Study On Feeding Ecology Of Three Migratory Fishes Species In Zhejiang Coastal Waters

Fish feeding ecology occupies an important place in the study of marine ecosystems.Muraenesox cinereus is the main bycatch species in the Zhejiang coastal waters,and has relatively stable utilization potential.Trichiurus japonicus is the species with the highest catch of single fish species in the Zhejiang coastal waters.As one of the fastest growing fish species in the East China Sea,Scomber japonicus is one of the most important pelagic fish resources.Therefore,the study of the feeding ecology of these three species of fishes in the Zhejiang coastal waters is of profound academic and practical importance.This study sampled fishery resources based on various sub-seasonal fishery surveys conducted in Zhejiang coastal waters from July 2019 to April 2021.The feeding ecology of typical migratory fish such as M.cinereus,T.japonicus and S.japonicus in the Zhejiang coastal waters was preliminarily studied by two methods of stomach contents analysis and carbon and nitrogen stable isotope techniques.The research included food composition and diet type,the variation of feeding intensity with seasons and ontogeny,characteristics of carbon and nitrogen stable isotope ratios and their changes with ontogeny,nutritional structure,and calculation of trophic level with stomach contents and nitrogen stable isotopes,etc.The main results were as follows:1.Food composition and feeding type:At least 35 species of food organisms,including stomatopoda,pisces and shrimps,were consumed by M.cinereus in Zhejiang coastal waters throughout the year,with the dominant species including Oratosquilla oratoria and Larimichthys polyactis.T.japonicus fed on at least 35 species of bait organisms including Pisces,euphausiacea,stomatopoda and cephalopods,with T.japonicus being the most dominant bait species（IRI%=28.42%）,followed by O.oratoria and Euphausia pacifica.S.japonicus fed on a total of 36 species of bait organisms throughout the year,and the dominant baits were the Megalopa of crabs,Oxycephalus clausi,Mysidacea,Lestrigonus schizogeneios and E.pacifica.Compared with historical data,the feeding habits of T.japonicus in the Zhejiang coastal waters has undergone a shift,with the absolute dominant bait changing from the euphausiacea to pisces,while the phenomenon of self-mutilation within the T.japonicus remains very obvious.Compared with historical studies,all of the species of the three fish identified in this study had declined,and there were differences in food groups.From the analysis of dietary types,M.cinereus was the diet of benthonic animals and nekton organisms,T.japonicus mainly feeded nekton organisms,and feeded both benthonic animals and zooplankton,and S.japonicus was the diet of zooplankton.2.Changes in feeding intensity:There were significant or extremely significant differences in the feeding intensity of three fish species with seasonal changes and ontogeny.The feeding intensity of M.cinereus showed a trend of first decreasing and then increasing with ontogeny,and the feeding intensity was lower when the anal length was close to the length of primary sexual maturity.The feeding intensity of T.japonicus was lower in summer and autumn,higher in spring and winter,and fluctuated greatly with the length of the anal length,with lower values in the large,medium and small individual groups.The feeding intensity of S.japonicus was the highest in spring,and the lowest in winter.It first decreased,then increased and then continued to decrease with ontogeny.The feeding intensity of fish before and after the first sexual maturity was the highest.3.Trophic level:The trophic level of the three fish were calculated based on the analysis of stomach contents and nitrogen stable isotope ratios.The results showed that the trophic levels of M.cinereus were 4.13 and 3.78,T.japonicus were 3.92 and 3.65,and S.japonicus were 3.28 and 3.48.The trophic level calculated by nitrogen value gradually increased with the individual development of fish,and there were extremely significant differences in the trophic level between groups with different body lengths（P<0.01）.Comparing the two methods,the trophic levels of M.cinereus and T.japonicus estimated by nitrogen values were slightly lower,and the trophic levels of S.japonicus calculated according to the composition of feed were lower.4.Carbon and nitrogen stable isotope ratio characteristics:The range ofδ¹³C of M.cinereus in the Zhejiang coastal waters was-24.79‰～-15.58‰,and the average was（-17.11±1.54）‰.The range ofδ¹⁵N was 9.56‰～13.38‰,and the average was（11.71±1.13）‰.Theδ¹³C of M.cinereus in different seasons showed extremely significant difference（P<0.01）,but there was no significant difference betweenδ¹⁵N（P>0.05）.Anal length of M.cinereus had no significant correlation withδ¹³C（P>0.05）,but had a very significant positive correlation withδ¹⁵N（P<0.01,r=0.594）.The range ofδ¹³C of T.japonicus was-21.89‰～-16.07‰,and the average value was（-18.42±1.38）‰.The range ofδ¹⁵N was 8.60‰～14.02‰,and the average value was（11.22±1.65）‰.There were extremely significant differences between theδ¹³C andδ¹⁵N of T.japonicus in different seasons and different anal length groups（P<0.01）.The anal length of T.japonicus had a very significant negative correlation withδ¹³C（P<0.01,r=-0.485）,and a very significant positive correlation withδ¹⁵N（P<0.01,r=0.652）.The range ofδ¹³C of S.japonicus was-22.95‰～-15.55‰,and the average was（-19.01±1.98）‰.The range ofδ¹⁵N was8.93‰～11.63‰,and the average was（10.65±0.61）‰.There were extremely significant differences betweenδ¹³C andδ¹⁵N of S.japonicus in different seasons（P<0.01）.The fork length of S.japonicus had a very significant negative correlation withδ¹³C（P<0.01,r=-0.674）,and a significant positive correlation withδ¹⁵N（P<0.05,r=0.512）.5.Nutritional structure:The food source diversity（CR）of the M.cinereus was 9.21,the trophic length（NR）was 3.82,the total ecological trophic niche area（TA）was 21.05,and the corrected ellipse ecological niche area（SEAc）was 5.56.The trophic ecological niche indexs and the breadth of trophic ecological niche were the smallest in the M.cinereus in the large group.The overlap of trophic ecological niche showed that there was a clear overlap between the small and medium anal-length groups,while the separation with the large group was more pronounced.The CR of T.japonicus was 5.82,the NR was3.18,the TA was 21.97,and the SEAc was 7.07.The niches of T.japonicus in the small and medium anal-length groups were completely overlapped,and the large individuals were completely separated from the other two groups.This was because the T.japonicus of the large individual group had a higher nutritional level and a stronger ability to utilize resources and adapt to the environment.The CR of S.japonicus was 7.40,the NR was 2.70,the TA was 13.94,and the SEAc was 3.63.The trophic niches breadth of S.japonicus in the small and large fork-length groups were more significantly expanded in theδ¹³C dimension than that in the middle individual group,and the trophic niche breadth of the medium individual group was smaller.All three fork length groups overlapped,but the large individual group overlapped to a smaller extent with the other two groups.6.Interspecies trophic relationship:The dietary overlap between M.cinereus and T.japonicus was significant（0.63）,while the dietary overlap coefficient between M.cinereus and S.japonicus,and T.japonicus and S.japonicus was less than 0.3 and the overlap was not significant.The overlap of T.japonicus with M.cinereus and S.japonicus was obvious,but the overlap between M.cinereus and T.japonicus was very small.The breadth of trophic ecological niche of M.cinereus and T.japonicus was larger,indicated that they occupied more ecological resources in the Zhejiang coastal waters,and their resource utilization and environmental adaptability were stronger than S.japonicus.In summary,combining stomach contents and stable isotope analysis methods,there was food competition between M.cinereus and T.japonicus in Zhejiang coastal waters,mainly in the common dominant bait O.oratoria.Unlike S.japonicus,as bottom or lower middle pelagic aggressive fish,the trophic levels of M.cinereus and T.japonicus were high and spanned a wide range,and the overlapping characteristics of trophic ecological niches embodied in different body length ranges are similar,and the large individual groups had obvious separation than other individuals,while the trophic levels and adaptability to resource utilization and environment were significantly higher than other individuals.There was a certain overlap in the trophic ecological niches of T.japonicus and S.japonicus,which was mainly reflected in the feeding competition between them on Euphausia pacifica.Meanwhile,the correlation between theirδ¹³C and their respective anal lengths or fork lengths were similar,both of them have highly significant negative correlation,which was related to the existence of feeding transformation of the organism during the growth process.Due to the large differences in distribution stratum and food sources,M.cinereus and S.japonicus in Zhejiang coastal waters were not have obvious competitive relationships,but both assumed important roles in the food web of the ecosystem in the sea.The competitive relationship between the species was weakened by increasing the vertical range of the habitat layer and the diversity of food sources.The differences in trophic level results calculated by stomach contents and stable isotope analysis methods were due to the differences and characteristics of the two methods,and in future trophic level calculations of fish,both methods would be combined to make more comprehensive and accurate estimates of fish trophic levels.Compared with historical studies,the bait organism species of the three fish species identified in this paper have decreased,and there are differences in the bait species.It was suggested that in future studies of fish feeding ecology,the distribution and changes of their bait population resources should be increased,and the two complement each other to provide a scientific basis for the sustainable development of important economic species in Zhejiang coastal waters,and to provide basic data reference for the study of the food web in this sea area.