| Small yellow croaker（Larimichthys polyactis）is one of the important economic fish species in China’s coastal waters.Since the 1960s,due to overfishing,marine environmental pollution,climate change and increased extreme weather,the amount of resources has declined.In recent years,although the resources of small yellow croaker have been restored by taking measures such as the summer fishing moratorium and establishing spawning ground reserve,the biological characteristics of small yellow croaker have also changed accordingly with climate change.Therefore,it is of great significance to study the biological and ecological characteristics and resource status of small yellow croaker.Based on the survey data of bottom trawling along the Zhejiang coast from 2014 to 2019,this study analyzed and studied the population structure,growth heterogeneity,reproductive biology,feeding ecology and resource status of small yellow croaker.The main findings are as follows:In order to grasp the growth dynamics of small yellow croaker in spring and autumn along the coast of Zhejiang Province more accurately,this thesis used the survey data of bottom trawling in spring（April）and autumn（November）from 2014 to 2019 along the coast of Zhejiang Province to study the heterogeneity of small yellow croaker growth by constructing a generalized linear model（GLM）and nine linear mixed effect models（LMEM）.The results showed that the average body length was 124.12 mm of small yellow croaker,the body length ranged from 15 mm to 210 mm,and the advantage body length group ranged from 110 mm to 140 mm.The weight ranged from 0.04 g to 156.2 g,the average weight was 33.28 g,and advantage body weight group ranged from 30 to 50 g.According to the AIC minimum criterion,the LMEM model with the random effects of months and waters on growth parameters a and b was the best,and the results of cross validation also showed that the prediction effect of this model was the best.In the optimal model,the fixed value of growth parameter a was 0.61×10-4,the range of parameter a after adding month and region random effect was（0.32-1.91）×10-4,the fixed value of parameter b was 2.73,and after adding the random effects of month and water area,the parameter b ranged from 2.49 to 2.86,indicating that small yellow croaker was negative allometric growth.Season and water had significant effects on the relationship between body length and body weight of small yellow croaker.From a seasonal perspective,the growth rate of small yellow croaker in spring was faster than that in autumn.From different water area perspective,the shorter the offshore distance was,the faster the growth rate of small yellow croaker was.Therefore,in order to conserve the fishery resources of small yellow croaker in Zhejiang fishing ground more effectively,it is suggested that the management departments should focus on protecting small yellow croaker in spring.In the mean time inshore small yellow croaker also should be protected,especially spawning groups and larvae,to maintain its reproduction and growth.In order to understand the reproductive characteristics of small yellow croaker in Zhoushan fishing ground and surrounding waters,according to the trawl and drift net catch samples in Zhoushan fishing ground and adjacent waters from March to May in 2020 and2021,the reproductive population characteristics of small yellow croaker and the relationship between reproductive capacity and growth indicators were analyzed,and the changes in reproductive capacity were compared and analyzed.The results showed that the body length of small yellow croaker population was 107～200 mm from 2020 to 2021,and the dominant body length group was 130～170 mm,with an average body length of143.98±16.78 mm.The body weight ranged from 18.4g to 108.3g,and the dominant body weight ranged from 30g to 70 g,with an average body weight of 46.78±16.54 g.The absolute fecundity ranged from 7944 to 31077,with an average of 15545±6019.The relative fecundity of body length ranged from 53 to 182 grains/mm,with an average of98±31.29 grains/mm.The relative fecundity of body weight ranged from 126 to 433grains/g,with an average of 258±60.91 grains/g.The fish eggs diameter ranged from 0.41mm to 2.77 mm,with an average of 1.01±0.36 mm.Studies have shown that absolute fecundity has a power function correlation with body length,and a linear correlation with body weight and commercial body weight.According to the survey data of bottom trawl fishery resources in Zhoushan Islands and offshore waters from 2019 to 2021,the feeding habits of small yellow croaker were studied by the combination of stomach contents and stable carbon and nitrogen isotopes.The factors affecting the feeding habits of small yellow croaker were analyzed by GAM model.The results showed that there were 52 kinds of feeding species of small yellow croaker.According to the percentage of key food organisms（IRI%）,four kinds of food organisms were selected,including Euphausiacea,Decapoda,Mysidacea and Fishes,and the dominant species were Euphausiacea,Mysidacea,Leptochela gracilis,Amblychaeturichthys and Benthosema pterotum.Carbon and nitrogen stable isotope results showed that the body length of small yellow croaker was positively correlated withδ13C andδ15N values,and the feeding habits of small yellow croaker changed with the change of body length.GAM models fitting results showed that the probability of feeding key prey species of small yellow croaker was mainly related to season,bottom salt,bottom temperature and depth,and also had a great correlation with the distribution of prey species.Based on the survey data of bottom trawl fishery resources in Zhoushan fishing ground and adjacent sea areas in spring（April）and autumn（November）from 2016 to 2019,this thesis analyzed the spatial and temporal distribution characteristics of biomass resource density of small yellow croaker in this sea area,and used the two-stage generalized additive model to study the environmental factors affecting its spatial and temporal distribution.The results showed that the density of small yellow croaker resources showed obvious seasonal changes,and it was significantly higher in spring than in autumn.The main factors affecting the resource density of small yellow croaker in this sea area are season,water depth and bottom water temperature.Small yellow croaker is mainly distributed in the range of 19 to 66 m water depth and the range of 11 to 22℃.With the increase of water depth,the resource density of small yellow croaker shows an upward trend,and the resource density was the highest near the water depth of 60 m.The resource density showed a downward trend at 16℃to 20℃,and the resource density was high at 12℃to 16℃.This thesis reveals the influence of seasonal variation of water depth and bottom water temperature in the sea area on the density distribution of small yellow croaker resources,and provides basic data for further understanding of its spatial distribution and quantitative management of resources.In conclusion,heterogeneity for the growth of small yellow croaker,breeding characteristics,feeding habit,the resources of time and space distribution and its relationship with environmental factors,the study found season and waters has significant effect on the growth of small yellow croaker,breeding group showed a trend of miniaturization,chaff feeding krill main kinds of small yellow croaker,Euphausiacea,Decapoda,Mysidacea and Fishes bait four groups,Small yellow croaker key bait feeding biological probability is mainly related to season,bottom salt,bottom temperature and depth,density of small yellow croaker resources show obvious seasonal and interannual variations,the main factors affecting the distribution of the products as the interannual factor,water depth and the bottom water temperature,the results can provide some reference for administration and protection of small yellow croaker and basis,For the future study of small yellow croaker resources evaluation,reserve management and maintenance to provide basic data. |
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