Preliminary Study On The Feeding Habits Of Trichiurus Japonicus In The Wen-tai Fishing Ground And Its Physiological Changes During Gonad Developmental Period

Wen-tai Fishing Ground is one of the significant fishing areas in China,abundant in bait resources and is a breeding ground for many valuable marine species.However,due to extensive and persistent fishing activities,the fishery resources have decreased.Trichiurus japonicus,a crucial economic species in China,has been affected with miniaturization,premature sexual maturity,and under-aging.This threatens the sustainability of the fishery resources and calls for immediate action.The nutritional intake affects fish development and reproductive processes,which is essential in evaluating population status and sustainability.To comprehend nutrient utilization and mobilization during different ovarian development phases in Trichiurus japonicus,we collected samples of female fishes for two consecutive years in spring from 2020 to2021 from Wen-tai Fishing Ground.Through high-throughput sequencing and nontargeted liquid chromatography(LC)-mass spectrometry(MS)analysis,we compared and analyzed prey items and small molecule metabolite composition during different ovarian developmental stages of Trichiurus japonicus in the Wen-tai Fishing Ground.Our study revealed Trichiurus japonicus’ s feeding preference,trophic level,metabolite composition changes,and metabolic pathways involved during ovarian development.This improved our understanding of nutritional ecology and reproductive biology theory of Trichiurus japonicus and provided useful insights in formulating effective fisheries management policies.The detailed results of this study are as follows:(1)Analysis of feeding habits revealed that Trichiurus japonicus was a carnivorous fish with a broad spectrum of dietary preferences,mainly consuming fish with a small quantity of crustaceans and cephalopods.Trichiurus japonicus,Engraulis japonicus,Scomber japonicus,Euphausia pacifica,and Decapterus maruadsi were vital bait for each stage in the two-year studied.Comparison of the stomach contents of sexually immature and mature Trichiurus japonicus over the two years demonstrated that the diet altered congruent with gonad maturation.Specifically,the relative proportion of Trichiurus japonicus in the stomach contents composition significantly increased,while that of Engraulis japonicus significantly decreased.(2)Bait diversity and trophic level analysis showed that the mean values of H’ and evenness J for sexually immature Trichiurus japonicus in 2020 at the Wen-tai Fishing Ground were 1.37 and 0.44,respectively.Correspondingly,the mean values for sexually mature Trichiurus japonicus were 1.85 and 0.53.In 2021,the mean values of H’ and evenness J for sexually immature Trichiurus japonicus were 1.31 and 0.37,respectively,while the mean values for sexually mature Trichiurus japonicus were 3.44 and 0.94.The diversity indices of H’ and evenness J for sexually mature Trichiurus japonicus in 2020 and 2021 were superior to those for sexually immature Trichiurus japonicus.The Trichiurus japonicus’ s trophic level was computed by the accumulation of bait organisms’ trophic levels and was found to be 4.89 in 2021,higher than the trophic level of 4.69 in 2020.(3)Metabolomic analysis revealed that lipids and lipid molecules,comprising glycerophospholipids,steroids and steroid derivatives,aryl alcohol lipids,and fatty acyl groups,were the first major group of differential metabolites found in the ovaries at different developmental stages.These substances have significant effects on fish reproductive performance and egg quality.The second major group of substances included amino acids,peptides,and analogues,with protein intermediate metabolites,oligopeptides,or dipeptides being discovered.Tryptophan,phenylalanine,lysine,leucine,valine,alanine,serine,glutamic acid,arginine,and aspartic acid were among the amino acids identified,indicating their potential role in ovarian development.Several metabolic pathways associated with ovarian development were also detected in the present study,including the neurotrophin signaling pathway,adipocytokine signaling pathway,tryptophan metabolism,steroid hormone biosynthesis,and Gn RH signaling pathway.The study revealed that the process of ovarian development from stage II to stage III may be a critical period for nutrient accumulation in Trichiurus japonicus,with differential metabolites regulating yolk accumulation and oocyte growth and maturation.Estradiol is a key regulatory hormone of ovarian development in fish,which can promote yolk production.Estrone can be converted to estradiol by the action of enzymes,and the concentration of estradiol was significantly reduced in the stage III compared with stage II in this study.The level of 5-methoxytryptamine(5-MT),closely related to the secretion of FSH and LH,was significantly increased during the initiation of ovarian development.The cytochrome P450 metabolic pathway associated with yolk growth was significantly decreased during the development of ovaries from stage III to stage IV,indicating the maturity of the oocytes.During this process,the oocytes needed to consume a large amount of ATP,and c AMP was able to be converted to ATP under the regulation of AMPK.In the present study,the concentration of c AMP in the ovary decreased significantly.In addition,ceramide content increased gradually from stage II to stage IV,and it has been shown that the increase in ceramide content can advance the development of ovary.