Comparative Study In Feeding Ecology Of Large Head Hairtail(Trichiurus Japonicus)in The Beibu Gulf,Northern South China Sea

Beibu Gulf Fishing Ground is one of China’s excellent traditional fishing grounds.Due to the overexploitation of this ground,fishery resources have been damaged to varying degrees.Fish populations show signs of decline such as individual miniaturization,low age,and sexual maturity advance.Various adaptive changes in biological characteristics,feeding habits,and trophic structure.Restricted targeted protection of fishery resources due to the limited scientific research on adaptive changes.In this research,the traditional dominant species of T.japonicus in this sea area is selected as the research target.Comparative analysis of changes in biological characteristics,feeding habits and trophic structure of T.japonicus in 2008 and 2018 by the method of stomach content analysis and stable carbon and nitrogen isotopic analysis.Revealing T.japonicus trophic status and trophic ecological adaptability in the marine fish food web of Beibu Gulf.The results are as follows:1.From 2008 to 2018,the T.japonicus’s dominant length and dominant weight have decreased.Growth parameters value b from 2.62 to 3.05,the growth state are negative allometric growth and positive allometric growth,and the difference gender is not obvious.The average Fulton condition factor from 1.50 to 1.44.The 2008 gonadosomatic index is slightly higher than 2018,and the Length at 50%maturity from 267.47mm to 174.63mm.T.japonicus appear to be miniaturized and sexual maturity advances.2.From 2008 to 2018,the feeding item species of T.japonicus is reduced,and the richness of feeding item is reduced,and the dominant fish species in feeding items are from Decapterus maruadsi,Bregmaceros rarisquamosus,Bregmaceros nectabanus,Anchoviella heterolobam and Acetes chinensis to Bregmaceros rarisquamosus,Anchoviella commersoni,Greenband anchovy and Stolephorus chinensis.The proportion of fish in the item composition increased from 79.77%to 98.77%.According to the change of bait composition in different growth stages,T.japonicus may have two feeding habit changes,which the preanal length at second feeding habit change occur in anal length 259mm and 199mm.The trophic levels base on stomach content are 3.8 and 3.7.The niche breadths base on stomach content are 1.68 and 1.59.3.In 2008,the wide range forδ¹³C andδ¹⁵N of T.japonicus are-19.62～-13.60‰and10.19～15.54‰,in 2018 they are-20.25～-15.76‰and 11.02～16.69‰.The average isotopic trophic levels are 3.8 and 3.7 remains stable as 3.38 and 3.43.The range ofδ¹³C shift to nearshore item in trophic niche.The total trophic niche（TA）,core niche（SEA）and mean distance to centroid（CD）dropped from 20.20,4.68 and 1.48 to 18.40,2.39 and 1.10,trophic similarity（MNND）and trophic evenness（SDNND）in 2008 is twice that of 2018.The overall trophic niche shows a downward trend.In summary,from 2008 to 2018,T.japonicus has appeared to be miniaturized,sexual maturity advance,and the species of feeding item is reduced,the overall trophic niche shows a downward trend.But the trophic levels is still stable.Explain T.japonicus maintain a stable population structure through method of sexual maturity advance and feeding habit change advance.The trophic status of T.japonicus in the Beibu Gulf ecosystem has not changed.