Trophic Niche Of Fishes In Wanshan Islands In Autumn

Wanshan Islands is an area with a unique ecosystem in the Pearl River Estuary.Under the joint action of the dilute water of the Pearl River and offshore tidal currents,it has complex hydrodynamic conditions,high primary productivity,rich nutrients,and diverse fishery resources,making it an important place for Marine organisms to forage and inhabit.However,in recent years,intensified human activities have led to the decline of fishery resources in this area,such as over-exploitation,miniaturization and simplification of fish,and serious eutrophication in the waters near some islands and reefs.At present,the ecological status of this area is worrying.Understanding the current situation of fish food web structure in the waters of Wanshan Islands,and grasping the feeding ecology and nutritional structure characteristics of main fish are helpful to provide basic data support for the protection and management of ecosystem in the waters of Wanshan Islands in the future.In this study,the stable isotope technique was used to analyze the feeding habits and interactions of main fishes in the Wanshan Islands from the perspective of trophic niche.The results are as follows:1.Based on the stable carbon and nitrogen isotopes of fish in Wanshan Islands in autumn 2021,we studied and analyzed the trophic niche of fish in Wanshan Islands.Theδ¹³C values of fish range between-21.23‰（Mugil cephalus）～14.00‰（Mugil cephalus）,average was 16.96±0.41‰,δ¹⁵N value range between 9.93‰（Trachinotus ovatus）～14.83‰（Pennahia anea）,average was 12.52±0.34‰.The mean value of trophic level of fish was 3.30,the CR was 7.23,the NR was 4.90,the TA was 23.64,the SEAc was 3.73,the CD was 1.30,the MNND was 0.24,and the SDNND was 0.24.Based on the cluster analysis of Carbon and Nitrogen stable isotopes,the main fish species could be divided into three groups,including 4 species（3.57%）of planktonic feeding fishes,7species（6.25%）of benthic feeding fishes and 101 species（90.18%）of mixed feeding fishes.According to the trophic level,the main fish species in Wanshan Islands were divided into three groups,11 species（9.82%）of omnivorous fish,97 species（86.61%）of low carnivorous fish,and 4 species（3.57%）of intermediate carnivorous fish.Using the Bayesian mixed model to calculate the contribution percentage of food sources in Wanshan Islands,the results show that the overall carbon source in Wanshan Islands is mainly from zooplankton,with a contribution percentage of 62.54%.This was followed by Algae（25.28%）,Mollusc（10.34%）and Crustacean（1.83%）.2.In order to better explore the differences between islands,the study selected Guishan Island,Wailingding Island,Dongao Island and Wanshan Island for analysis and comparison.Theδ¹³C value of fish in Guishan Island was-17.01±1.21‰,δ¹⁵N value was12.78±1.09‰.Theδ¹³C value of fish in Wailingding Island was-16.61±0.72‰,δ¹⁵N value was 12.49±0.81‰.Theδ¹³C value andδ¹⁵N value of fish in Dongao Island were-17.21±1.02‰and 12.23±0.92‰,respectively.Theδ¹³C value andδ¹⁵N value of fish in Wanshan Island was-16.62±0.84‰and 12.85±0.92‰.Based onδ¹³C andδ¹⁵N value comparison and analysis of the nutritional niche indicators of 4 islands,Dongao Island occupied the widest SAc,and the highest CR,NR and CD.It indicated that the fish in the Guishan ecosystem occupied a longer trophic level,a wider range of food sources,and a higher diversity of nutrient structure.The nutrient index,food source diversity and species richness of Wailingding Island were all at the lowest level,indicating that the trophic niche occupied by Wailingding Island was the narrowestern in the Wanshan Islands,and the trophic level of fish was low,which might be related to the seasonal migration of fish.The MNND and SDNND of Wanshan Island were the lowest,indicating that the nutrient redundancy of Wanshan Island was high,the trophic niche distribution was more uniform,and the ability to resist external disturbance was strong.3.The Bayesian mixed model was used to calculate the nutrient contribution ratio of algae,zooplankton,mollusks and crustaceans to the four islands.The results showed that zooplankton was the most important food for the four islands,and the proportion of zooplankton decreased from 62%on Dongao Island,51%on Gisha Island,43%on Mansan Island,and 32%on Wailingding Island.However,the proportion of mollusks in these islands showed a trend of increasing successively,reaching 45%in Wailingding Island,indicating that the food web structure had changed significantly from zooplankt-dominated food web to mollusk-dominated food web.This is related to the water quality of different sea areas,human disturbance,the amount of bait biological resources and other factors.At the same time,it may also be directly related to the migration of fish at this time.4.In order to further elucidate the interactions of fish in this area,the dominant species Siganus canaliculatus and Halichoeres nigrescens were selected to analyze their nutritional niche characteristics.The result shows theδ¹⁵N values ranged from 9.15‰to14.09‰of S.canaliculatus and 10.34‰to 14.60‰of H.nigrescens;Theδ¹³C values ranged from-19.87‰to-14.54‰of S.canaliculatus and-18.16‰to-15.60‰of H.nigrescens;and the trophic level of S.canaliculatus and H.nigrescens was 2.97 and 3.23,respectively.The analysis ofδ¹³C-δ¹⁵N nutrient niche structure showed that there was a resource competition between the two populations.The trophic niche breadth and trophic diversity of S.canaliculatus were higher than those of H.nigrescens,indicating that S.canaliculatus had more food sources and stronger ability to compete and use resources.There was no significant correlation between body length andδ¹³C of the two populations（p>0.05）,but there was a significant correlation between body length andδ¹⁵N.There was a significant positive correlation of S.canaliculatus between body length andδ¹⁵N（p<0.05）,while there was a very significant negative correlation of H.nigrescens（p<0.01）.The results indicated that the feeding patterns of the two fish changed with ontogeny,and the S.canaliculatus tended to be high-nutrient diet organisms,while the H.nigrescens tended to be low-nutrient diet organisms.There was a sharp increase in the trophic niche width of S.canaliculatus and H.nigrescens in the range of 90～100 mm and100～110 mm,respectively,which was just near the length of their first sexual maturity or sexual reversal,suggesting that the feeding habits of the two species changed during the period of gonadal development and maturity.