidence
of males maturing earlier than females in both sites, which is consistent with the findings from
previous studies (Joll, 1976; Leporati et al., 2015).
Only five mature females were recorded during the four months of this study which
may have been due to a migration out of the study area by maturing females or a change in
female catchability as they mature; mature females actively brood eggs, do not feed and are
not actively hunting at this time, and are thus not adventuring into traps (Leporati et al., 2015).
The migration of female octopuses to find mates and suitable dens for brooding eggs during
the breeding season has been suggested for several species, including the east coast O. tetricus
(Anderson, 1997), O. insularis in Brazil (Lima et al., 2014) and O. vulgaris in South Africa
(Oosthuizen & Smale, 2003).
55
Both sexes exhibited very similar monthly maturity proportions in the shallow and
deeper water of North Fremantle. The remarkable similarity of the maturity stages indicates
that the difference between the depth region is not large enough to demonstrate the migration
of mature individuals. The study design divided the two depth areas into ≤10 and >10 m to a
maximum of 20 m. In Larson (2008) the two depth regions examined were a shallow inshore
habitat (<20 m) and an offshore deeper habitat (30-40 m). The findings in this study are
consistent with Larson (2008) results for the inshore habitat, and the distinct lack of mature
females within the North Fremantle site demonstrates that the depth range is not adequate to
observe the size-related offshore migration that females undertake while maturing.
Mature males were observed in every haul in North Fremantle, with the highest number
being recorded in May. The presence of mature males for much of the year suggests that they
have adapted an opportunistic reproductive strategy. Octopus aff. O. tetricus mates all year
round, with two distinct pulses every six months (Larsen, 2008; Leporati et al., 2015) similar
reproductive patterns are seen with O. vulgaris (Katsanevakis & Verriopoulos, 2004).
Moreover, the storm incidents in May and June 2020 in the North Fremantle study area
may have accelerated the maturation of O. tetricus. Turbidity caused by high wind speeds and
wave action reduces light penetration within the water. In studies of the sexual maturation of
O. vulgaris it was found that light inhibits the release of the gonadotrophin hormone from the
optic gland (Mangold, 1983) which delays the onset of maturing gonads.
Males reach maturity earlier than females and are typically present within the
population throughout most of the year (Leporati et al., 2015). Female O. tetricus reach
maturity at ~12 months, as they have a semelparous life history and longevity of ~ 18 months
there is a ~6-month spawning window (Joll, 1976; Leporati et al., 2015). However, females
may mate at an earlier maturity stage and are able to store the spermatophores in the oviducts
for up to 16 weeks (Joll, 1976). Laboratory mating trials at the University of Western Australia
of O. tetricus revealed that multiple paternity is common among broods of O. tetricus (Morse,
2008). Theories behind female promiscuity are that either, females mate with multiple partners
to ensure phenotypic diversity among their offspring, or females may be sperm limited, and as
they only have a singular opportunity to reproduce, mating with multiple partners ensures the
fertilisation of larger broods (Joll, 1976; Morse, 2008). There is strong evidence of the latter,
with observations of female octopods (O. oliveri) initiating copulation with the first male they
are introduced to and become more selective as more mates are presented (Ylitalo et al., 2019).
56
Previous studies revealed that in captivity, female O. tetricus brood their eggs for ~1
month and then survive for 1–3 weeks after their eggs hatch (Joll, 1976). The spent individual
caught in April thus indicates a potential spawning event around February or March. The exact
length of the paralarval stage for O. tetricus has not been determined however it is likely to be
similar to O. vulgaris, which has a duration of 47-54 days at 21.1 °C and 30-35 days at 23 °C
(Iglesias et al., 2007). Furthermore, the number of very small juvenile (<80 mm ML) octopuses
observed was notably lower than in previous years (P. Jasinski, pers. comm.). The lower
number of small octopuses may be attributed to different water temperature conditions in the
previous year. Preliminary investigations of the interaction of SST and successful recruitment
were conducted for O. vulgaris in the Strait of Sicily, showed a significant correlation of
recruitment success and higher SST (Garofalo et al., 2010).
Due to the large proportion of small, immature octopus caught within the waters of
North Fremantle, it is likely that this site acts as a nursery area for juvenile octopuses. The
significant proportion of small octopus’ within the North Fremantle study site indicates an
ontogenetic migration of juveniles occurs to the shallower regions of North Fremantle,
potentially due to the differences in prey availability. It is well established that juvenile
octopuses frequently prey on crustaceans, and shift to teleosts and other cephalopods as body
size increases (Smith, 2003; Villanueva et al., 2017; Vincent et al., 1998). This is consistent
with the preliminary analysis of diets conducted in this study which shows that >40% of
octopuses had consumed at least one crustacean. These preliminary findings contrast with those
on O. tetricus diet from an abalone sea ranch in Flinders Bay, approximately 300 km south of
Cockburn Sound. Octopus consumed much greater amounts abalone, non-cephalopod
molluscs, on the sea ranch (Greenwell et al. 2019) than in North Fremantle. Future studies of
O. tetricus should examine the differences in the dietary composition of juvenile and adults,
including samples from deeper depth regions than those evaluated in this research.
4.4 Occurrence of nematodes
This study was the first to examine the presence of anisakid nematodes in O. tetricus.
Little information and very few publications are available on infestations of nematodes in
cephalopods, especially octopuses. Nearly 60% of octopuses sampled from North Fremantle
site were infested with nematodes. Modelling on nematode prevalence showed that the
probability of infection doubled for medium and larger octopus and that the incidence of
57
nematodes was much greater in March than June (about double). Temporal variation in
nematode prevalence does not appear to be related to octopus’ size, as the smallest mean sizes
was recorded in May. Potentially, the difference in prevalence due to size could be attributed
to changes in feeding regimes between the warmer and cooler months for O. tetricus, as
seasonal variation in diets has been identified for O. bimaculatus (Villegas et al., 2014) and E.
cirrhosa (Regueira et al., 2017). Additionally, the nematode reproductive cycle is known to
slow at lower temperatures, which may reduce the incidence of infection during the cooler
months (Fazio et al., 2012; Nagasawa et al., 1994).
The prevalence of nematodes in early April was almost three times in North Fremantle
than those from Cockburn Sound. The probability of infection by at least one nematode for
North Fremantle octopus was nearly ten times that of octopus in Cockburn Sound. The regional
differences of nematode infection between Cockburn Sound and North Fremantle may
demonstrate the existence of two separate populations of O. tetricus. Anisakis infections have
been applied as biological tags to assess sub-populations of pelagic and demersal octopus
species from the Mediterranean Sea (Mattiucci et al., 2015; Rello et al., 2009; Roumbedakis et
al., 2018a).
There is a high probability that O. tetricus ingested the nematodes from predating on
crustaceans, which are usually the first intermediate host of anisakid nematodes (Mattiucci et
al., 2018; Roumbedakis et al., 2018a). This coincides with the preliminary dietary analysis
performed on O. tetricus which demonstrated that a major component of the stomach contents
of the North Fremantle population was crustaceans. Predation on other cephalopods could also
result in the spread of nematodes throughout the population, as the incidence of cannibalism
detected in the diet analysis was quite high.
Infested crustaceans are likely to inhabit the same areas as O. tetricus due to the limited
migration of the species. At present, the extent of O. tetricus broad-scale movement is
unknown, however demersal octopuses tend to have restricted migrations (tens of kilometres)
(Semmens et al., 2007), and mark-recapture trials on similar species such as O. vulgaris have
demonstrated movement was limited to a home range of <1 km (Arechavala-Lopez et al., 2019;
Mereu et al., 2015). Minimal broad-scale movement accounts for the vast difference in
infection levels between the two sites, as they are unlikely to be sharing feeding grounds. Other
fish and cephalopods, particularly squid, can exhibit large broad-scale movement from feeding
58
and mating ground, therefore there is a higher likelihood that they will intermix with other
populations (Nadolna & Podolska, 2014; Semmens et al., 2007).
Octopus aff. O. tetricus is a suitable host due to their trophic position and morphological
adaptation. Coleoid cephalopods are susceptible to parasites due to the loss of an external shell,
which has allowed high-speed locomotion but increased their vulnerability, and the complex
structure of their skin, which is prone to lesioning (Kinne, 1990). Moreover, their function as
mesopredators and voracious feeding behaviours mean they easily accumulate multi-host
parasites through predation (Boyle & Rodhouse, 2005; Kinne, 1990; Roumbedakis et al.,
2018a).
Although it was not possible to identify the species of anisakid nematode, this would
be possible with molecular sequencing techniques. Nematodes in the genus Anisakis
(Anisakidae) are some of the most common and abundant parasites known to cause
pathological effects on fish and cephalopods, including ulceration and potential castration
(Abollo et al., 1998; Abollo et al., 2001). Examination of anisakid infection on a squid species
Illex coindetii, Todaropsis eblanae and Totarodes sagittatus found nematodes within the
gonads had caused partial destruction and alteration of the tissue, with partial inhibition of the
formation of reproductive cells. Within the North Fremantle site, only one octopus, a maturing
female, was found to have had a nematode infestation in the gonads. A small proportion of
octopus’ (3%) were found with nematodes encysted in the salivary glands. It not currently
known what impact a nematode infestation of the salivary glands has on the health of the
octopus; however, other parasitic infections have been known to cause severe damage to
salivary glands. Cestode (Prochristianella sp.) infection on the salivary glands of O. maya
caused deterioration of the tissues and the glands were dysfunctional, producing little to no
secretions (Guillén-Hernández et al., 2018). Consequently, the damage caused to organs by
parasites hinders the hosts capacity to feed and reproduce (Guillén-Hernández et al., 2018).
Multi-host parasites have a complex life history and are transmitted through the food
web, with cephalopods and fish acting as secondary intermediate hosts, and marine mammals
as definite hosts. A recent review of parasitism in cephalopods outlined several cuttlefish and
squid species known to have been infected by nematodes but does not identify any known
octopus species (Mattiucci et al., 2018; Roumbedakis et al., 2018a). Currently, there have only
been three other published incidents of larval nematode infection within octopuses, O. vulgaris
59
(Angelucci et al., 2011; Gestal et al., 1999), Eledone cirrhosa and E. moschata (Guardone et
al., 2020).
The presence of anisakid nematodes is also relevant to human health as the third stage
anisakids are aetiological agents of painful gastrointestinal diseases in humans if raw or
undercooked fish or cephalopods are consumed (Abollo et al., 2001). Anisakidosis, a parasitic
infection in humans, occurs when larvae penetrate the gastrointestinal mucosa, which can
trigger anaphylactic shock, and may require an endoscopic procedure or surgery (Abollo et al.,
2001; Shamsi, 2014). Additionally, anaphylactic reactions can still occur even a dead nematode
is ingested (Audicana et al., 2002). The first recorded incident of anisakidosis in Australia was
in 2011 and was attributed to the consumption of locally caught mackerel (Shamsi & Butcher,
2011). Due to the popularity of seafood consumption in Australia, in particular sushi and
sashimi, more undocumented incidents of anisakidosis in Australia are likely (Shamsi, 2014).
The extent of nematode infection on the arms of O. tetricus is not known and therefore the risk
of infection from consuming octopus from Perth waters cannot be assessed.
Due to time constraints of this thesis, and the challenges of nematode taxonomy, the
species of nematodes could not be determined. Future studies should perform molecular
identification on the nematodes and collect further samples from Cockburn Sound to identify
the true extent of the infestation within the commercial fishery.
Conclusions and Recommendations
The examination of the spatial and temporal distribution of O. tetricus in North
Fremantle, Western Australia, based on data collected between March and June 2020
demonstrates that the nearshore area of North Fremantle acts as a nursery area for juvenile and
maturing octopus. The changes in sex ratio between March to June were consistent with the
known composition of O. tetricus within the commercial fishery. Due to their early maturation,
mature males were observed throughout the entirety of the study. The distinct lack of mature
females within either depth region is consistent with the current biological hypothesis that O.
tetricus females have a size-related migration during maturation to find mates and suitable dens
for brooding.
The data on the mantle length distribution suggest a slower growth period for O. tetricus
between autumn and winter as SST is declining, and the presence of small juvenile octopus
60
from April onwards indicates a spawning event in January or February. This slower growth
during the time of lower SSTs is consistent with a previous study of the abundance and growth
of O. tetricus (Larsen, 2008), and with the findings for other octopus species. However, a more
detailed examination of the effect of environmental conditions on the population carried out
over an 18 to 24-month period is required more accurately patterns of abundance and growth
rates in the population.
Female mantle length was significantly longer in Cockburn Sound than North
Fremantle, and the overall proportion of maturing individuals was greater, which suggests that
Cockburn Sound may have spawned earlier or had a faster growth rate, potentially due to food
availability, differing environmental conditions. Moreover, some of the samples from
Cockburn Sound could have come from areas where females were maturing or brooding their
eggs. It may be worthwhile for future studies evaluate the potentially distinguishing growth
rates between the two sites and identifying where reproduction and egg brooding takes place,
with the utilisation of samples throughout the year to enable identification of seasonal
variances.
This study provides the first record of anisakid nematodes in the viscera of O. tetricus.
Nematodes occurred in the viscera of 60% of the octopus’ from North Fremantle, and
modelling indicated that the probability of North Fremantle octopus having at least one
nematode was almost ten times higher than the Cockburn Sound population. The full extent of
infection was not determined as the octopus’ arms were not examined during this study. While
the ingestion of seafood infected with anisakids by humans is known to cause anisakidosis, the
incidences of this in Australia are very rare, and it is likely to be low risk to the public at this
time. Molecular identification of the nematode species may help to determine what, if any,
potential risks may be associated with the consumption of O. tetricus by people. Examination
of any potential pathological effects on the octopus infested with nematodes would be
beneficial to monitor the health of the O. tetricus population.
The priority for future research is to perform a detailed analysis of the dietary
composition of O. tetricus and to assess any differences that may occur between different size
classes, sexes or depth regions. For a better understanding of the ingestion of nematodes by
octopuses, comparative assessment on diets between North Fremantle and Cockburn Sound
populations would be very valuable Furthermore, taking invertebrates from identified feeding
grounds of octopus may help identify the first intermediate nematode hosts.
61
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