| According to the trawl survey data of 8 voyages conducted in the spawning ground reserve and nearby waters of northern Zhejiang from April(spring)and November(autumn)of 2017 to 2020,the relative importance index,resource density,species diversity and Generalized additive model(GAM)and other methods are used to analyze the species composition characteristics of fish eggs,larvae and juveniles in the spawning grounds of northern Zhejiang and adjacent waters,the temporal and spatial changes of resource density and diversity,and the impact of environmental factors on species diversity.The analysis aims to provide scientific theoretical support for the protection of fishery resources in the spawning ground reserve in northern Zhejiang.The main findings were as follows:1.Species composition: In terms of the inter-annual changes in species composition,the number of fish eggs and larvae and juveniles in the waters of the spawning ground protection zone in northern Zhejiang showed a downward trend from 2017 to 2020.In terms of spatial composition,the dominant species of fish eggs collected in the spring Dazhenyang Spawning Ground Reserve and Daiquyang Spawning Ground Reserve was Mullet(Liza haematocheilus),and the dominant species of larvae and juveniles was Sea bass(Lateolabrax japonicus).The main dominant species of fish eggs collected in the Spawning Ground Reserve of the Saddle Islands and the Spawning Ground Reserve of Zhoushan Fishing Grounds were large yellow croaker(Larimichthys croceus),hairtail(Trichiurus lepturus)and mullet,etc.The main dominant species of larvae and juveniles was anchovy(Engraulis japonicus).In the autumn,the main species composition of the four spawning ground reserves was the same.The eggs were mainly Dragon head fish(Harpadon nehereus),and the larvae and juveniles were mainly anchovy.There was little difference in the species composition of the sea area outside the protected area,and the species of Perciformes were the most.Comparing the composition of important species and protected species of fish eggs,larvae and juveniles in and outside the protected area in2019 and 2020,it is found that the protected species were distributed in the protected area and outside the waters,and the relative importance index of the protected species in the protected area is higher than the protected species.There were clear differences in the composition of important species outside the protected area and outside the area.The dominant species of roe in spring from 2017 to 2019 were all mullet,no obvious dominant species will appear in 2020,mullet is an important species of that year,and the dominant species of roe in autumn were dragon head fish.The replacement of dominant species of larvae and juveniles is obvious.In spring,the economic species of small yellow croaker(Larimichthys polyactis)is mainly replaced by Latimeria goby(Chaeturichthys stigmatias)in 2018 and 2019 and mullet in 2020.The big yellow croaker and dragon head fish in 2018 were replaced by the anchovies in 2019 and2020.2.Resource density: From 2017 to 2020,the average resource density of fish eggs,larvae and juveniles in the sea area of the northern Zhejiang spawning ground had a large inter-annual change.The overall resource density in 2019 and 2020 was relatively high.The average resource density of fish eggs,larvae and juveniles in the waters of the protected area in spring was lower than that in the waters outside the protected area,and the opposite was true in autumn.In terms of spatial distribution,fish eggs,larvae and juveniles were mainly distributed in the waters near the Zhoushan Islands in April,and in November,they were mainly concentrated in the spawning area of Zhoushan Fishing Grounds and nearby waters.In terms of seasonal distribution,the resource density of roe and larvae in autumn was lower than that in spring.In terms of inter-annual changes,the egg densities of protected species of small yellow croaker,silver pomfret and hairtail collected in spring showed an overall trend of fluctuations,but the overall trend was increasing,and the resource density of larvae and juveniles showed a downward trend;protected species of large yellow croaker eggs collected in autumn And the larvae and juvenile hairtail fish egg density changes greatly,and the overall trend was declining.In terms of spatial distribution,protected species are mainly distributed in the waters near the spawning ground protection area of Zhoushan Fishing Ground.3.Environmental factors: In terms of the inter-annual changes in the level of diversity,the diversity of fish eggs,larvae and juveniles in the offshore spawning ground reserve in northern Zhejiang from 2017 to 2020 showed an overall downward trend,and the diversity index and richness index fluctuated greatly.The evenness index fluctuates relatively little over the past four years and was relatively stable.The generalized additive model was used to study the relationship between the diversity of fish eggs and larvae and environmental factors in the northern waters of Zhejiang.The results show that water depth and surface water temperature were the main environmental factors that affect the diversity of fish eggs in the surveyed sea area,and water temperature affects larvae and juveniles.The main environmental factor of diversity. |
