Shrimp community structure and influence of environmental variables in the East China Sea and Yellow Sea in spring.

Based on data collected from research vessel cruises performed in May 2020 off the East China Sea (ECS) and the southern Yellow Sea (YS) (26°30'-35°00' N, 120°30'-127°00' E), we analyzed the shrimp community and its relationships with environmental variables by using index of relative importance, biodiversity indices, and multivariate techniques. A total of 29 species were recorded, belonging to 11 families and 19 genera. The dominant species were Metapenaeopsis longirostris, Leptochela gracilis, Solenocera melantho, Crangon hakodatei, Parapenaeus fissuroides, Plesionika izumiae, and Trachypenaeus curvirostris, which together accounted for 82.9% of the total biomass and 90.8% of the total abundance of shrimps. Results of Cluster and NMDS analyses showed that three groups were identified for the shrimp community in the ECS and YS in spring, including group A (inshore of northern ECS and YS group), group B (offshore of northern ECS group) and group C (southern ECS group). ANOSIM and SIMPER analysis showed significant differences between group A and B, gourp A and C, and group B and C, with the dissimilarity of 92.2%, 95.8% and 91.6%, respectively. The typical species were T. curvirostris, C. hakodatei, L. gracilis and Palaemon gravieri in group A, and S. melantho in group B, and M. longirostris, P. fissuroides, P. izumiae and Solenocera alticarinata in group C. Significant differences were also detected in biomass, diversity index, species richness index and evenness index among groups, with significantly greater values in group C than those in A and B. Environmental variables and the substrate also displayed significant differences among groups. Results of canonical correspondence analysis showed that bottom temperature, bottom salinity, depth, and the substrate were the main environmental variables affecting spatial structure of shrimp community. Water mass characteristics and substrate type had important influences on the distribution of shrimp community in the ECS and YS in spring.

[Utilization and management of Trichiurus japonicus resources in East China Sea based on Beverton-Holt model].

In this paper, the population structure of Trichiurus japonicus in East China Sea was analyzed based on the data of its age composition and anus length in 2002-2004, and the rational utilization of T. japonicus resources in the Sea was approached by using the catch (in mass) per recruit theory of Beverton-Holt model. The results revealed that: (1) there was a miniaturization trend of the T. japonicus population in East China Sea. The range of age composition changed from 0-6 year in the late 1950's to 0-4 year in the early 21st century, and the population was dominated by the group of 0-1 year now. The percentage of 2-year old T. japonicus decreased from 12.84% in the late 1950s' to 6.91% in the early 21st century, and that of 3-year old T. japonicus decreased from 4.92% in the late 1950s' to 0.57% in the early 21st century; (2) the exploitation rate of T. japonicus in the period of 2000-2003 was 0.864, which was beyond of the optimum exploitation rate of 0.51, suggesting that the T. japonicus in East China Sea was under over-fishing; and (3) to enhance the age value at first capture (t(c)) was the best measure of increasing the catch (in mass) per recruit of T. japonicus. The age at recruitment (t(r)) and the t(c) of T. japonicus in East China Sea was 0. 25 and 0.5 at present, respectively. If the t(c) changed from 0.5 to 1, the unit catch would be increased by 55.38%, and if the t(c) changed to 1.5, 2 or 2.75, it would be increased by 100.81%, 130.52% or 145.23%, respectively. It was suggested that due to the difficulties in greatly reducing catching intensity, it could be available to properly increase the to value while decrease the catching intensity to protect the T. japonicus resources in East China Sea and realize their sustainable use.