Autoecology of the marble marmoratus in the Province of Verbano

Cusio Ossola

Codice Azione A.2. Collection of abiotic, hydromorphological and biological Titolo information necessary to plan concrete actions Codice Subazione -

Titolo - 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 A B C D E F necessary to plan concrete actions

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 - 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. Collection of abiotic, hydromorphological and biological information Action Title 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 CNR – Istituto per lo Studio degli Ecosistemi implementation Expected end date 15.06.2018

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 (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-) 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 () 12.8 ± 2.0 19.2 ± 3.6 23.8 ± 3.9 27.5. ± 4.1 32.3 ± 2.2 36.0 ± 00 Zakojska Vincenzi et al; 2007 11.5 ± 1.5 17.6 ± 2.7 21.1 ± 2.9 24.2 ± 3.1 28.0 ± 3.5 32.9 ± 2.4 (Slovenia) 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 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.

A B

C D

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E F

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.

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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.

Min Max Mean Min Max Mean Min Age Max Age Brown t. Site N° marble t. lenght lenght ±S.E. Weight Weight ±S.E. years years 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 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 100.0 102.8 ± 3.1* 53.6 ± 10.4* 62.7 ± 12.1* literature)

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 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., Migratory lacustrine Lake Maggiore, 49.6 ± 2.7 cm 62.8 ± 4.9 cm 2017 marble trout Toce River Merlo, 1956 “” 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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