Study On Feeding Ecology Of Migratory Coilia Nasus In The Lower Reaches Of The Yangtze River

As a key habitat for fish,the lower reaches of the Yangtze River are the hub for communication between rivers,lakes and tributaries and estuaries,and a necessary passage for migratory fish,with good ecological environment and rich fishery resources.In recent years,with the rapid development of regional economy,human wading activities such as water pollution,water-related engineering,channel improvement,bank slope hardening,and overfishing have become more frequent,resulting in obvious decline of fish resources and adaptive changes in community structure,feeding strategies and nutritional structure.As a typical small migratory fish in the Yangtze River Basin,sword fish enter the tributaries of the Yangtze River and Tongjiang lakes in early spring for reproductive migration.At present,domestic scholars have carried out relevant studies on the growth characteristics,intestinal microenvironment,gene sequence and otolith microchemistry of migratory Coilia nasus.Research on the feeding behavior of Coilia nasus is mainly in lake fish and Coilia nasus juvenile fish,and the feeding behavior of adult fish is not fully understood.In this study,Coilia nasus was the research area in the lower reaches of the Yangtze River,and samples were collected in four sampling areas of Chongming,Taizhou,Anqing and Poyang Lake in the lower reaches of the Yangtze River during the 2021 flood season(March to July).In this study,a total of 416 samples were collected for migration history confirmation,gastrointestinal(intestinal)content analysis and stable isotope analysis,and the food composition,food selectivity and feeding intensity of the Coilia nasus were studied,and their nutritional level and nutritional niche characteristics were determined,in order to provide scientific basis for the conservation of the Yangtze River Coilia nasus resources.The results of the study are as follows:1.Confirmation of the migration of Coilia nasus:a total of 416 samples were collected,with a body length range of 168.90～362.88 mm,and an average body length of290.66±37.21 mm;The weight range was 17.35～211.81 g,and the average weight was93.37±44.77 g.There are 262 females and 154 males,with a male-to-female ratio of1.70:1.Through the combination of three methods:jaw length and head length comparison,seawater parasite screening and otolith microchemical identification,it was determined that the samples of Coilia nasus used for the experiment were all migratory Coilia nasus.2.Food composition of Coilia nasus:A total of 6 major categories were detected in Coilia nasus,among which copepods and clades accounted for the absolute dominance,with frequency of 88.70%and 76.44%,and the number percentages were 58.26%and27.49%,respectively;The empty stomach rate of Coilia nasus collected in the Taizhou River section was the highest(89.76%),and the empty stomach rate in the Poyang Lake section was the lowest(3.53%).The Poyang Lake sample had the highest fullness index(71.00%)and the Taizhou River section sample had the lowest fullness index(20.67%).The main dominant species are the genus Elephant(17.44),the copepod larvae(5.89)and the genus Daphnia(3.97).With the increase of body length,the dominant bait changed from the original zooplankton to shrimp,and it was found that the intraspecific predation phenomenon of Coilia nasus was very obviously.There are great differences in the isotope values of Coilia nasus in different regions,and aquatic environmental factors may affect the reasons why the isotope values of aquatic organisms are different.In general,shrimp contributed the most to the food source of Coilia nasus,followed by zooplankton.The overall trophic level range of Coilia nasus was 1.88～3.91,and the total span was2.03.3.Feeding ecology:Coilia nasus have obvious selectivity for copepods and clades,and prefer to prey on relatively large individuals in the water body,and as the body length of the Coilia nasus increases.The I_i value of copepods and clades is between 0～1,and Coilia nasus prefer to feed on both types of food.When the I_i value of protozoa and rotifers is between-1～0,Coilia nasus tend to avoid these two feeding organisms.There were significant differences in feeding intensity with individual development and spatial changes.During ontogenetic development,the feeding intensity of Coilia nasus increases with the increase of gonadal development and body length.The probability of preying on juvenile fish and juvenile Shrimp will also increase.The overall trophic level range of Coilia nasus is 1.88～3.91,the total span is 2.03,the average value is 2.64±0.47,with the increase of body length,the nutrition level also increases,consumers of higher trophic levels in the food web are generally present.4.Simplifying the structure of the food web:fish in the waters of Poyang Lake occupy a high trophic level between 1.64～3.84,which belongs to three trophic levels;Shrimp contributed the highest proportion among the food sources of Coilia nasus;The highest niche width is the bream,which overlaps with the Baek niche.