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ICES CM 2002/M:28

Theme Session on Oceanography and Ecology of Seamounts

Indications of Unique Ecosystems

The role of Zenopsis spp. as a predator in seamount and shelf habitats

Heike Zidowitz, Heino O. Fock, Hein v. Westernhagen

Alfred Wegener Institute for Polar and Marine Research, PO Box 12 01 61,

D- 27515 Bremerhaven, Germany [tel + 49 471 4831 1382, fax +49 471 4831

1425, email: initial first name+surname@awi-bremerhaven.de]

Abstract

The genus Zenopsis Gill 1862 consists of three species. Zenopsis conchifer occurs in the

Atlantic and Indian Oceans whereas Zenopsis nebulosus has a wide distribution in the (Indo-)

Pacific Ocean. Zenopsis oblongus, described in 1989, is closely related to Z. nebulosus and

only known from the Nazca Ridge in the SE Pacific. Feeding studies are reviewed for all three

species. At the Great Meteor Seamount (GMR) (NE Atlantic), Z. conchifer shows a shift in

prey selectivity with ontogenetic development. Smaller specimens (<46 cm TL) preyed on

constituents of the sound scattering layer (SSL) (myctophids, stomiids). Diet of larger

specimens (>52 cm) predominantly consisted of bentho-pelagic Macroramphosus spp., which

was the most abundant fish species on that seamount. A similar ontogenetic shift was

observed for Z. conchifer on the Namibian shelf. However, prey for the larger specimens

consisted mainly of the pelagic species Trachurus trachurus and Synagrops microlepis. In the

Pacific, Z. nebulosus (oblongus) obtains a similar trophic position with regard to migrating

components of the SSL. However, though being also very abundant at the Nazca-Ridge

seamounts, no Macroramphosus spp. were eaten. Large specimens of Z. nebulosus were

found to prey on bentho-pelagic rockfishes (Sebastidae). Zenopsis spp. appears to be an off-

bottom pelagic predator with preference for mesopelagic food components in all areas

considered. It is suggested that larger specimens abandon the off-bottom pelagic feeding

mode and that body size thus determines the capabilities of Zenopsis spp. to prey on bentho-

pelagic species.

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Keywords: Zenopsis conchifer, Zenopsis nebulosus, Zenopsis oblongus, feeding ecology,

sound scattering layer,

Abbreviations: GMR – Great Meteor Seamount (follow Wilson and Kaufmann 1987), SSL –

sound scattering layer

Introduction: The genus Zenopsis spp. Gill 1862 consists of three species, Zenopsis conchifer, Zenopsis

nebulosus and Zenopsis oblongus. These species are very similar in body plan with laterally

flattened bodies with an upwardly pointing face and a silvery coloration (Swaby & Potts

1999).

Z. conchifer occurs in the Atlantic and Indian Oceans (fig.1). In the eastern Atlantic it

inhabits shelf areas from Ireland to South Africa, most abundant at 20° N (north-west Africa)

(Maurin & Quéro 1982). The distribution in north-western European areas expanded since the

1960ies reaching Ireland in the 1980ies (Swaby & Potts 1999). Besides the shelf habitats it is

also found around Islands (Canary islands) and on seamounts far off the continental rises e.g.

at the Great Meteor Seamount (GMR). In the western part of the Atlantic its distribution

ranges from Nova Scotia to North Carolina (Scott & Scott 1988) where it gets more abundant.

It is also known from Brazil (Haimovici et al. 1994) to Argentina (Quigley & Flannery 1995).

In the Indian Ocean Z. conchifer is found off the SW coast of India and off southern Africa

from Walvis Bay to Kenya (Smith & Heemstra 1986) up to Somalia and in Indonesia (Froese

& Pauly. Eds. 2002. FishBase).

Z. conchifer reaches a total length of 80 cm (Smith & Heemstra 1986) and a body weight up

to 3.2 kg (Robins & Ray 1986). Specimens of ca. 60 cm are estimated to be 12-14 years old

(this paper), larger specimens probably get older than 20 years.

Z. nebulosus is known in the Indo-Pacific region, from Japan, Northwest shelf of Australia to

Broken Bay in New South Wales, New Zealand (Kailola et al. 1993), and elsewhere in the

region (Philippines (Anon 2001)) (fig.2). In the Eastern Pacific it is known off central and

southern California, USA (Eschmeyer et al. 1983) and from several seamounts in the western

Pacific (e.g. Hancock Seamount, Kammu Seamount (Hawaiian Ridge); Jumeau Seamount

(Richer de Forges 2000), Stylaster Seamount (Norfolk Ridge) (Froese & Pauly. Eds. 2002.

FishBase) Specimens caught at seamounts of the Nazca Ridge (SE Pacific) have been

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investigated by Parin et al. 1988. An analysis of otolith increments revealed an age of 13

years at a standard length of 46 cm. Different from Z. chonchifer, Z. nebulosus reaches only a

size up to 70 cm in total length and a maximum weight of 3.0 kg (Williams 1990), so that it

can be estimated that the species reaches an age of more than 20 years. These investigations

on the ecology of Z. nebulosus of the Nazca Ridge (Parin et al. 1988) are more likely to refer

to Z. oblongus.

Z. oblongus was first described in 1989 and is very closely related to Z. nebulosus, but differs

by a lower body and larger number of osseous scutella above the anal fin (Parin 1989). It is

only known from the Nazca Ridge in the Eastern Pacific Ocean as a probable endemic species

of the area (Parin et al. 1997) (fig.3).

Z. conchifer was caught in depths ranging from 50 – 730 m (Saldanha 1968) but mainly

occurs on the upper slope at 200 – 400 m (Quigley & Flannery 1995, Quéro 1998). It is

frequently encountered in coastal waters (Quéro et al. 1990) and found in midwater or near

the bottom (Swaby & Potts 1999) and it is probable that it occurs in schools (Berry 1978,

Whitehead et al. 1986). Z. conchifer is regarded as a bentho-pelagic species of the deep-water

(Froese & Pauly. Eds. 2002. FishBase), a mesopelagic species (Scott & Scott 1988) or

bathypelagic species (Quéro & Pariente 1977).

Z. nebulosus is considered as a bathydemersal deep-water species with occurrences in depths

ranging from 30 - 800 m (Froese & Pauly. Eds. 2002. FishBase).

Parin et al. (1997) consider Z. oblongus as a bentho-pelagic species in a wide sense inhabiting

depths between 180 – 330 m. They also indicate the species as an off-bottom pelagic species,

which can swim far away from the bottom, rising in midwater during diel vertical migrations.

Compared to fisheries of shelf areas, deepwater fisheries show a shift in families of fishes

which are commercially exploited. On the continental shelves primary families of exploited

fishes are Gadidae, clupeoids, salmonids, scombrids and Pleuronectidae whereas deepwater

fisheries are based on entirely different orders, such as Beryciformes, Zeiformes and

Scorpaeniformes (Koslow et al. 2000). Differences at this taxonomic level indicate

fundamental shifts in body plan and ecological strategy as well as in evolutionary lineage

(Koslow et al. 2000). Many deepwater species aggregate on seamounts and form a distinct

guild based on common features of their body plan, proximate composition, physiology and

metabolism, ecology and life history (Koslow 1996, 1997). They tend to be robust and deep-

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bodied in order to manoeuvre in the strong currents characteristic of this environment

(Koslow et al. 2000).

These fishes generally do not migrate vertically, but depend on the influx of meso- and

bathypelagic organisms past the seamount and on intercepting mesopelagic migrators on their

downward migration (Isaac and Schwartzlose, 1965; Genin et al. 1988; Koslow 1997). Some

predators developed an expectant hunting strategy along plateau margins and an increased

habitat dependent resource utilisation rate at locations of sound scattering layer interception

(Fock et al. 2002). To explain living conditions for seamount populations in often

impoverished nutritional conditions in the ambient oceanic regions, the sound scattering

layer-interception hypothesis (Isaacs and Schwartzlose 1965) has been developed. It implies a

primarily pelagic food utilisation for bentho-pelagic fishes, increased habitat dependent

utilisation rates at locations of interception with the sound scattering layer, diel changes in

utilisation rates due to availability of prey and sufficient resource partitioning among species

in order to avoid competitive exclusion (Fock et al. 2002b). It was suggested that it represents

a large enough prey source to maintain populations at seamounts (Hesthagen 1970, Rogers

1994, Parin et al. 1997).

A very important reason for the success of the deepwater families is the far K-selected end of

the life-history spectrum (Koslow et al. 2000). It is characterised by longevity, slow growth

and delayed maturity. With these features the species fill a gap in the distribution of teleost

life-history patterns (Roff 1984). Because in deepwater habitats the recruitment appears to be

episodic and e.g. orange roughy and Sebastes spp. undergo extended periods (decade or more)

of very low recruitment to the adult population (Leaman and Beamish 1984), Murphy 1968

and Stearns 1976 hypothesised an evolutionary link between longevity and recruitment. The

adaptations on the life-history level could constitute the population’s ability to withstand

extended periods of poor recruitment (Koslow et al. 2000). Episodic recruitment can also be

assumed for commercially not exploited genera like Zenopsis, Antigonia, Capros.

Morids (Moridae), cusk-eels (Brotulidae) and hakes (Merlucciidae) are robust-bodied

Gadiformes and active predators (Koslow et al. 2000). Hakes form a small but widely

distributed family (Koslow et al. 2000). Species of Merluccius are voracious predators

inhabiting the continental shelf and upper slope (Froese & Pauly. Eds. 2002. FishBase) and

are often dominant piscivores over upper portions of the continental slope and typically

migrate vertically into the upper waters at night to feed (Bulman and Blaber 1986). Their

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productivity is thereby linked directly to the near-surface food web (Koslow et al. 2000)

which makes them less adjusted to seamount habitats. Fock et al. (2002) showed that the

Gadiformes contributed an extremely low share to overall catch at the Great Meteor

seamount although they dominate on the shallower shelf of the NE Atlantic.

Materials and methods Sampling

Sampling was carried out at GMR during R/V Meteor cruise M 42/3 in September 1998. The

GMR is located at 30°N, 28.5°W and is an isolated, flat-topped seamount. Sampling

procedures applied on 15 station were bottom trawls in depths between 286 and 435 m with

an ENGEL bottom trawl. Eight hauls were performed during daytime, two at dusk and dawn

and five during night time.

Biological standard measures

A total of 94 specimens of Zenopsis conchifer were caught on the GMR. Of these, 62

specimens were provided for stomach content analysis. Specimens caught were measured in

total length (TL) to the nearest centimetre and weighted in the whole, using an electronic

digital laboratory scale. After preparation single organs (stomach, liver, ovaries) were

weighted as wet weight. Organs and heads of specimens were then deep-frozen.

Stomach content analysis

To investigate a shift in prey selectivity with ontogenetic development, specimens were

subdivided in size classes. Small fish <36 cm, medium-size fish 36 – 46 cm, medium-size fish

47 – 52 cm and large fish >52 cm. The medium-size classes comprise 82 specimens (87% of

overall catch). 52 individuals of 36 – 46 cm in total length were caught (55% of overall

catch), of which 11.53% were male specimens. In size class 47 – 52 cm 32% of total catch

with 30 individuals were found (6.6% males). The size class >52 cm was represented by 11

specimens which were exceptionally females that made up 11.7% of the total catch. Only one

juvenile fish was caught and sent to the ichthyological collection of Madeira, so that the small

size class was abandoned for further investigations.

Stomach contents were accomplished by defrosting stomachs and adherent water was blotted

off with tissues. For comparison with the weight taken onboard, wet weight was taken to the

nearest 0.005 g on electronic digital scales.

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All containing food items and regurgitated items in the pharynx with clear signs of digestion

were counted and weighted. Specimens with totally regurgitated stomachs without any food

items were not considered for further analysis.

Stomach fullness, which is recommended as an index for feeding periodicity (Hyslop 1980),

was estimated by definitions shown in table 1. Only stomachs with no food item at all were

considered as empty. Cephalopod beaks were only counted but not weighted. Prey items were

identified to the lowest possible taxon. Freely occurring otoliths were used for identification

of already digested prey items or prey in an advance state of digestion by using otolith

catalogues or by comparison with otoliths of successfully identified fishes. Myctophids and

other mesopelagic fishes were identified by a catalogue of A. Kotthaus (1972) of previous

studies of GMR ichthyocoenosis. Trachurus sp. was identified by the catalogue of W.

Schmidt (1968).

Age determination

Otoliths of Z. conchifer were prepared out of heads and sent to the Central Ageing Facility of

the Marine and Freshwater Resources Institute in Queenscliff, Australia for further

preparation and reading.

Whole otolith reading and sectioned otolith reading were accomplished. Each otolith was read

using a dissecting microscope at up to 40x magnification with reflected light. Otoliths were

distally ground on both sides to reveal growth increments and samples were viewed with

transmitted light. Increments were counted from the primordium to the edge of the largest

lobe and measured using an image analysis system.

Analysis of fish diet

The percentage of frequency of occurrence of each prey item (%F), the percentage of

abundance (%A) and percentage of weight (%W) were calculated (Hyslop 1980). %F was

determined as percentage of stomachs of fish with prey each item compared to all non-empty

stomachs. %A was calculated as percentage of abundance of prey item (N) compared to the

total abundance of all prey items (Ntot). %W was calculated respectively.

For overall comparison of prey utilisation the “relative importance index (RI)” (George &

Hadley 1979) was calculated for each prey item, which is based on the absolute importance

index (AI) as follows:

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AI = %frequency of occurrence + %total numbers + %total weight,

RI [%] = 100 AI/ AI ∑n

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Where n is the number of different food categories.

Results Size composition

94 specimens of Zenopsis conchifer were caught at GMR with a biomass of 104 kg. Z.

conchifer was tenth in position of the most abundant fishes and fifth in weight ranking of all

fishes caught (Zidowitz 2001).

The specimens caught were between 17.2 cm and 59 cm in total length (TL), but the fish were

predominantly between 35 and 59 cm (mean length 45.1 cm, mean weight 1109.2 g) There

was only one catch of a juvenile (17.2 cm) but no postlarvae or “fingerlings“ . Fig. 4 displays

the length-frequency of Z. conchifer with maxima in length class >40–42.5 cm with 28

individuals, and in length classes >45–52.5 cm with 13, 12 and 11 individuals. The maxima

probably attributes to cohorts of episodic recruitment. A Kolmogorov-Smirnov-Test showed

no normal distribution.

Age

Grinding and reading of otoliths of Z. conchifer revealed a lifespan of 12 – 14 years for the

largest specimens. Increments counted of whole otoliths were relatively clear from the

primordium to the edge although large otoliths were very opaque in the primordial region

(fig.5). Increments visible in transverse sections were very diffuse so otoliths were distally

ground in an attempt to increase increment clarity. A comparison of age estimates obtained

from whole and sectioned otoliths found that higher ages were derived from sectioned

otoliths. This may be because more increments were visible in the primordial region of

sectioned otoliths and because increments formed on the edge were also clearer in sectioned

samples. After examination of whole and transversely and distally sectioned otoliths, it was

evident that more accurate estimates of age were obtained from whole otoliths. Steward

(1992) found that increments formed in the otoliths of the mirror dory (Zenopsis nebulosus)

were also very diffuse when sectioned and that consistent age estimates were obtained from

whole otoliths. Similar age estimates were determined for Z. conchifer as for Z. nebulosus for

similar sized fish. Assuming that the counted increments were annual, age estimates obtained

from the Z. conchifer otoliths ranged from 4 to 14 years of age (table 2., fig. 6).

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Food items and diet composition

The food spectrum of Zenopsis conchifer comprise 14 food categories (table 3). The diet is

characterised by pelagic, mesopelagic and bentho-pelagic fishes. Seven genera of fishes could

be identified, of these four species. Due to an advanced state of digestion a lot of fishes could

not be determined to the lowest taxon but categorised as “unidentified fishes“.

Stomachs contained five genera of the family Myctophidae (Lampanyctus sp., Lepidophanes

sp., Ceratoscopelus sp., Diaphus sp., Hygophum sp.). Stomiiform fishes were represented by

families Stomiidae (Chauliodus danae), Phosichthyidae (Vinciguerria nimbaria) and

Sternoptychidae (Argyropelecus sp.) with a few individuals. Capros aper (Caproidae,

Zeiformes) and the genus Trachurus sp. (Carangidae, Perciformes) could be verified in

stomach contents. Most important food item in the diet of Z. conchifer was Macroramphosus

spp. (Macroramphosidae, Syngnathiformes).

Besides fishes molluscs (Cephalopoda, Decabrachia) could be identified by cutinisied jaws

and a rest of a buccal-apparatus. Crustacea in the diet belonged to the decapods.

For food categories the percentage of abundance (%A), percentage of frequency of occurrence

(%F), percentage of weight (%W) and the relative importance index (%RI) were calculated.

The most important food item was Macroramphosus spp. with 39 % relative abundance.

Myctophidae accounted for 32 %, „unidentified fishes“ 10 % of overall prey items. Other taxa

were only of minor relevance with one to five percent of relative abundance.

Prey item Macroramphosus spp. was found in 44 %, „unidentified fishes“ in 18% and

myctophids in 16 % of investigated stomachs. Capros aper was found in 10 % and Trachurus

sp. in seven percent of stomachs. Other prey categories accounted for two to five percent of

relative occurrence.

Looking at the relative weight of the prey categories of Zenopsis conchifer, Macroramphosus

spp. was again the dominant food with 44 %. The few individuals of Trachurus sp. Were the

second important in weight category with 38,6%, because of their large body size.

“unidentified fishes” were third in percentage of weight due to a few larger, and heavier

individuals with 10% of prey biomass. Capros aper, reaching a higher individual weight

compared to Macroramphosus spp., took eight percent of weight prey. The numerous

myctophids only made up two percent of biomass just like the decapos. The mesopelagic

fishes Chauliodus danae und Vinciguerria nimbaria accounted only for 0,4 % and 0,04 % of

weight.

The relative importance index shows that Macroramphosus spp. was the main prey item of

Zenopsis conchifer at GMR with 39% (fig.7).

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Myctophids were of relatively high significance with 18 % although of little weight (2 %) and

therefore the second important food category. Trachurus sp. Had a share of 17 %, due to its

size and weight but small number, in the overall comparisons, “unidentified fishes” 7%,

Capros aper 8 %. All other taxa, Chauliodus danae, V. nimbaria and decapods, were only

between 1 - 3 % of relative importance index.

Diurnal feeding rhythm

At GMR a habitat dependent resource utilisation was observed (Fock et al. 2002 b). Among

other fishes Z. conchifer shows a preference in consuming constituents of the sound scattering

layer. By partitioning habitats and the analysis of the consumed food in these areas, a

connection to the SSL-trap phenomenon was revealed.

The intensity of ingestion varied. Differences in feeding activity over course of time, were

acquired by degree of stomach fullness at different day and night times. The data revealed a

change in stomach fullness and periods of maximal feeding activity could be made visible

(fig. 9). At night time a high percentage of stomachs were empty and no or only little food

was consumed. At dawn and in the morning hours half of the stomachs were full or had a

moderate filling. This was the main feeding period of Z. conchifer. In the afternoon the filling

decreased and more intense digested food items could be observed. At dusk (18-21 h) a

second but weaker feeding period was determined.

At the Great Meteor seamount the analysis of the horizontal distribution of seamount fishes

over the plateau revealed that most of the populations were related to habitats at the edge of

the plateau (Fock et al. 2002). At plateau margins the probability of interception with the

horizontally advected SSL is increased and vertically migrating SSL ideally passes the

marginal habitats twice a day during its ascent and descent, while the summit plateau is only

supplied with advected planktonic prey during the descending phase of the SSL (Fock et al.

2002) (fig.10). Besides enhanced interception probability with the SSL, plateau margins are

further affected by topographically induced circular currents around the summit (Taylor-

Column) and local upwelling phenomena caused by these current anomalies, which are

typical at GMR (Meincke 1971, Mourino et al. 2001).

Ontogenetic shift in diet composition

The comparative presentation of relative importance indices in fig. 10 illustrates the

relevancy of single food categories in the nutrition of the different sized specimens.

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Specimens of 36 – 46 cm fed on six food categories, whereat no item extremely dominated in

the diet. Food items Trachurus sp., Myctophidae and Macroramphosus spp. took the main

body of prey, whilst cephalopods, Capros aper and “unidentified fishes” were of less

important trophic meaning. With increasing size a change in the diet composition could be

observed. In size class 47 – 52 cm the significance of Trachurus sp. decreased. Instead,

Macroramphosus spp. predominated the diet of Z. conchifer and “unidentified fishes” were

second in meaning. Myctophids still took an important role but by comparison with the

smallest size class (36 – 46 cm) were less important. In the prey spectrum of the moderate

size class other mesopelagic fishes turn up, but only in small numbers so that they were

summarised to “other fishes”. Decapods were found but with little importance. In size class

>52 cm a distinct taxonomic impoverishment of the prey spectrum occurs. Only two food

categories, Macroramphosus spp. and unidentified fishes were ascertainable though only two

specimens of Z. conchifer were available.

Discussion Individuals of Zenopsis conchifer reaching a maximum length of 59 cm were relatively large.

Because no larvae, postlarvae and small juveniles were caught in pelagic trawls above the

GMR and also very large specimens with total lengths over 60 cm were missing a regular

recruitment can be doubted. Ehrich (1974) suggests a self preserving recruitment of Z.

conchifer at GMR but there is no evidence that the population is independent from other

habitats. Irregular current situations may cause an influx of pelagic larvae from other areas

and therewith fill up the population at GMR. Maxima of several length classes could indicate

cohorts of episodic recruitment. This ability of withstanding episodic recruitment is one of the

factors for the ecological success of the genus Zenopsis and with this adaptation to prevail in

insecure recruitment phases is the causes for successfully inhabiting seamounts.

Like Z. conchifer, Z. nebulosus was one of the most important fishes of seamounts of the

Nazca Ridge. It occupied fourth to sixth place in lists of dominant species on these seamounts

and accounted for up to 5 % of the total catches (Parin et al. 1988). Catches of Z. nebulosus

were between 4 and 48 cm, with a mean length of 32-33 cm. They also had remarkably few

catches of small and juvenile specimens, in four years only two “fingerlings“ (postlarvae)

with standard lengths (SL) of 44 and 60 mm and only three juvenile females with standard

lengths of 140 to 160 mm. Parin et al. (1988) assume that the infrequency of the discovery of

the juveniles is explainable by their pelagic mode of life.

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The results of the analysed size classes revealed an increasing selectivity with ontogenetic

development from a diverse diet to a one-sided diet consisting of the most abundant species

Macroramphosus spp.. From an relatively even utilisation of all food categories Z. conchifer

shifts to a tighter utilisation with a predominant food category. It can be regarded as an

opportunistic feeder probably due to the possibilities of body size.

At GMR only Heptranchias perlo was the most competitive predator to Z. conchifer. H. perlo

fed predominantly on bentho-pelagic teleost fishes like the snipefishes Macroramphosus spp.,

boarfish Capros aper, congrid eels and other unidentified teleosts. Besides this cephalopods,

mainly octopods, a few elasmobranchs and remains of echinoderms and sponges were food

items of importance (Frentzel-Beyme & Koester 2002).

The diet of Z. conchifer (n=69) at the Namibian shelf consisted predominantly of fishes but

also of euphausiids. The major food component was represented by the family Myctophidae

and minor components were formed of Synagrops microlepis and Trachurus trachurus. The

larger the individuals were the less important became the crustaceans. In smaller individuals

(20 – 29 cm TL) the food were made up of 87.1 % of weight by myctophids mainly composed

of Diaphus dumerilli and D. taaningi. In larger specimens other fishes became more

important. In individuals of 30 – 39 cm TL crustaceans only accounted for 0.2 %, Trachurus

trachurus already reached 32.2 %, whereas the myctophids constituted only 67.7 % (29.2 %

D. dumerilli) of the food. Specimens larger than 40 cm only fed on Synagrops microlepis

(33.4 %), Trachurus trachurus (34.2 %) and the myctophid Symbolophorus boops (32.4 %)

but no crustaceans (MacPherson 1983). The stomach contents revealed a shift in diet

composition with ontogenetic development similar to the shift of Z. conchifer at GMR, from a

main share of mesopelagic food items to a more one-sided diet of a few pelagic and

mesopelagic components. At GMR the range of the size-shifting was in a different magnitude

because the specimens caught were larger in comparison and especially non-myctophid

components consisted of other species than those from the Namibian shelf. The food analysis

of Z. conchifer in the Northwest Atlantic (NOAA area Nova Scotia – South of Cape Hatteras)

showed a 100 % fish diet (Bowman et al. 2000). Of this a share of 32.1 % was identifiable as

Stenotomus chrysops (Sparidae), 67.9 % were unidentifiable osteichthyes. Because of the

small number of specimens (n=5) no shift could be pointed out.

In investigations of the upper slope demersal ichthyofauna in Brazil Zenopsis conchifer was

caught in 34 hauls. Stomach content analysis revealed mesopelagic Maurolicus muelleri,

Diaphus dumerilii and Euphausia similis as main food components (Haimovici et al.1994).

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Bigelow & Schroeder (1953) reported that they found two butterfish, 15 – 18 cm long, and

one squid in the stomach of a large specimen about 50 cm long captured in a trawl off the

American Atlantic coast.

In terms of optimal foraging (Hart 1986) the SSL-interception hypothesis firstly implies

reliance on pelagic prey. Secondly, an increased resource utilisation rate as cause for the

aggregation of fishes in plateau margins as locations of potential interceptions is predicted.

Thirdly, the SSL-interception hypothesis predicts either cessation of feeding during periods of

absence of migrating prey or a switch between diets depending on their diel availability (Fock

et al. 2002 b)..

Z. conchifer shows in all investigated areas an intense food utilisation of pelagic and

mesopelagic food items. At GMR and a positive relationship to plateau margins (Fock et al.

2002 b) and an increased resource utilisation rate at the plateau margins and a diurnal feeding

rhythm which corresponds to the availability of migrating prey could be determined. This

preference were suggested to be attributed to differential feeding modes with respect to the

SSL-interception hypothesis (Fock et al. 2002 b).

Seamount-associated predators such as Zenopsis spp. are less mobile and have a body plan

constituted for a different hunting strategy than the fast swimming neritic hakes. Zenopsis spp.

is considered as a feeble swimmer (Wheeler 1969) and profits of a expectant strategy

(Zidowitz 2001). Zenopsis spp. showed in all regions of occurrence a selectivity for

mesopelagic items during SSL decent with a therefore adapted diurnal feeding rhythm.

Smaller specimens of Z. conchifer at GMR, on the Namibian shelf and smaller and medium-

size specimens of Z. oblongus show furthermore an adaptation in ontogenetic development for

this favoured feeding strategy. Larger specimens abandon preference for the SSL and the off-

bottom pelagic feeding mode and turn to a more one-sided diet by feeding on the most

abundant species. Diet of larger specimens of Z. conchifer at GMR constitutes almost

exclusively of Macroramphosus spp., the most abundant fish on the seamount, probably

caused by body size enabling to prey on bentho-pelagic species. MacPherson (1983)

appraised the presence of fast swimmers like Trachurus trachurus in several stomachs, that it

shows the great efficiency of the hunting strategy of Zenopsis conchifer.

Z. nebulosus (oblongus) reaches a maximum size of 70 cm and therefore has a lower average

size compared to Z. conchifer. Like Z. conchifer the diets consists of the most abundant

species, whether “thalassobathyial bentho-pelagic” fishes (in particular Maurolicus), “oceanic

mesopelagic” animals (euphausiids, myctophids and squids) and “pseudoneritic and oceanic

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nektonic” fishes (Trachurus, Emmelichthys, Cubiceps) (Parin et al. 1988) thus making them to

opportunistic feeders with preferences depending on body size. Though being also very

abundant at the Nazca Ridge seamounts (Parin et al. 1997), no Macroramphosus spp. were

eaten here. In Californian waters Z. nebulosus was found to prey on rockfishes (Sebastidae).

Conclusions The species of the genus Zenopsis spp. are similar types of predators in the investigated

habitats. They fill the same ecological niche benefiting in seamount habitats of their typical

zeid body plan and life history with special adaptations. Besides a few sharks they play an

important role in food web structures as higher-level carnivores, whereas in shelf habitats

other fast swimming nektonic species dominate.

Literature Anon. (2001). Fish collection database of the National Museum of Natural History

(Smithsonian Institution).. Smithsonian Institution - Division of Fishes

Bowman, R. E., Stillwell, C. E., Michaels, W. L., Grosslein, M. D. (2000). Food of Northwest

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17

Figures and Tables

Fig

Fig

Fig

. 1: Distribution of Zenopsis conchifer (FishBase mapper).

. 2: Distribution of Zenopsis nebulosus (FishBase mapper).

. 3: Distribution of Zenopsis oblongus (FishBase mapper).

18

Table 1: Definitions for estimation of stomach fullness, estimated values.

degree of stomach fullness Definition

0 empty

< ¼ little

¼ moderate

½ half full

4/4 full

Empty

A few, or single remains <10%

Filled up to 30 %

Filled between 30 % and 70 %

Filled between 70 % and 100 %

13

8

28

9

13 12 11

3 3 4

0

5

10

15

20

25

30

17 18,5

>35 -

37,5

>37,5

- 40

>40 -

42,5

>42,5

- 45

>45 -

47,5

>47,5

- 50

>50 -

52,5

>52,5

- 55

>55 -

57,5

>57,5

- 60

length [cm]

num

ber [

n]

Fig. 4: Size composition (TL) of Zenopsis conchifer at GMS.

Ageing Transect

Fig. 5: A whole Z. conchifer otolith viewed with a dissecting microscope, reflected light and a black background. Magnification = 15.75x

19

Table 2. Sample information of otoliths. Total Length Otowght Age

40 0.001 440 0.001 440 0.002 440 0.002 4

642 0.001 542 0.001 643 0.001 643 0.001 649 0.004 950 0.003 850 0.003 853 0.004 957 0.006 1059 0.005 1459 0.007 1159 0.006 12

42

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14 16

Whole age estimates (yrs)

Tota

l Len

gth

(cm

)

Fig. 6: The relationship between estimated age (determined from whole otoliths) and total length of Z. conchifer.

20

Table 3: Number of prey items, frequency of occurrence (%F), percentage of abundance (%A), percentage of weight (%W) and “relative importance index (RI)” for food categories of Zenopsis conchifer.

Taxon number prey item[n]

percent abundance (A) [%]

percent frequency (F) [%]

percent weight (W) [%]

rel. importance index (RI) [%]

Pisces Syngnathiformes Macroramphosidae Macroramphosus spp. Zeiformes Caproidae Capros aper Stomiiformes Stomiidae Chauliodus danae Phosichthyidae Vinciguerria nimbaria Sternoptychidae Argyropelecus sp.* Myctophiformes Myctophidae Lampanyctus sp.* Lepidophanes sp.* Ceratoscopelus sp.* Diaphus sp.* Hygophum sp.* Perciformes Carangidae Trachurus sp. Unidentified fishes

142 59 7 6 2 1 48 15 13 14 5 1 4 15

94,6 39,3 4,7 4 1,3 0,7 32 10 8,6 9,3 3,3 0,6 2,7 10

43,5 9,7 3,2 1,6 1,6 16,1 9,7 4,8 11,3 4,8 1,6 6,5 17,7

40,3 7,5 0,4 0,04 -- 2,2 -- -- -- -- -- 38,6 9,7

39 7,8 2,7 1 -- 17,9 -- -- -- -- -- 17,1 7,1

Mollusca Cephalopoda Decabrachia+

3

2

4,8

--

--

Arthropoda Crustacea Decapoda

5

3,3

1,6

1,8

2,4

* identification bay otoliths, no weight data available

+ no cephalopod beaks weighted

7 %

17 %

18 %

1 %3 %

8 %

39 %

2 % Macroramphosus spp.

Myctophidae

Capros aper

Chauliodus danae

V. nimbaria

Trachurus sp.

Pisces indet

Decapoda

Fig. 7: relative importance index (George & Hadley 1979) of food components in the diet of Zenopsis conchifer.

21

0%

20%

40%

60%

80%

100%

0 - 3

h3 -

6 h

6 - 9

h

9 - 12

h

12 - 1

5 h

15 - 1

8 h

18 - 2

1 h

21 - 2

4 h

Time

empty full

little moderate half full

other fishes

Cephalopodaunident. fishes

Chauliodus sp.Capros aperMacroramphosus spp.

Fig. 8: Diurnal feeding rhythm based on degree of stomach fullness.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

36 - 46 cm 47 - 52 cm > 52 cm

size classes TL [cm]

rel.

Impo

rtan

ce in

dex

[%] Decapoda

Trachurus sp.

Myctophidae sp.

Fig. 9: Comparison of food composition of the 3 size classes of Z. conchifer based on the relative importance indices. 36 – 46 cm: n = 32, 47 – 52 cm: n = 24, > 52 cm: n=6.

22

F

ig. 10: DSL (SSL) trap at Great Meteor Seamount (from Hesthagen 1970).