Reproductive Biology And Feeding Ecology Of Saurida Tumbil In The Beibu Gulf

Greater lizardfish（Saurida tumbil）,an important economic species in the Beibu Gulf,has a very important fishery status.In the context of the decline of fishery resources in the Beibu Gulf,the resources of S.tumbil are also facing the dilemma of decline.The population structure,reproductive strategies and feeding habits of Saurida tumbil were changed under high fishing pressure to maintain the stability of the population.Therefore,understanding the current adaptive response to the population structure,reproductive biological characteristics,feeding habits and nutritional niche of S.tumbil is conducive to the development of S.tumbil research and the rational development and protection of the resource.In this study,a total of 1214 samples were collected from random sampling of bottom trawling landcatch in Jianghong Port,Zhanjiang City,Guangdong Province for one year in spring（March and April）,summer（June,July and August）,autumn（September,October and November）and winter（December and January）in 2020.Through conventional biological methods,the population structure and reproductive strategy of S.tumbil were analyzed,and the biological status of S.tumbil was studied.Then,the feeding habits of S.tumbil were studied by using traditional gastric content analysis and molecular biology techniques,and the nutritional ecology of S.tumbil was studied by using carbon and nitrogen stable isotope techniques.The main research results are as follows:1.Population structure analysis:In 2020,the standard length（SL）of S.tumbil in Beibu Gulf ranged from 11.4 cm to 26.9 cm,with an average of 18.7 cm,and the body weight ranged from 13.1-285.6 g,with an average of 81.6 g.The allometric parameter b was 3.31,indicating that S.tumbil had positive allometric growth.The average fatness was 1.14,and the fatness decreased with the seasonal variation and increased with the increase of SL.In recent years,the resources of S.tumbil in the Beibu Gulf are still declining,and the individual miniaturization still exists.2.Analysis of reproductive biology characteristics:S.tumbil in the Beibu Gulf can lay eggs throughout the year.The main spawning peak was in spring and summer.The oocyte development in S.tumbil was asynchronous.The absolute fecundity of S.tumbil was 2774-158986,with an average of 37084.The relative fecundity to the body weight was 36-774,with an average of 267.The individual fecundity was lower than the historical level.The size at first maturity(L₅₀)has been recovered in recent years,with19.1 cm for female and 19.0 cm for male.In terms of the number of males and females,the female population accounted for a higher proportion,and there were more females than males in each season.There were differences in the ratio of females to males in different growth stages,which the sex ratio of S.tumbil before sexual maturity was generally 1:1 and the proportion of females in the sexually mature population was relatively high.3.Analysis of feeding habits:S.tumbil was a kind of ubiquitous carnivorous fish,feeding mainly on fishes,including Stolephorus sp.,Sardinella aurita,Secutor ruconius and Thryssa dussumieri,and also feeding on cephalopods and crustaceans,including Loligo sp.,Oratosquilla sp.And Solenocera sp..There were seasonal differences in food composition,and the diversity of bait species was higher in summer and autumn.During the growth process,there were changes in feeding habit.When the SL was 14.0cm and 18.0 cm,the feeding habits changed significantly,and the individual diet specialization（IS）ranged from 0.05 to 0.63.From young stage to mature stage,with the increase of feeding species,the IS increased,which led to the decrease of the intensity of intraspecies competition.The feeding intensity was different in different seasons,and the feeding intensity was still high in the spring and summer breeding period.Further,the feeding intensity changed significantly with the individual growth,feeding intensity in immature population decreases with increasing SL,while feeding intensity in mature population increases with increasing SL.After spawning（Ⅵstage）the population’s feeding intensity was higher than that of the population at other developmental stages.In addition,the combination of morphological identification and DNA barcoding is superior to a single method in the identification of gastric species.4.Nutritional structure analysis based on stable carbon and nitrogen isotopes:theδ¹³C values of S.tumbil in the Beibu Gulf ranged from-19.54‰to-16.39‰,with an average of-17.77‰,δ¹⁵N values ranged from 10.99‰to 15.63‰,with an average of13.24‰,and the trophic levels ranged from 2.62 to 3.98,with an average of 3.28.In addition,the trophic level of different SL groups ranged from 3.05 to 3.36,and the trophic level increased with the increase of SL.There were differences in the nutrient niche width（TA）among different seasons,with the widest（5.67）in autumn and the narrowest（2.00）in spring and which increased with growth and development,ranging from 3.45 to 6.23.S.tumbil belongs to middle and high trophic fish,and there was partial overlap in the trophic niche of different developmental stages.The competition degree of S.tumbil was reduced by the difference of food composition and the diversity of trophic levels.The resources of S.tumbil in the Beibu Gulf are still in a declining trend,but the individual size and the L₅₀ have been restored to a certain extent.Continuous spawning in batches and high number of females are important strategies to maintain the stability of the population.S.tumbil has a very diverse diet,which means that the nutrition source is wide and the nutrition level is high.Intraspecies feeding competition existed in each growth stage.The food competition between different growth stages can be reduced by the difference of food composition and the diversity of nutrient levels.