Feeding Ecology Of Decapterus Maruadsi And Trachurus Japonicus In The Southern Coastal Area Of Zhejiang

With the influence of multiple factors such as climate change,environmental pollution and overfishing,the fishery resources are declining and the ecological environment is deteriorating.The marine ecosystem is facing great pressure,so its protection is becoming more and more urgent.Fish feeding provides necessary nutrition and energy for its own growth and reproduction.Feeding habit is an important part of feeding ecology research.It can understand the interaction relationship of fish community structure and the change of resources in the waters.The ultimate goal of marine fish feeding ecology research is to provide scientific basis for the sustainable development of marine fishery resources and realize ecosystem-based fishery management.Therefore,the study of fish feeding ecology is very important.Decapterus maruadsi and Trachurus japonicus are important pelagic economic fish in the southern coastal area of Zhejiang.As prey for high-trophic species and as predators for low-trophic species,they play a key role in the food web through nutritional interactions.Understanding their feeding ecology can be combined with stomach content analysis and stable isotope analysis to more comprehensively analyze the feeding changes of D.maruadsi and T.japonicus.Stable isotope analysis was used to analyze the trophic niche of D.maruadsi and T.japonicus to clarify the nutritional relationship between them in order to grasp the changes of ecosystem structure and function.It is helpful to further explore the nutritional dynamics characteristics of the southern coastal area of Zhejiang,so as to provide basic data for the research and scientific management of food web in this sea area.The specific research contents and results are as follows:（1）In order to deeply analyze the dynamic changes of feeding habits of D.maruadsi in the southern coastal area of Zhejiang.According to the bottom trawl surveys conducted in May,August,November 2020 and January 2021,the feeding habits of D.maruadsi in the southern coastal area of Zhejiang were analyzed by both stable isotope(carbon,δ¹³C,and nitrogen,δ¹⁵N)and stomach content analyses.Results showed that theδ¹³C value of D.maruadsi ranged from-17.76‰to-15.25‰,with the mean of（-16.55±0.60）‰,while theδ¹⁵N value ranged from 9.06‰and 13.03‰,with the mean of（11.76±0.88）‰.There was a significant negative correlation between theδ¹³C values and fork length,while a positive relationship was found between theδ¹⁵N with fork length.Results from stomach content analysis indicated that the main prey groups of D.maruadsi in the southern coastal area of Zhejiang were fish,shrimp,crabs,Cephalopod,Polychaete,and small crustaceans.As for the stable isotope analysis,the nutritional contribution rate of shrimp was the highest（40%-84%）among all prey groups,followed by Polychaete,small crustaceans,crabs,Cephalopods and fish.Significant ontogenetic dietary changes were found for D.maruadsi in this study.As the fork length of D.maruadsi increased,it tended to eat higher trophic level prey items.（2）Based on the bottom trawl surveys carried out in the southern coastal area of Zhejiang in August 2019 and August 2020,this paper studies the feeding ecology of T.japonicus in the southern coastal area of Zhejiang by using stomach content analysis and carbon and nitrogen stable isotope analysis.The SIMMR model was used to analyze the stomach content analysis results as a priori information,combined with stable isotope values to better analyze the feeding situation of T.japonicus.The results show that:in 2019,theδ¹³C value of T.japonicus is-（15.88±0.67）‰,ranging from-17.72‰to-14.30‰.The average value ofδ¹⁵N was（12.09±0.40）‰,ranging from10.89‰to 13.03‰.The results of one-way ANOVA showed that there was no significant difference inδ¹³C values（P>0.05）,δ¹⁵N value was significantly different（P<0.05）;In 2020,theδ¹³C value of T.japonicus is-（16.50±0.73）‰,ranging from-17.77‰to-15.22‰.The average value ofδ¹⁵N was（12.18±0.72）‰,ranging from9.93‰to 13.72‰.The results of one-way ANOVA showed thatδ¹³C value andδ¹⁵N were no significant difference in values（P>0.05）.The results of two-way ANOVA showed that theδ¹³C value was significantly different（P<0.05）,whileδ¹⁵N value had no significant difference（P>0.05）.according toδ¹⁵N value calculated that the average trophic level of the T.japonicus in the southern coastal area of Zhejiang in 2019 was（3.98±0.12）,and in 2020 was（4.01±0.21）.The results of stomach content analysis showed that the main food groups of the T.japonicus in the southern coastal area of Zhejiang were fish,shrimp,crabs,Cephalopods and small crustaceans.The results of stable isotope analysis showed that the nutrient contribution rate of small crustaceans in both years to the T.japonicus in the southern coastal area of Zhejiang was the highest,followed by shrimp.The feeding habits of T.japonicus have obvious age changes,and the average contribution rate of its feeding fish has increased.（3）Based on 79 samples of D.maruadsi and 33 samples of T.japonicus collected in the southern coastal area of Zhejiang in August 2020,the trophic niche indexes were calculated by carbon and nitrogen stable isotope values,and the interspecific niche differences and trophic niche changes at different developmental stages were analyzed to analyze the interspecific competition and resource utilization.The results showed that the feeding habits of D.maruadsi were similar to those of T.japonicus,and the isotope niche overlap between them was high,with an overlap area of 0.08,accounting for 82.5%and 74.6%of the ecotopic areas of D.maruadsi and T.japonicus,respectively,showing a certain possibility of interspecific competition.The food source diversity of T.japonicus was slightly higher than that of D.maruadsi.From the perspective of individual growth and development,the niche overlap of different developmental stages of D.maruadsi is higher,and they have similar feeding prey.Both of them show that the trophic niche width decreases with individual development,and they are more inclined to eat higher trophic levels of food organisms,from small crustaceans to fish or shrimp.