autoecology of the marble trout - idrolife...this river portion has a medium flow, alternating...
TRANSCRIPT
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F C B D E A
Codice Azione
A.2.
Titolo Collection of abiotic, hydromorphological and biological
information necessary to plan concrete actions
Codice Subazione
Titolo
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Tipo di elaborato Deliverable
Stato di avanzamento Final Version
Data 14.02.2020
Autori CNR – Istituto per lo Studio degli Ecosistemi
Responsabile dell’azione CNR – Istituto per lo Studio degli Ecosistemi
Collection of abiotic, hydromorphological and biological information
necessary to plan concrete actions
Autoecology of the marble trout Salmo
marmoratus in the Province of Verbano
Cusio Ossola
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LIFE Nature and Biodiversityproject
LIFE15 NAT/IT/000823
Project title: Conservation and management of freshwater fauna of EU
interest within the ecological corridors of Verbano-Cusio-Ossola
Project acronym: IdroLIFE
Name of the Member State: IT - Italy
Start date: 15-11-2016
End date: 14-11-2020
Coordinating beneficiary CNR - Institute of Ecosystem Study (abbrev. CNR-ISE)
Associated beneficiaries Provincia del Verbano-Cusio-Ossola (PROVCO)
Ente Parco Nazionale della Val Grande (PNGV)
G.R.A.I.A. srl - Gestione e Ricerca Ambientale Ittica Acque (GRAIA)
Action A.2.
Action Title Collection of abiotic, hydromorphological and biological information
necessary to plan concrete actions
Subaction -
Subaction Title -
Title of the product Autoecology of the marble trout in the Province of Verbano Cusio Ossola.
Type of product Deliverable
Progress Final version
Date 14.02.2020
Authors CNR – Istituto per lo Studio degli Ecosistemi
Beneficiary responsible for
implementation CNR – Istituto per lo Studio degli Ecosistemi
Expected end date 15.06.2018
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Contents
1 INTRODUCTION ..................................................................................................................................... 4
2 STUDY SITES .......................................................................................................................................... 8
3 METHODS .............................................................................................................................................. 13
4 RESULTS AND DISCUSSION .............................................................................................................. 14
5 CONCLUSIONS ..................................................................................................................................... 22
6 REFERENCES ........................................................................................................................................ 24
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1 INTRODUCTION
The present paper has the aim to describe the ecology of Marble trout (S. marmoratus) in the
Verbano-Cusio-Ossola province (VCO). In the following sections, after a brief species description,
a framing about the species ecology in VCO waters has been shown. This has been possible by
using all the 2017 data from IdroLIFE activities, integrating with previous data from surveys,
anglers catches and visual census.
Distribution
Marble trout, Salmo marmoratus (Cuvier, 1817), is an endemic salmonid, with of particular
conservation interest, that inhabits a very restricted geographical area belonging to the Adriatic
Basin from Po river (only northern tributaries) to Slovenian drainages of Soča and Rižana rivers
(Bianco 1987; Sommani 1961; Forneris et al. 1990; Schoffmann 1994; Povz 1995).
Morphological traits
First descriptions of the species (Gridelli, 1936; Pomini, 1937) highlighted the main differences
with the “stream trout”, the brown trout (Salmo trutta). Compared to this last one, marble trout has a
typical marbled pattern on its back and flanks and distinguishes itself for a big head (22-25% of the
Standard length) and for the absence of ocellated red spots and parr marks on its body (adult
specimens). It owns cycloid scales of small size, four teeth on the head of the vomer and vomer
stem is provided with 8 to 15 teeth, aligned but with the tips oriented right and left alternately. Fins
are well developed and caudal fin is homocercal, with back edge straight or slightly sunken
(Gandolfi et al. 1991; Delling et al. 2000; Kottelat and Freyhof, 2007).
Ecology
Besides the morphological aspects marble trout also differs from other ecological peculiarities; in
addition to the size (marble trout can reach 15-20 kg and exceed 1 meter length) this species prefers
the middle-low portion of cold water rivers (foothill area) overlapping its distribution with other
reofil species such as grayling (Thymallus sp.), insubrian barbel (Barbus caninus) and bullhead
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(Cottus gobio). Exceptionally in december 2008 the first ever marble trout (95 cm length) from the
sea (Igrane-Croatia) has been recorded (Soldo, 2013). Furthermore, S. marmoratus (compared to
brown trout) seems to be more suited (over 60-70 cm length) to an ichthyophagous diet. In fact, in
the first years of life S. marmoratus feeds mainly of larvae and adults of aquatic and terrestrial
insects and with the increase in size, the diet is progressively oriented towards fish (conspecific
included).
Reproduction
Reproduction takes place from November to January, with water temperatures normally between 6
° and 8° C. Generally, the reproductive period lasts about a month, even if in some watercourses it
takes place only for a few days. Spawning is more intense in the early morning on new moon days.
Spawners run upstream in big rivers and their main tributaries to reach spawning sites. Males arrive
a few weeks before females. The deposition sites are located in shallow water (60 - 80 cm), with a
substrate characterized by pebbles and gravels and with a stream of about 0.4 - 0.5 m / s. The
female digs a shallow depression into the substrate and then lays her eggs. After fertilization by the
male, the mother covers the brood with nest’s gravel. Parental cares do not exist. Each female lays
an average of 1,700 - 2,800 eggs per kg of weight, with a diameter of about 5 - 6 mm and an orange
or yellowish color. Embryonic development requires a relatively long period depending on the
temperature of the water (about 40 days at 10 ° C). The fry remain buried in the gravel until the
yolk sac is reabsorbed. Male specimens reach sexual maturity towards the third year of age, females
at the same age or one year later (Zerunian, 2004).
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Growth
Regarding S. marmoratus (Fig. 1) some of growth rates data available in literature are reported in
the following table (Tab. 1).
Tab. 1 – Age classes and corresponding lenght (mean ± S.D.) for Marble trout.
Author River 1+ 2+ 3+ 4+ 5+ 6+
Ielli, 1989 Avisio (Italy) 9-14 15-22 22-30 28-37 32-41 36-42
Vincenzi et al; 2007 Górska (Slovenia) 12.8 ± 2.0 19.2 ± 3.6 23.8 ± 3.9 27.5. ± 4.1 32.3 ± 2.2 36.0 ± 00
Vincenzi et al; 2007 Zakojska
(Slovenia) 11.5 ± 1.5 17.6 ± 2.7 21.1 ± 2.9 24.2 ± 3.1 28.0 ± 3.5 32.9 ± 2.4
Vincenzi et al; 2007 Gatsnik (Slovenia) 10.7 ± 1.5 16.3 ± 2.2 21.4 ± 2.6 27.0 ± 2.5 - -
Sumer et al; 2001 Trebusčica (Slovenia) 13.3 ± 1.0 17.4 ± 1.4 21.8 ± 1.0 28 ± 0.5 - -
Status
Species’ status at present time is considered critical due to introgression by brown trout strains.
Hybrids between the two forms are fertile and actually more abundant in many rivers of the species’
range (Meldgaard et al., 2007). This genetic affinity with S. trutta, moreover, underlines the species
'fragility' and the realistic risk of a complete genetic introgression leading to its disappearance.
Lastly, also the human impact represents a heavy factor responsible for the species decline: water
withdrawals, reduction of minimum vital flows, overfishing, pollution and alien species
introduction (brown trout over all) have contributed to list marble trout in the Annex II of the
European Union Habitats Directive 92/43/EEC and in the Red List (IUCN, 1996).
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Fig. 1 - Marble trout (Salmo marmoratus), Toce river.
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2 STUDY SITES
In this section Tab. 2 shows sites name and numbers with corresponding altitude, while the map
(Fig. 2) shows their geographical position. The figure includes data from the past (Regione
Piemonte, 1991 and G.R.A.I.A. s.r.l. samplings, 1995-1999) about Salmo marmoratus distribution
in Verbano-Cusio-Ossola (VCO) Province.
Fig. 2 - Numbers from 1 to 13 represent IdroLIFE sites; circular spots (red and white) represent past data about
marble trout distribution in the VCO Province.
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Tab. 2 - IdroLIFE sites. (U)=upstream, (D)=downstream.
Toce River site (IdroLIFE activities) Site number m.a.s.l.
Gravellona Toce 1 211
Migiandone bridge (D) 2 215
Pieve Vergonte 3 232
Vogogna 4 226
Prata Dam (D) 5 227
Prata Dam (U) 6 229
Six Arches bridge (D) 7 239
Six Arches bridge (U) 8 240
Mizzoccola bridge (D) 9 261
Mizzoccola bridge (U) 10 263
Lake Tana (D) 11 302
Lake Tana (U) 12 304
Crodo 13 412
Sites description
1) Gravellona Toce
It is the last section of Toce river, close to the mouth. The riverbed reaches considerable amplitudes
(more than 60/70 m). Water depth is between 1 m and 5 m and banks are made up of large boulders.
This site is characterized by a low heterogeneity of habitats and a smooth flow.
2) Migiandone bridge (D)
It is the first main obstacle that fish find on their migrations. Under the bridge there’s a jump of 2 m
and only with high water levels it is possible to overcome this barrier (mostly for big fish). The
sampling, due to the persistent high water level, has been conducted 300 m downstream in a lateral
portion of the river, with a smooth flow and characterized by a substrate made by sand, gravel and
boulders. (Max depth 1m).
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3) Pieve Vergonte
This river portion has a medium flow, alternating riffle and pools with a maximum depth of 1.8 m.
The substrate is made up by sand (80%), gravel (10%) and rocks (10%). On the hydrographic right
there is an artificial bank characterized by big boulders (Fig. 3C).
4) Vogogna
This sampling site is nearby the mouth of the Anza stream and it is characterized by a riverbed with
an amplitude of 30 m and maximum depth of 1.5 m. The substrate is made by sand and rocks while
the hydrographic left consists of a big bank made by artificial boulders. In this site water
temperature and transparency are influenced by Anza’s flow.
5) Prata Dam (D)
This site is located before the dam in an area where fishing is forbidden. The substrate is various,
with sand, gravel and big boulders. The depth can reach 3 m in the middle of the pool. The high
heterogeneity of this site allows the fish to find different types of refuge (Fig. 3A).
6) Prata Dam (U)
This site is located 300 m upstream the dam. The riverbed has an amplitude of 50 m, a smooth flow
and a substrate composed of sand and sporadic macrophytes. The maximum depth reaches 2 m (Fig.
3B).
7) Six Arches bridge (D)
This site is made by several pools downstream the natural barrier created by big boulders. The
substrate is made by rocks, gravel, sand and some boulder. The flow is various and alternates fast
flowing water with smooth flows and pools (Fig. 3D).
8) Six Arches bridge (U)
This spot has the main riverbed on the hydrographic right while on the left it owns a narrow
secondary arm with a complex system of vegetation on its banks and consequently with a high
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percentage of shading. The main arm has a medium flowing water and a substrate made by sand,
gravel, rocks and some sporadic boulder.
9) Mizzoccola bridge (D)
This site is placed near the town of Domodossola, in the middle section of the river. A cochlea for
electricity production (and its fish passage) influences the riverbed hydrology on the left arm, while
on the right arm, several boulders placed alternately constitute a “natural” fish passage. The
substrate is made by gravel and rocks and the flow is medium-fast. Still on the left side of the bed
(in the left arm) there is a sectioned bank made by big boulders that during the sampling activities
have proven themselves as good shelters (Fig. 3E).
10) Mizzocola bridge (U)
This river area is made by a long flat section with a smooth flow and low depth (60-70 cm max.)
The substrate consists of gravel and rocks with some sporadic boulder. At the end of the sampling
site (hydrographic right) there is a deep pool (1.8 m) with some submerged boulders. This deep
refuge is due to the presence of an oil pipeline pylon.
11) Lake Tana (D)
Site 11 is placed exactly downstream the dam that creates the Lake tana. This is most probably the
last potential river area that a fish may potentially reach in its migration from downstream (or from
Lake Maggiore). The riverbed is highly artificial with banks made of concrete as well as the
substrate. Sand is present due to the lake deposit which moves downstream when dam’s gates are
completely open. The maximum depth reaches 1.7 m in the middle of the pool.
12) Lake Tana (U)
This site starts upstream the Lake tana, where the river has a smooth flow with the presence of a
couple of pool (maximum depth of 1.7 m). The substrate presents all the granulometry types, from
sand to some rare boulder. At the end of the site, hydrographic right, Toce meets the water of an
artificial canal, which on summer season brings cold and oxygenated waters exactly before the Lake
tana entrance (Fig. 3F).
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13) Crodo
It is the site at the highest altitude. The riverbed is narrow and the hydrology is more typical of a
fast flowing alpine stream, where riffle zones are followed by smooth flows. The substrate
alternates gravel, rocks and boulders. The maximum depth reaches 70 cm maximum.
Fig. 3 - Six IdroLIFE sites: A) Prata dam downstream, B) Prata dam upstream, C) Pieve Vergonte, D) Six Arches
bridge downstream, E) Mizzoccola bridge downstream, F) Lake Tana upstream.
D
5
E F
C
A B
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3 METHODS
Sampling activities have been conducted following the downstream-upstream gradient each
transect. The electrofishing device was a built-in-frame ELT60 IIGI (Scubla Acquaculture, 1,300
W, 600 V, DC current) set up with a copper cathode (width 2.5 cm, length 300 cm) and with a steel
ring anode (thickness 0.8 cm, diameter 40 cm). The fish collected were all individually measured
(total length, LT), weighed (total body mass, WT) and scales were taken for age determination by a
special magnifier (33x).
The body mass-length relationship was calculated using the equation:
W = a x Lb
The "b" coefficient usually is included between 2.6 and 3.4, representing a synthetic indicator of the
corpulence of the population’s individuals, and indirectly indicates their health state (Froese, 2006).
Length-at-age data were used to estimate the parameters of the Von Bertalanffy (1938) growth
function (VBGF) according to the equation:
LT= Linf (1-e-K(t-t0)
)
where LT is total length of the fish at time t, Linf is the theoretical maximum length an average fish
could achieve, k is the growth constant which determines how fast the fish approaches L∞, and t0 is
the hypothetical age at LT = 0.
Lastly, Linf, length at first maturity Lm (the length where the 50% of the fish in a population reach
sexual maturity) and optimal capture length Lopt (the intermediate length class where the product of
individuals times their average weight, reaches a maximum) have been obtained also through an
empirical equation from a specific excel spreadsheet created by Froese & Binohlan (2000).
As a management tool the PSD (Proportional Stock Density Index) index has been used (Volta,
2010). PSD is calculated as:
PSD= (Ni≥ Lm)/(Ni≥Lstock)*100
Ni = number of individuals
Lm = minimum quality length = mean length at maturity.
Lstock = minimum stock length = Lm-(LTrophy-Lm)/3
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Ltrophy = Ltot ≥ 0.8*(Linf)
Linf is the site-specific maximum length reached by the species in that environment.
In order to obtain a more detailed picture on the species ecology in Toce river (in particular about
growth dynamics and on the related parameters), additional information from grey literature,
recreational and professional fishing have been considered and used in the excel spreadsheet.
4 RESULTS AND DISCUSSION
In the following table (Tab. 3), all data from IdroLIFE sampling activities have been resumed. Sites
progression from 1 to 13 follows the altitudinal gradient (1 lowest site, 13 the highest).
Tab. 3 - Results of sampling campaigns. Sites number (code), number of fish caught, morphometric
characteristics, measured lengths and weights have been shown. Additionally, brown trout presence is indicated
in each sampling site.
Site N° marble t. Min
lenght
Max
lenght
Mean
±S.E.
Min
Weight
Max
Weight
Mean
±S.E.
Min Age
years
Max Age
years
Brown t.
presence
1 0 - - - - - - - - X
2 22 7.3 8 7.6 ± 0.3 3.9 5.1 4.5 ± 0.6 0+ 0+ X
3 22 13.7 15.9 14.8 ± 1.1 27.4 41.2 34.3 ± 6.8 1+ 1+ X
4 21 15 65 23.7 ± 2.3 33.1 2554.3 234.6 ± 117.7 1+ 8+ X
5 21 28.5 65 44.4 ± 2.1 230.0 2650.0 1067.2 ± 168.1 3+ 6+ X
6 0 - - - - - - - - X
7 25 6.3 55 24.4 ± 4.1 2.0 1100 282.4 ± 99.4 0+ 5+ X
8 15 15 27 20.2 ± 1.9 33.2 189.2 89.4 ± 26.7 1+ 3+ X
9 28 8.5 32 21.2 ± 2.8 7.0 367.0 156.6 ± 54.4 0+ 3+ X
10 26 10 18 14.2 ± 1.4 9.9 61 34.0 ± 8.9 0+ 1+ X
11 0 - - - - - - - - X
12 16 24.8 68 40.2 ± 2.2 158.0 2580 723.6 ± 139.8 3+ 9+ X
13 7 20 38.5 26.8 ± 2.6 74.0 367.0 195.0 ± 42.9 2+ 3+ X
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On 13 sites totally sampled, marble trout (203 specimens in total) has been recorded in 10 sites
whilst in 3 sites (1-Gravellona, 6-Prata dam upstream and 11-Lake Tana downstream) was absent.
On the contrary, the brown trout was sampled in every site, although in a limited number.
The highest maximum length has been recorded in site 12 (Lake Tana upstream), while the fish
with the maximum weight has been found in site number 5 (Prata dam). The maximum age
recorded (9 years) is from site 12 (Lake Tana upstream) while the “young of the year” class appears
in four sites on ten in which S. marmoratus showed its presence.
In the next lines, length-age relationship for marble trout has been reported (Fig. 4)
Fig. 4 - Age-length relationship for S. marmoratus (Toce river).
Length-weight relationship follows in the next figure (Fig. 5). Looking at the b parameter of the
relationship, S. marmoratus from Toce river has a value that falls within the widest range of values
(from 2 to 4) considered acceptable (Carlander, 1969). Furthermore, b parameter (3.0118) is slightly
higher than 3, indicating substantially an allometric growth.
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Fig. 5 - Length-weight relationship for S. marmoratus (Toce river).
Focusing on the analysis on life-traits parameters, three different scenarios (A, B & C) are
generated in regard to the body growth of the fish (Tab. 5):
SCENARIO A: deriving the parameters shown in Table 5 by using the Von Bertalanffy
equation, the highest Linf among all is obtained. If subsequently this Linf value is entered in
the excel spreadsheet (Froese & Binholan, 2000), it is possible to obtain Lm and Lopt, and
even in this case both values are the highest of all 3 scenarios.
SCENARIO B: processing all IdroLIFE data through Froese & Binholan (2000), starting
from the maximum length of marble trouts caught by electric fishing in Toce river
population (68 cm), the lowest values for all the parameters (Linf, Lm and Lopt) are
obtained,
SCENARIO C: this last scenario has been originated by using data from anglers. After a
brief but detailed research it has been possible to get some information about the maximum
lengths recorded for S. marmoratus in the Toce river in these last 3 years. There are few
records of fish around and over 1 meter of length and several over 90 cm (Fig. 9). The
reasonable Lmax used for this scenario is at least 100 cm. The values for parameters Linf,
Lm and Lopt are placed halfway between the values from scenarios A and B.
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Tab. 5 - Lmax, Linf, Lm and Lopt for Marble trout in Toce river (95% ± C.I.).*Calculated by using the
empirical equation from (Froese & Binholan, 2000).
Source Lmax (cm) Linf (cm) Lm (cm) Lopt (cm)
A) Von Bertalanffy, 1938 68.0 164.7 ± 17.5 81.8 ± 20.4* 100.3 ± 20.7*
B) Froese & Binholan, 2000 68.0 70.4 ± 1.7* 38.1 ± 6.1* 42.9 ± 8.0*
C) Extra info (anglers, grey
literature)
100.0 102.8 ± 3.1* 53.6 ± 10.4* 62.7 ± 12.1*
Robustness and accuracy of the Von Bertalanffy equation (and consequently of the curve and the
parameters derived from it) increase with the increase in the number of data for each individual age-
class. The reason why parameters derived through Table 5 are higher (Scenario A) compared to
others (B and C) depends on the reduced data pool. In particular, small abundances of individuals
belonging to age-class 4+ and to the following ones are reflected in the curve’s coefficient, which
generates likely an overestimation of the Linf parameter and consequently of Lm and Lopt when
computed with the excel spreadsheet.
Lowest parameters calculated in the B scenario by using data achieved from field samplings and
Froese and Binholan (2000) spreadsheet (starting from the maximum length sampled), may depend
instead on a sampling bias. In fact, a sampling carried out without the use of a boat don’t allow to
reach the deepest pools, where often larger marble trouts hide themselves. Because of this possible
bias, an underestimation of the maximum length (Lmax) recorded from data pool is obtained and,
consequently, low values for the remaining parameters derived from the excel spreadsheet equation.
By using data from anglers (scenario C), it has been possible to enrich the data set of river Toce
population even with large-sized specimens. Based on this addition, the parameters obtained are
more reliable and fit better to the ecology of a large foothill river fish, a river that in this case is
interconnected with a lake. In fact, Lake Maggiore is an important suitable habitat for marble trout
as pointed out by Gibertoni et al., (2017) the existence of a migratory lacustrine marble trout (Fig.
8) that during its life migrates (mainly for spawning) from Lake Maggiore to Toce river and
viceversa, has been documented. This lacustrine form seems to have a particular morphology
(larger dimensions, silvering colour and caudal fin with a “swallow tail” shape) compared to the
riverine marble trout (Fig. 9). Overall its growth rate seems to be significantly higher compared to
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resident specimens (Tab. 6). The rapid growth rate of lacustrine trout ecotypes has also been found
in other environments (Merlo,1956; Gratton et al., 2004, Turin et al., 2006).
Tab. 6 - Lmax, Linf, Lm and Lopt for Marble trout in Toce river (95% ± C.I.).*Calculated by using the empirical
equation from (Froese & Binholan, 2000).
Source Species Water body Class 3+ Class 4+
This paper Marble trout Toce river 29.5 ± 4.4 cm 40.0 ± 5.7 cm
Gibertoni et al.,
2017
Migratory lacustrine
marble trout
Lake Maggiore,
Toce River
49.6 ± 2.7 cm 62.8 ± 4.9 cm
Merlo, 1956 “Lake trout” Lake Garda 37.8 ± 1.0 cm 41.6 ± 1.0 cm
Fig. 8 -Migratory lacustrine marble trout. Toce river (2017).
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Fig. 9 – Two very large specimens from Toce river basin caught by amateur anglers (year 2017).
Regarding the growth rate of marble trout, Toce river fish (Tab. 4) have higher average length at
age than other rivers for which growth data are available (see Tab. 1). Furthermore, the structure of
the population shows more age classes, including fish with an age up to 9+, while the maximum
reported from other rivers is 6+ (Fig. 7).
Tab. 4 - Marble trout (Toce river) age classes and corresponding total lenght (mean ± S.D.).
0+ 1+ 2+ 3+ 4+ 5+ 6+ 7+ 8+ 9+
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Toce river 8.0 ± 1.5 16.0 ± 2.0 21.2 ± 3.7 29.5 ± 4.4 40.0 ± 5.7 43.4 ± 3.7 53.7 ± 3.8 57.0 65.0 68.0
Fig. 7 - Comparison between Toce river lengths (mean values) separated by age-classes and other rivers values
available from literature.
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Fig. 8 – Marble trout (young of the year).
After this previous multiple scenarios analysis the PSD index has been calculated starting from the
C scenario’s parameters. The PSD obtained starting from a Linf of 102.8 cm is 50. As a very
general approach, a population of fish is considered “in balance” if the PSD value is between 30 and
70 (Guy et al., 2007). In fact, if the PSD value is very low then there are few large fish in the
population, whereas a very large PSD indicates that there are few small fish in the population. In
this specific case of study the PSD shows reference conditions, beeing exactly in the middle of the
optimal range and it indicates that marble trout from Toce river has a balanced population.
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Fig. 6 - Marble trout Length-frequency distribution (Toce river)
5 CONCLUSIONS
The purpose of this deliverable was to provide a picture of the autoecology of the marble trout in
the VCO province, and specifically in the Toce River, one of the last rivers in Italy inhabited by this
endemic salmonid. Thanks to the sampling activities related to action A.2 of the IdroLIFE project, it
has been possible to collect a dataset that helped to analyse the main biological characteristics of
this species and hypothesize strategies to its conservation on the long term.
In the following lines some suggestion on the management of the species are indicated.
the minimum capture lengths established in the VCO province for marble trout and for the
“lake trout” in general are respectively 35 cm and 40 cm (Provincia VCO, 2018). These
lengths are totally inadequate if compared with what emerges from this study. In fact,
looking at the scenario B (a scenario that underestimates all the parameters deriving from
the maximum recorded length) a Lopt of more than 40 cm is still obtained and by
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completing the dataset with information derived from recreational anglers, the optimal
length of capture increases further by over 60 cm.
Concerning population dynamics, the presence of a structured population highlighted in this
study may depend to a relevant contribution of local hatcheries, annually introducing several
hundred thousand of fry along the river. At present we did not go deeply in genetic analyses
of the fish captured in the river and those from the anglers hatcheries, so it is not clear how
much the restocking from the hatcheries has a role in maintaining a balanced population in
the river.
The brown trout (Salmo trutta, atlantic strain - Fig. 10) is still present in all the sampling
site, although not very abundant. As already pointed out by Gibertoni et al. (2017) up tp
45% of the genetic structure of phenotypically identified marble trout in Toce River
indicates the presence of alleles of atlantic origin. In this regard, the brown trout is still a
threat to the existence of a pure marble trout population.
Fig. 10 - Salmo trutta
Management of Marble trout in the Toce river Basin: suggestions to improve its
conservation status
On the base of our results, we do think that different approaches must be taken in order to
improve the conservation status of marble trout in toce River Basin:
1. The first step to protect S. marmoratus is to increase the minimum length of capture
close to the optimum length. Increasing the minimum capture length (based on the Lopt
obtained from these data) allows the species to reach sexual maturity with a greater
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percentage of individuals compared to percentage reached with the current minimum
capture length, thus helping to increasing the spawning stock and the resilience of the
population in regard to all stressors. Moreover, investing more to protect natural
reproduction is always preferable to the result achieved from artificial reproduction. In
fact, the fish grown in the hatchery may have a negative correlation with parameters
such as genetic variability, survival rate, growth rate, antipredator behaviour and fitness
in general (Reisenbichler and McIntyre, 1977; Kostow 2004; Jackson and Brown 2011;
Araki et al., 2008).
2. Prohibit further brown trout introductions in the lower stretch of the Toce River and in
Lake Maggiore. Avoiding the introduction of brown trout in Lake Maggiore, in the main
course of the river Toce and in the last stretches of the main tributaries would allow the
marble trout to re-establish an ever less introgressed population through the years. It
must be noted that the marble trout, due to its faster growth than brown trout, could
outcompete the latter when it is not massively stocked (Sommani 1961).
3. Increase the quality and genetic diversity of fish used for repopulation by a preliminary
selection of pure marble trout.
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