Evolutionary pressures under the current fishing practices in European hake (Merluccius merluccius)

Abstract There is a rising concern involving the impact of modern fisheries on the dynamics and viability of the exploited fish populations. The increasing fishing pressure has been recorded to cause declines in age and size at maturation in variety of marine fish species. In this study I have aimed to detect heritable phenotypic changes of the European hake (Merluccius merluccius) under the currently prevailing fishing intensity. Using dataset of 64 individual-based growth parameters of the studied species and an individual-based simulation, which incorporates quantitative genetics, ecological processes and biological characteristics, I have simulated ecological and evolutionary dynamics of hake. Species` phenotypic traits were observed through three different scenarios which showed significant ecological consequences for hake population under the current fishing strategies but detected no evolutionary changes in life-history traits, e.g. growth and age and size at maturation. Even though, the current fishing intensity does not appear to select for a specific life-history type, the ecological impact observed under under the current fishing practices can pose a severe risk through direct population declines.Popular science summary Is the European hake “shrinking” under the current fishing practices? Did you know that the current fishing practices have caused significant declines in body size of many exploited fishing stocks? This effect of fishing management has become one of the major threats for commercially exploited fish species and it can have severe negative consequences for the offspring of the species as well as the ecosystem`s stability. As the demand for fish grows, the fishing gear, such as fishing nets, become more effective in harvesting older and bigger fish since they are the most valuable fish on the market. By removing these individuals, there is more food and space for smaller fish to grow and feed. This will eventually lead for the smaller ones to grow faster and achieve sexual maturity earlier than it occurs in populations not experiencing the pressure by fisheries. As the fish matures earlier and at a smaller size, so does their offspring become smaller and such biological characteristics can become heritable over time. In the long run, these changes induced by fishing intensity, can cause higher variations in the population dynamics and, in the end, lead to population collapse. Following this theory, I have investigated the state of European hake (Merluccius merluccius) stocks across geographic areas in order to detect whether current fishing practices are causing heritable changes in size and age at maturation. For this purpose, I have simulated fish populations and exposed them to three different scenarios which allowed me to compare the effects of: (a) non-fishing conditions, (b) fishing with and (c) without evolutionary processes. Opposite of our expectations, the overall results of the simulations show a biologically nonsignificant decline in size and age at maturation for the European hake. However, the ecological impact of fisheries shows drastic drop in biomass and recruitment. In other words, the results of the study suggest that either the current fishing practices are not potent enough to induce heritable changes for this species or that hake`s specific life strategy has developed mechanisms that can counteract the effects of fishing. Nevertheless, currently prevailing fisheries management could pose a risk of loss through the severe population declines. Therefore, the results of this study should be interpreted with caution. There is a definite need to investigate further the processes that lie behind the observed resistance in populations of the European hake. Advisor: Anna Kuparinen (University of Helsinki) and Per Carlsson Master project 45 credits in Aquatic Ecology Department of Biology, Lund Universit

Diet composition of the spiny dogfish (Squalus acanthias L.) in the northern Adriatic Sea

Analizirana je ishrana 157 jedinki kostelja prikupljenih iz ulova pridnenih koća u periodu od 2005. do 2007. godine, u vanjskim vodama Kvarnera i Zadarskog akvatorija između 44°91’ i 43°59’ sjeverne zemljopisne širine, te 14°06’ i 14°49’ istočne zemljopisne dužine. Ukupna dužina tijela istraživanih životinja bila je između 26,1 i 112,0 cm (prosječna dužina = 57,51; SD = 20,3). Ulovljeni morski psi su morfometrijski obrađeni, nakon čega je slijedila izolacija probavnog trakta te prikupljanje sadržaja probavila koji je konzerviran u 75%-tnom etanolu. Iz sadržaja probavila izolirane su i determinirane skupine plijena, a potom im je određena mokra masa, maseni udio i učestalost. Analizom sadržaja probavila utvrđeno je prisustvo 6 skupina plijena: puževi (Gastropoda), školjkaši (Bivalvia), glavonošci (Cephalopoda), mnogočetinaši (Polychaeta), rakovi (Malacostraca) i ribe zrakoperke (Actinopterygii). Dobiveni rezultati ukazuju na dominantnu zastupljenost dviju skupina plijena, riba zrakoperki zastupljenih s masenim udjelom od 68,3% i učestalošću od 78,3%, te glavonožaca s masenim udjelom od 23,0% i učestalošću od 14,6%. Ostale skupine su zastupljene s ukupnim udjelom od 8,7%. Utvrđena je pozitivna korelacija ukupne dužine tijela (TBL) i mokre mase probavila (rs = 0,247; p = 0,002) te smanjenje udjela zrakoperki (rs = - 0,278; p = 0,002) s porastom TBL. Nisu utvrđene statistički značajne razlike u ishrani kostelja između sezona. Analize razlika u ishrani između spolova ukazali su na postojanje statistički značajnih razlika u masama i masenom udjelu zrakoperki (p < 0,01), te razlika u ishrani juvenilnih i adultnih jedinki pri čemu je udio zrakoperki kod spolno nezrelih životinja bio značajno veći (p = 0,001). Vremenska i veličinska raspodjela životinja ukazuju na sezonsko korištenje istraživanog područja te na njegov značaj kao kritičnog staništa morskog psa kostelja u sjevernom Jadranu.We analyzed diet composition of 157 spiny dogfish sampled by a bottom trawl, during a period from 2005 to 2007, in an area which occupies the aquatic ecosystem of Kvarner and Zadar, between 44°91’ i 43°59’ N and 14°06’ i 14°49’ E. Total body length of the samples ranged from 26.1 to 112.0 cm (mean = 57.51, SD = 20.3). Caught sharks underwent morphometric analysis including digestive tract content sampling which was afterwards preserved in 75% ethanol. Prey items were isolated from every stomach content and identified, also grouped in systematic categories and weighed. We recorded 6 prey groups in food remains: sea snails (Gastropoda), bivalves (Bivalvia), cephalopods (Cephalopoda), polychaetes (Polychaeta), crustaceans (Malacostraca) and ray-finned fish (Actinopterygii). The highest ranked prey groups included ray-finned fish with 68.3% of total wet weigth and 78.3% of occurence, and squid with 23.0% w.w. and 14.6% of occurence. Other prey groups represent altogether 8.7% of total wet weight. We noted a positive correlation between total body length (TBL) and total wet weight (rs = 0.247; p = 0.002), and also, a decrease in weight percentage of ray-finned fish (rs = - 0.278; p = 0.002) with an increase of TBL. There were no statistical significant differences in the diet between seasons. Analysis of the diet between sexes showed statistical significant differences in weigth and weigth percentage of ray-finned fish (p < 0.01) and in the diet between juvenile and adult dogfish, where the percentage of ray-finned fish was considerably larger in the diet of juvenile sharks in comparison to the diet of adult sharks. Temporal and TBL distribution of samples showed seasonal use of the researched area and also, its role as a critical habitat of the spiny dogfish in the northern Adriatic Sea

Fisheries sustainability in the presence of predation by marine megafauna

Overexploitation is still a leading problem of many commercially targeted fish species. In addition to the high harvest rates and increasing biomass removals, harvested marine ecosystems have become a stage for the dynamic interplay of evolutionary and ecological processes. Removal through size selective fishing gear can cause negative pervasive effects on individual as well as population level. Observations of the individual phenotypic traits show a general trend of decreasing size and age at maturity that can have further negative effects on fecundity and population productivity. As these phenotypic changes become heritable (i.e., fisheries-induced evolution or FIE), this can further diminish the fish available to fisheries and render future fishing yields unsustainable. Current management requires additional measures to include avoidance and detection of evolutionary changes. In order to understand which fishing objectives precede evolutionary change in individual traits, in my thesis I explored how different fishing strategies of the European hake (Merluccius merluccius) fishery reflect on ecological and evolutionary processes. While management focusing on the protection of juvenile fish can minimise the negative ecological impact of fishing, it increases the potential for evolutionary change in fish phenotypic traits. In contrary to this, fishing mortality targeting a wider range of age–size classes avoids evolutionary shifts in individual traits, however such fishing strategy demonstrates higher biomass removals. In the wild, fisheries continuously interact with other predators, such as marine mammals, which can prey upon the same fish species or stock. The impact of these direct and indirect biological interactions between the marine mammals and fisheries is harder to detect and quantify, especially in synergy with other natural or anthropogenic stressors. In the context of fisheries-induced evolution, changes observed on an individual and population level caused by fisheries will also affect the prey size selectivity and prey availability to natural predators. My synthesis of recent research and findings on marine mammal–fisheries biological interactions demonstrates the need for improvement on data regarding marine mammal dietary and energetic requirements as well as their representation in model-based approaches. Moreover, combining different sources of knowledge about marine mammal–fisheries competition can aid to better quantify fish mortality caused by predation. Subsequently, this information would improve the fish stock assessments and provide insight on a sustainable window of opportunity to catch fish for fisheries and natural predators. Thus far, attempts to quantify predation and fish availability for fisheries and natural predators exist through studies using mainly ecosystem and fisheries models. To explore how predation and fisheries shape and direct individual as well as population parameters, I have used an individual-based model to simulate hake growth trajectories with regards to its own biological characteristics. As an individual grows, its life history is formed by ecological and evolutionary processes which also take into account the reproductive cost of survival and sexual size dimorphism (SSD). With co-evolved interactions between hake and the bottlenose dolphin (Tursiops truncatus) as the predator, fishing is introduced through a limited time period in order to observe prey recovery and resilience on an individual and population level. Although different types of predation give insight to discrepancies in the intensity of predation mortality, mere presence or absence of predation determines the projected values reached by prey individual and population parameters. Moreover, the joint effect of predation and fishing reveal contra-intuitive trends in hake individual traits and population parameters. The combination of duration and intensity of both size-selective removals, predation type and SSD determine the potential for persistent phenotypic and demographic changes after a period of overexploitation. Additionally, not all individual traits are equally susceptible to fisheries-induced evolution where the accountability of SSD and predation type can play a critical role. While fisheries remain the most detrimental source of mortality and size-selective removal for the harvested species, the indirect effects of fishing intensity diminish predator survival, thus having direct implications for top predator conservation. In conclusion, increasing the biological realism of the targeted species and incorporating different predation types with respect to evolutionary processes provide a more holistic approach to fisheries management: as it helps to avoid potential FIE and an overestimation of fish available to fisheries that can prevent top predator collapse. This will, ultimately, lead to a more ecosystem-based management with sustainable harvest rates and optimised fishing effort as well as the minimal cascading effects of size-selective removals.Monet kaupallisesti kalastetut kalakannat kärsivät edelleen liikakalastuksesta. Korkean kalastuskuolleisuuden ja suoran kalabiomassan poiston lisäksi kalastettuihin meriekosysteemeihin vaikuttavat lisäksi monet samanaikaiset ekologiset ja evolutiiviset prosessit. Valikoiva kalastus vaikuttaa negatiivisesti sekä yksilöihin että populaatioihin. Yksilöiden ilmiasun ominaisuuksien, kuten sukukypsyysiän ja –koon, on havaittu pienenevän, mikä vähentää kalojen lisääntymiskykyä ja kalakantojen tuottavuutta. Koska yksilöominaisuudet ovat osin periytyviä, kalastuksen aiheuttama evoluutio voi edelleen vähentää kalastettavissa olevaa kalakantaa ja vähentää kalastuksen kestävyyttä tulevaisuudessa. Kalastuksen aiheuttaman evoluution havaitseminen ja välttäminen vaatii kalastuksen sääntelyyn uusia työkaluja. Väitöskirjassani tutkin, millaiset kalastuksen sääntelyn tavoitteet johtavat evolutiivisiin muutoksiin yksilöiden ominaisuudessa; tutkimuksen erityisenä kohteena on kummeliturskan (Merluccius merluccius) kalastus ja sen ekologiset ja evolutiiviset vaikutukset. Jos kalastuksen sääntely keskittyy poikasten suojeluun, tämä saattaa vaikuttaa suotuisasti kalakannan kehitykseen mutta toisaalta lisätä evolutiivisten muutosten mahdollisuutta. Jos taas kalastus kohdistuu laajaan kirjoon eri ikäluokkia ja kokoja, evolutiivisten muutosten riski on pienempi, mutta kalastuksen vaikutukset kalakannan kokoon ovat suuremmat. Luonnossa kalastus on jatkuvassa vuorovaikutuksessa muiden saalistajien kuten merinisäkkäiden kanssa; merinisäkkäät saalistavat usein samoja kalalajeja ja –kantoja, joihin myös kalastus kohdistuu. Tämän kaltaisten suorien ja epäsuorien biologisten vuorovaikutusten havaitseminen ja merkityksen arvioiminen on vaikeaa, etenkin kun otetaan huomioon myös muut kalakantoihin vaikuttavat luonnolliset ja ihmisten aiheuttamat tekijät. Kalastuksen aiheuttaman evoluution seurauksena syntyneet muutokset kaloissa ja kalakannoissa vaikuttavat myös saalistajille tarjolla olevien saaliskalojen määrään ja kokoon. Katsaukseni viimeisimpään tutkimukseen merinisäkkäiden ja kalastuksen vuorovaikutuksesta osoittaa kuinka tarpeellista on mallinnuspohjainen tutkimus sekä tutkimusaineistot merinisäkkäiden saalislajeista ja ravinnon käytöstä. Eri lähteistä saatavaa tietoa merinisäkkäiden ja kalastuksen kilpailusta yhdistelemällä voidaan parantavaa arvioita kalojen saalistuskuolleisuudesta. Tällainen tieto edistää kalakantojen arviointia ja kalastuksen kestävyyttä, huomioiden sekä kalastuksen että saalistajien tarpeet. Kalojen riittävyyttä kalastukseen ja saalistajille on toistaiseksi arvioitu käyttäen kalakanta- ja ekosysteemimalleja. Väitöskirjassani käytän yksilöpohjaista mallinnusta arvioimaan saalistuksen ja kalastuksen vaikutuksia kalojen yksilö- ja populaatioparametreihin. Malli perustuu kummeliturskan havaittuihin kasvukäyriin ja muihin lajien biologisiin ominaisuuksiin. Yksilöiden kasvuun vaikuttavat sekä ekologiset että evolutiiviset prosessit, huomioiden esimerkiksi lisääntymisen vaikutus selviytymiseen sekä sukupuoleen liittyvät kokoerot. Mallinnuksessa yhdistellään evolutiivisesti muuttuvaa vuorovaikutusta pullonokkadelfiinien (Tursiops truncatus) ja kummeliturskan välillä, kalastuksen vaikutuksia näihin rajoitetulla aikavälillä sekä kummeliturskan kykyä kestää kalastusta ja palautua kalastuksen loputtua yksilö- ja populaatiotasoilla. Vaikka erilaiset saalistusvaihtoehdot kuvaavatkin vaihtelua saalistuksen voimakkuudessa, suurimmat erot mallinnuksessa johtuvat yksinomaan saalistuksen läsnäolon vaikutuksesta saalistettavan lajin yksilö- ja populaatio- ominaisuuksiin. Saalistuksen ja kalastuksen yhdysvaikutus johtaa myös ennalta-arvaamattomiin muutoksiin kummeliturskayksilöissä ja populaatiossa. Saalistuksen keston, saalistustyypin ja sukupuolien kokoerojen yhdysvaikutus määrittää mahdollisuudet kalastuksen aiheuttamiin pysyviin yksilö- ja populaatiotason muutoksiin. Yksilöominaisuuksia alttius kalastuksen aiheuttamiin evolutiivisiin muutoksiin myös vaihtelee saalistuksesta ja sukupuolten kokoeroista riippuen. Vaikka kalastus onkin suurin kokovalikoiva kuolleisuuden lähde, kalastuksen epäsuorat vaikutukset heikentävät myös saalistajan selviytymistä ja siten vaikuttavat sen suojeluun. Johtopäätöksenä voidaankin sanoa, että biologisesti realistinen, saalistuksen ja evoluution huomioiva näkökulma tarjoaa kokonaisvaltaisemman kuvan kalastuksen sääntelyyn: näiden tekijöiden huomioiminen auttaa välttämään kalastuksen aiheuttamaa evoluutiota, estää yliarvioimasta kalojen riittävyyttä kalastukseen sekä auttaa välttämään saalistajapopulaation romahtamisen. Näin kalastusta voidaan paremmin säädellä ekosysteemi ja kestävän kalastuksen periaatteet huomioiden sekä optimoida kalastuspaine ja pienentää kokovalikoivan kalastuksen vaikutukset ravintoketjussa

Diet composition of the spiny dogfish (Squalus acanthias L.) in the northern Adriatic Sea

Analizirana je ishrana 157 jedinki kostelja prikupljenih iz ulova pridnenih koća u periodu od 2005. do 2007. godine, u vanjskim vodama Kvarnera i Zadarskog akvatorija između 44°91’ i 43°59’ sjeverne zemljopisne širine, te 14°06’ i 14°49’ istočne zemljopisne dužine. Ukupna dužina tijela istraživanih životinja bila je između 26,1 i 112,0 cm (prosječna dužina = 57,51; SD = 20,3). Ulovljeni morski psi su morfometrijski obrađeni, nakon čega je slijedila izolacija probavnog trakta te prikupljanje sadržaja probavila koji je konzerviran u 75%-tnom etanolu. Iz sadržaja probavila izolirane su i determinirane skupine plijena, a potom im je određena mokra masa, maseni udio i učestalost. Analizom sadržaja probavila utvrđeno je prisustvo 6 skupina plijena: puževi (Gastropoda), školjkaši (Bivalvia), glavonošci (Cephalopoda), mnogočetinaši (Polychaeta), rakovi (Malacostraca) i ribe zrakoperke (Actinopterygii). Dobiveni rezultati ukazuju na dominantnu zastupljenost dviju skupina plijena, riba zrakoperki zastupljenih s masenim udjelom od 68,3% i učestalošću od 78,3%, te glavonožaca s masenim udjelom od 23,0% i učestalošću od 14,6%. Ostale skupine su zastupljene s ukupnim udjelom od 8,7%. Utvrđena je pozitivna korelacija ukupne dužine tijela (TBL) i mokre mase probavila (rs = 0,247; p = 0,002) te smanjenje udjela zrakoperki (rs = - 0,278; p = 0,002) s porastom TBL. Nisu utvrđene statistički značajne razlike u ishrani kostelja između sezona. Analize razlika u ishrani između spolova ukazali su na postojanje statistički značajnih razlika u masama i masenom udjelu zrakoperki (p < 0,01), te razlika u ishrani juvenilnih i adultnih jedinki pri čemu je udio zrakoperki kod spolno nezrelih životinja bio značajno veći (p = 0,001). Vremenska i veličinska raspodjela životinja ukazuju na sezonsko korištenje istraživanog područja te na njegov značaj kao kritičnog staništa morskog psa kostelja u sjevernom Jadranu.We analyzed diet composition of 157 spiny dogfish sampled by a bottom trawl, during a period from 2005 to 2007, in an area which occupies the aquatic ecosystem of Kvarner and Zadar, between 44°91’ i 43°59’ N and 14°06’ i 14°49’ E. Total body length of the samples ranged from 26.1 to 112.0 cm (mean = 57.51, SD = 20.3). Caught sharks underwent morphometric analysis including digestive tract content sampling which was afterwards preserved in 75% ethanol. Prey items were isolated from every stomach content and identified, also grouped in systematic categories and weighed. We recorded 6 prey groups in food remains: sea snails (Gastropoda), bivalves (Bivalvia), cephalopods (Cephalopoda), polychaetes (Polychaeta), crustaceans (Malacostraca) and ray-finned fish (Actinopterygii). The highest ranked prey groups included ray-finned fish with 68.3% of total wet weigth and 78.3% of occurence, and squid with 23.0% w.w. and 14.6% of occurence. Other prey groups represent altogether 8.7% of total wet weight. We noted a positive correlation between total body length (TBL) and total wet weight (rs = 0.247; p = 0.002), and also, a decrease in weight percentage of ray-finned fish (rs = - 0.278; p = 0.002) with an increase of TBL. There were no statistical significant differences in the diet between seasons. Analysis of the diet between sexes showed statistical significant differences in weigth and weigth percentage of ray-finned fish (p < 0.01) and in the diet between juvenile and adult dogfish, where the percentage of ray-finned fish was considerably larger in the diet of juvenile sharks in comparison to the diet of adult sharks. Temporal and TBL distribution of samples showed seasonal use of the researched area and also, its role as a critical habitat of the spiny dogfish in the northern Adriatic Sea

Factors driving patterns and trends in strandings of small cetaceans

The incidence of cetacean strandings is expected to depend on a combination of factors, including the dis- tribution and abundance of the cetaceans, their prey, and causes of mortality (e.g. natural, fishery bycatch), as well as currents and winds which affect whether carcasses reach the shore. We investigated spatiotemporal patterns and trends in the numbers of strandings of three species of small cetacean in Galicia (NW Spain) and their relationships with meteoro- logical, oceanographic, prey abundance and fishing-related variables, aiming to disentangle the relationship that may exist between these factors, cetacean abundance and mor- tality off the coast. Strandings of 1166 common dolphins (Delphinus delphis), 118 bottlenose dolphins (Tursiops truncatus) and 90 harbour porpoises (Phocoena phocoena) during 2000–2013 were analysed. Generalised additive and generalised additive-mixed model results showed that the variables which best explained the pattern of strandings of the three cetacean species were those related with local ocean meteorology (strength and direction of the North– South component of the winds and the number of days with South-West winds) and the winter North Atlantic Oscil- lation Index. There were no significant relationships with indices of fishing effort or landings. Only bottlenose dolphin showed possible fluctuations in local abundance over the study period. There was no evidence of long-term trends in number of strandings in any of the species and their abun- dances were, therefore, considered to have been relatively stable during the study period.Versión del editor2,01

Exploring individual and population eco-evolutionary feedbacks under the coupled effects of fishing and predation

Intensive fishing that selects for large and old individuals can have pervasive effects on traits directly associated with the fecundity and survival of the target species. The observed reduction in fish body size can result in earlier sexual maturity at a smaller body size, leading to a lower individual reproductive output and population productivity in the long term. In addition, increased predation can induce similar responses in age and size at maturity due to the release of intraspecific competition and the lower population density. Thus, the combined impact of fisheries and predation is more difficult to predict due to their competition for fish, ultimately limiting and directing the prey size selectivity of each competitor. This can lead to various responses of fish on individual and population levels, depending on how the impacts of fisheries and predation affect each other. To explore this topic, we used an individual-based model to investigate the responses of the European hake (Merluccius merluccius) to the coupled pressures of the prevailing fishing regime and predation by bottlenose dolphins (Tursiops truncatus) as a study system. By simulating the growth trajectories of hake with consideration to its pronounced sexual size dimorphism (SSD), we applied Holling’s Type II and III functional responses (FRII and FRIII, respectively) and the Rosenzweig-MacArthur predator model to describe prey–predator dynamics. Our results demonstrate how the intensity, duration, and prey sizes selected by predation and fisheries determine the potential for evolutionary changes in hake life-history traits. Additionally, pronounced SSD in hake as well as the predation type play a pivotal role in hake resilience and recovery following a period of overexploitation. Different types of predation reveal the discrepancy in the intensity of competition between predators and fisheries for fish, as well as predator survival, which is relevant for sustainable fisheries management and effective predator conservation strategies.Peer reviewe

Diet composition of the spiny dogfish (Squalus acanthias L.) in the northern Adriatic Sea

Analizirana je ishrana 157 jedinki kostelja prikupljenih iz ulova pridnenih koća u periodu od 2005. do 2007. godine, u vanjskim vodama Kvarnera i Zadarskog akvatorija između 44°91’ i 43°59’ sjeverne zemljopisne širine, te 14°06’ i 14°49’ istočne zemljopisne dužine. Ukupna dužina tijela istraživanih životinja bila je između 26,1 i 112,0 cm (prosječna dužina = 57,51; SD = 20,3). Ulovljeni morski psi su morfometrijski obrađeni, nakon čega je slijedila izolacija probavnog trakta te prikupljanje sadržaja probavila koji je konzerviran u 75%-tnom etanolu. Iz sadržaja probavila izolirane su i determinirane skupine plijena, a potom im je određena mokra masa, maseni udio i učestalost. Analizom sadržaja probavila utvrđeno je prisustvo 6 skupina plijena: puževi (Gastropoda), školjkaši (Bivalvia), glavonošci (Cephalopoda), mnogočetinaši (Polychaeta), rakovi (Malacostraca) i ribe zrakoperke (Actinopterygii). Dobiveni rezultati ukazuju na dominantnu zastupljenost dviju skupina plijena, riba zrakoperki zastupljenih s masenim udjelom od 68,3% i učestalošću od 78,3%, te glavonožaca s masenim udjelom od 23,0% i učestalošću od 14,6%. Ostale skupine su zastupljene s ukupnim udjelom od 8,7%. Utvrđena je pozitivna korelacija ukupne dužine tijela (TBL) i mokre mase probavila (rs = 0,247; p = 0,002) te smanjenje udjela zrakoperki (rs = - 0,278; p = 0,002) s porastom TBL. Nisu utvrđene statistički značajne razlike u ishrani kostelja između sezona. Analize razlika u ishrani između spolova ukazali su na postojanje statistički značajnih razlika u masama i masenom udjelu zrakoperki (p < 0,01), te razlika u ishrani juvenilnih i adultnih jedinki pri čemu je udio zrakoperki kod spolno nezrelih životinja bio značajno veći (p = 0,001). Vremenska i veličinska raspodjela životinja ukazuju na sezonsko korištenje istraživanog područja te na njegov značaj kao kritičnog staništa morskog psa kostelja u sjevernom Jadranu.We analyzed diet composition of 157 spiny dogfish sampled by a bottom trawl, during a period from 2005 to 2007, in an area which occupies the aquatic ecosystem of Kvarner and Zadar, between 44°91’ i 43°59’ N and 14°06’ i 14°49’ E. Total body length of the samples ranged from 26.1 to 112.0 cm (mean = 57.51, SD = 20.3). Caught sharks underwent morphometric analysis including digestive tract content sampling which was afterwards preserved in 75% ethanol. Prey items were isolated from every stomach content and identified, also grouped in systematic categories and weighed. We recorded 6 prey groups in food remains: sea snails (Gastropoda), bivalves (Bivalvia), cephalopods (Cephalopoda), polychaetes (Polychaeta), crustaceans (Malacostraca) and ray-finned fish (Actinopterygii). The highest ranked prey groups included ray-finned fish with 68.3% of total wet weigth and 78.3% of occurence, and squid with 23.0% w.w. and 14.6% of occurence. Other prey groups represent altogether 8.7% of total wet weight. We noted a positive correlation between total body length (TBL) and total wet weight (rs = 0.247; p = 0.002), and also, a decrease in weight percentage of ray-finned fish (rs = - 0.278; p = 0.002) with an increase of TBL. There were no statistical significant differences in the diet between seasons. Analysis of the diet between sexes showed statistical significant differences in weigth and weigth percentage of ray-finned fish (p < 0.01) and in the diet between juvenile and adult dogfish, where the percentage of ray-finned fish was considerably larger in the diet of juvenile sharks in comparison to the diet of adult sharks. Temporal and TBL distribution of samples showed seasonal use of the researched area and also, its role as a critical habitat of the spiny dogfish in the northern Adriatic Sea