| الوصف: |
Promjene u ihtiofauni okarakterizirane su kao najznačajniji ekološki utjecaj globalnog zagrijavanja te dugoročno mogu uzrokovati kaskadne učinke u hranidbenim mrežama i posljedično dovesti do promjena u opskrbi morskim proizvodima što može negativno utjecati na gospodarski ribolov i tržište. Prodorom predatorskih termofilnih vrsta riba na sjeverni Jadran, odnosno u staništa gdje donedavno nisu bile prisutne, proširuje se spektar trofičke eksploatacije plijena. Iako je Medulinski zaljev, zbog raznolikosti staništa i produktivnosti, važno ribolovno područje na sjevernom Jadranu, oskudni literaturni podaciju ukazuju da je ribolov na tom području slabo istražen, naročito termofilne vrste koje se sve češće ondje izlovljavaju. Cilj ovog rada je ispitati sastav ulova termofilnih vrsta riba kao i ukupnog ulova ribe, utvrditi prostorno-vremensku distribuciju termofilnih vrsta, njihovu brojčanu i masenu zastupljenost u ulovu, istražiti njihove hranidbene navike te morfometrijske, merističke i biološko-ekološke značajke. Za potrebe prikupljanja uzoraka korišteni su uobičajeni ribolovni alati u gospodarskom ribolovu na moru (jednostruke mreže stajaćice i obalna potegača) tijekom trogodišnjeg perioda (od 2017. do 2019. godine). Istraživanjem je na tri postaje sakupljeno sedam termofilnih vrsta riba; gof (Seriola dumerili), strijelka skakuša (Pomatomus saltatrix), škaram (Sphyraena sphyraena), strijela bijelica (Lichia amia), strijela modrulja (Trachinotus ovatus), lampuga (Coryphaena hippurus) i plavi trkač (Caranx crysos) te 45 ostalih vrsta riba. Nakon brojanja i vaganja svake termofilne jedinke, sakupljeni su morfometrički i metrički podaci, izdvojeni su i izvagani jetra i gonade te je analiziran sastav probavila kako bi se odredio intenzitet prehrane, plijen i preklapanje u ishrani termofilnih vrsta. Za statističku obradu podataka korišteni su paketi PAST i CANOCO. Istraživanjem je utvrđeno da se termofilne vrste riba u Medulinskom zaljevu pojavljuju sezonski gdje se zadržavanju nekoliko mjeseci, najčešće od rujna do studenog iskorištavajući raspoložive energetske resurse na tom području. To je vidljivo po povećanju srednjih mjesečnih vrijednosti Fultonova indeksa kondicije (kod vrsta S. dumerili i S. sphyraena) i hepatosomatskog indeksa kod vrsta S. dumerili i L. amia. Hranidbeni indeksi ukazuju na preferenciju ishrane ribljim plijenom, a za razliku od ostalih termofilnih vrsta, u prehrani S. dumerili utvrđen je, uz pelagični i bentopelagični, još i demerzalni plijen. Analiza sastava prehrane termofilnih vrsta riba u Medulinskom zaljevu upućuje da među njima trenutno ne dolazi do preklapanja prehrambenih navika čime se dade zaključiti da je svaka termofilna vrsta zauzela svoju ekološku nišu. Rezultati ovog istraživanja doprinose boljem poznavanju promjena u ribljim morskim zajednicama i utjecaju klimatskih promjena na ribolov u Medulinskom zaljevu. The expansion of the range of thermophilic fish species towards the northern parts of the Mediterranean Sea and their impact on new habitats is a direct consequence of climate change. Changes in ichthyofauna are characterized as the most significant ecological impact of global warming. In the long run they may cause cascading effects in food webs, which can consequently lead to changes in the supply of seafood and negatively affect commercial fishing and the overall market. The ingression of predatory thermophilic fish species in habitats where they were not present until recently (such as in the northern Adriatic), expands the spectrum of their trophic exploitation. Although, due to its diversity of habitats and productivity, the Bay of Medulin is an important fishing area in the northern Adriatic, scarce literature data suggests that fishing in this area is poorly researched, especially thermophilic species that are more common in the last decades. The aim of this research is to examine the composition of catches of thermophilic fish species as well as the total fish catch, to determine the spatial and temporal distribution of thermophilic species, their numerical and mass representation in the catch, to investigate their feeding habits, and morphometric, meristic, and biological-ecological characteristics. Regular commercial fishing tools were used (gillnets and coastal purse seine) to collect thermophilic species during a three-year survey (from 2017 to 2019). In the research, seven thermophilic fish species at three stations have been collected: Greater Amberjack (Seriola dumerili), Bluefish (Pomatomus saltatrix), the Mediterranean Barracuda (Sphyraena sphyraena), the Leerfish (Lichia amia), the Pompano (Trachinotus ovatus), Dolphinfish (Coryphaena hippurus) and the Blue Runner (Caranx crysos). After counting and weighing each individual thermophilic species, morphometric and metric data was collected. Afterwards, liver and gonads were isolated and weighed. Gastrointestinal content was analyzed to determine feeding intensity, prey, and dietary overlap of thermophilic species. Assessment of the fish diet was based on the frequency of occurrence (F%), numerical frequency (N%), and mass frequency (W%) of the different diet components. The analysis of changes in feeding habits was performed using the fulness index (FI%) and vacuity coefficient (VI%). The index of absolute (IAIα) and relative importance IRI was calculated for each prey category to calculate the diet overlap. PAST and CANOCO packages were used for statistical data processing. The research has revealed that thermophilic fish species in the Bay of Medulin appear seasonally and remain in the area for several months, mostly from September to November, foraging on available energy resources. This is evident by the increase in mean monthly values of the Foulton condition index (for S. dumerili and S. sphyraena) and the hepatosomatic index for S. dumerili and L. amia. Feeding analysis suggests that important food items of L. amia were garfish, Belone belone (Linnaeus, 1760), and different species of Atherina, a pelagic prey consumed at the Kazela sampling station. At the Školjić and Dražice sampling stations, S. dumerili consumed benthopelagic organisms such as cephalopods, Mysidaceae crustaceans, common pandora, Pagellus erythrinus (Linnaeus, 1758), and demersal species represented by Mullus species. In the diet of S. sphyraena, the most important prey was represented by pelagic genus Trachurus from the Carangidae family. A number of gill rakers for S. dumerili, and mouth height and width for S. dumerili and L. amia, represent an important meristic trait in prey selectivity. Feeding indices indicate the preference of feeding on fish prey. In the diet of S. dumerili, in contrast to other thermophilic species, besides pelagic and bentopelagic pray, demersal species were also found. The analysis of the composition of the diet of thermophilic fish species indicates that there is currently no overlap of feeding habits between them, which leads to the conclusion that each species has found its ecological niche. The results of this research contribute to a better understanding of the changes in marine fish communities in the northern Adriatic and the impact of climate change on fishing. |
