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Trichiurus japonicus is an economically valuable species in the East China Sea, whose allowable capture size varies with fishing effort in different years. To clarify the optimum capture size for T. japonicus in the East China Sea, we collected samples and data from T. japonicus targeting fishing gears such as trawls, canvas spreader stow nets and longlines from 2016 to 2020. We estimated growth and mortality parameters using the FiSAT II software, and calculated the size limit standards for capture. The results showed that the inflection point of anal length, the critical anal length and the anal length of one-year-old T. japonicus was 382.84, 397.12, and 216.05 mm, respectively. The anal length of maturity was 230.38 mm, and the minimum capture size (anal length) was 219.23 mm. Based on the yield per recruitment analysis using Beverton-Holt model, the current fishery reference points were under overfishing condition ( t c =0.38 a, F =2.11), and the suggested optimum capture size was 364.64 mm. There would be a sharp decline of T. japonicus catches in the East China Sea if the suggested minimum capture size was substantially higher, which might be non-practical for local fishermen. Therefore, we suggested 220 mm as the capture size limit, which was close to the anal length of one-year-old fish and the anal length of maturity.

In order to understand the growth dynamics of Larimichthys polyactis in spring and autumn along the coast of Zhejiang Province, we used the survey data of bottom trawlers in spring (April) and autumn (November) from 2014 to 2019 along the coast of Zhejiang Province to examine the heterogeneity of L. polyactis growth by constructing a generalized linear model (GLM) and nine linear mixed effect models (LMEM). The results showed that body length of L. polyactis ranged from 15 mm to 210 mm, with an average of 124.12 mm. The dominant body length group ranged from 110 mm to 140 mm. The body weight ranged from 0.04 g to 156.2 g, with an average of 33.28 g. The dominant body weight group ranged from 30 to 50 g. According to the AIC minimum criterion, the LMEM model with the random effects of seasons and water zones on growth parameters a and b was the best. The results of cross validation also showed that the prediction of this model was the best. In the optimal model, the fixed value of growth parameter a was 0.61×10 -4 , which ranged in (0.32-1.91)×10 -4 after adding season and water random effect. The fixed value of parameter b was 2.73, and ranged from 2.49 to 2.86 after adding the random effects of season and water area, indicating that small yellow croaker was under negative allometric growth. Season and water zone 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 higher than that in autumn. From water zone perspective, the closer the offshore distance, the faster the growth rate of small yellow croaker.

We investigated population structure, resource density changes of Larimichthys polyactis and its relationship with environmental factors in spring, based on the survey data of bottom trawl in adjacent sea areas of Zhoushan fishery spawning ground protection area from 2014 to 2019. The results showed that the relationship between body length and body weight of Larimichthys polyactis was W =0.44×10 -4 × L 2.78 , and parameter b was less than 3, which indicated that L. polyactis had negative allometric growth in recent years. There was a negative correlation between fullness and body length, with body becoming slender. From 2014 to 2019, body length and weight of L. polyactis were the highest in 2014 and the lowest in 2019. Since 2014, population size of L. polyactis in Zhoushan fishery spawning ground protection area and adjacent sea area had gradually decreased, indicating that the miniaturization of L. polyactis had not altered in recent years. From the perspective of the annual change of resource density, resource density of L. polyactis was higher than that before the establishment of the reserve, indicating that the management and protection of the reserve area played a protective role in the recovery of L. polyactis resources. The fitting results of GAM model showed that water depth and bottom water temperature were the environmental factors closely related to the density distribution of L. polyactis resources. With increasing water depth, the resources showed a fluctuating upward trend and were the highest near the water depth of 60 m. In the range of 12-16 ℃ water temperature, the resources increased with the increases of bottom water temperature. When water temperature was above 16 ℃, resources decreased with the increases of bottom water temperature.

To elucidate the characteristics of fishery resources structure in the Dachenyang Spaw-ning Ground Reserve, the index of relative importance (IRI), biodiversity index and canonical correspondence analysis (CCA) were used to explore the relationship between crustaceans community and marine environment based on the bottom trawl survey data collected from April and November in 2018. A total of 38 crustaceans species were recorded, belonged to 25 genera in 14 families. The dominant species were Oratosquilla oratoria , Portunus trituberculatus , Charybdis bimaculata , and Parapenaeopsis hardwickii . The resource density of crustaceans was generally high in depths below 50 m in spring and in depths above 50 m in autumn. The density in the southern area was higher than nouthern area. The Margalef species richness index ( D ), Shannon diversity index ( H ) and Pielou evenness index ( J ) of crustaceans in spring was higher than that in autumn. The H of crustaceans in depths above 50 m was the highest. Based on cluster analysis and non-matrix multidimentional scaling analysis, the crustaceans could be classified into three groups in spring and four groups in autumn. The differences in crustacean community structure in spring were more significant than in autumn. The results of canonical correspondence analysis (CCA) showed that water depth, bottom temperature, salinity and dissolved oxygen were the main environmental factors affecting species composition and spatial structure of crustaceans in the surveyed area.

To understand the status and dynamics of macrozoobenthos community in Sanmen Bay, we collected benthos samples by Agassiz trawl in November 2015, and February, May and August in 2016. The results showed that: 1) A total of 119 macrozoobenthos species were recorded, with main groups of fishes, crustaceans and molluscs, accounting for 79% of the total number of species. 2) The dominant species of macrozoobenthos throughout the year were Leptochela gracilis , Hyperacanthomysis longirostris and Amblychaeturichthys hexanema . The identity of dominant species changed significantly in different seasons. 3) The annual average biomass and density were 0.025 g·m -2 and 0.07 ind·m -2 , respectively. 4) The Shannon's diversity index of macrozoobenthos in Sanmen Bay varied from 2.21 to 3.18, the Margalef's species richness index varied from 3.25 to 3.78, and the Pielou's evenness index varied from 0.53 to 0.79. Results from abundance-biomass curve analysis showed that the community was moderately disturbed in spring and winter, and slightly disturbed in summer and autumn. Results from Canonical correspondence analysis showed that water depth, temperature, salinity, and pH were the most important environmental factors affecting macrozoobenthos community.

The relative importance index (IRI), mean crowding, niche breadth, and niche overlap (Q ik ) of major fish species were calculated based on two fishery resource surveys from April to May 2016 (spring) in the coastal waters of central and southern Zhejiang Province. The results showed that there were 24 major fish species (IRI:100), the niche breadth values of which differed greatly and were positively correlated with IRI. The niche overlaps of fishes were extremely low in spring. The total amount of species pairs with niche overlap 0.6 (Q ik :0.6) were six in April and five in May, which represented 3.9% and 4.8% of the total pairs, respectively. Results from the redundancy analysis showed that the distribution of major fish species were mainly affected by temperature, salinity, water depth, and dissolved oxygen, which caused ecological differentiation of fish species.

A total of 453 Trachurus japonicus specimens with a fork length (FL) of 46-250 mm were sampled in the seasonal light seine net surveys in the East China Sea in May, August, and November, 2008 and in February, 2009. The stomach contents of the specimens were analyzed, and the seasonal and ontogenetic variations in the feeding habits of the T. japonicus were examined by the Kruskal-Wallis test, chi-squared test, and cluster analysis. There were 124 prey species (including not identified) ingested by the T. japonicus, among which, planktonic crustaceans and small-scale marine fish made up the dominant prey groups. According to the percentage index of relative importance (IRI%), Bregmaceros macclellandi was the most dominant prey, accounting for 39.2%, followed by Macrura mysis larva (18.4%), brachyura zoea larva (7.6%), and Euphausia pacifica (6.6%). The feeding intensity of the T. japonicus varied significantly with its FL and season, being the highest for the T. japonicus with a FL of 140-159 mm, higher for the T. japonicus juveniles with a FL of 45-99 mm, while lower for the T. japonicus of other size classes, and the highest in spring and the lowest in winter. Cluster analysis revealed there was an abrupt change in the diet composition for the T. japonicus with a FL of about 100 mm FL. The average trophic level of the T. japonicus in four seasons was 3.51, indicating that the T. japonicus in the East China Sea was of low-level carnivores feeding on plankton and nekton.