The mitre squid Uroteuthis chinensis is distributed widely in the Chinese coastal areas and contributes to the majority of the Chinese neritic squid fishery, especially in the South China Sea. However, little has been investigated about its life history traits, despite its commercial importance. In this study, using samples of U. chinensis collected through bottom trawlers from December 2019 to March 2021 in the northwestern South China Sea, biological traits including hatching date, growth pattern and dorsal mantle length at maturity of male individuals were explored by age determination based on statolith microstructure analysis. The results indicated that the U. chinensis showed a year-round spawning pattern with three main cohorts (spring, summer and autumn) that can be identified according to their hatching season. The range of the dorsal mantle length (DML) was 117–259 mm for females and 70–312 for males, and the body weight ranged from 55.1 to 480.5 g for females and from 19.3 to 560.2 g for males; the age ranges were estimated between 82 and 173 days for females and between 76 and 175 days for males. As for the length–weight relationship, males possessed a larger mantle length than females, while the body weight of females increased more compared to males at the same mantle length. The population recruits to the fishing ground, with individuals reaching sexual maturity at around 3 months, and the lifespan is less than 200 days. The growth model is well described by the exponential equation, which revealed that spring and autumn cohorts showed a higher growth rate than summer cohorts. As for male individuals, the smallest dorsal mantle length at maturity (DML_50%) was recorded in the spring cohort (DML_50% = 116 mm) compared with the summer (DML_50% = 129 mm) and autumn cohorts (DML_50% = 149 mm). This study provides key and updated fishery biological information of U. chinensis in the South China Sea and contributes to the understanding of U. chinensis resources.

Based on the data collected by four trawl surveys during 2016–2017, we applied biomass size spectrum (BSS) and abundance–biomass comparison (ABC) curve to assess the status of fish communities’ status in Daya Bay, China. Our findings indicated a unimodal pattern and biomass size ranged from −2 to 10 grain levels and the pattern of the Sheldon-type BSS of fish in Daya Bay. Moreover, fishes in the range of four to eight size class were relatively abundant. The highest peak belonged to the two to four grain level (log₂ size bins), mainly consisting of Leiognathus brevirostris, Callionymus meridionalis, Callionymus koreanus, Evynnis cardinalis, Trachurus japonicus, and other small fishes. The curves of the BSS in spring and winter were relatively flat and comprised a large curvature. The summer and autumn curves were comparatively steep, and the seasonal curvature was small. The curvatures of the curve were mainly related to a large number of small Evynnis cardinalis and a small number of large-sized Harpadon nehereus and Leiognathus ruconius. In our study, it was observed that the number and the size of the breeding population, trophic levels, migration habits, and other life history characteristics, as well as anthropogenic disturbances (especially overfishing), significantly affected the peak shape, slope, or curvature of the fish BSS, with overfishing being the main factor. The ABC curve exhibited that Daya Bay was in a critical state of disturbance throughout the year. The spring, summer, and autumn were in severe disturbance, while the winter was in moderate disturbance.

Cephalopod fisheries in the China Seas have witnessed an increasing trend in the catches of coastal cephalopods since the 1990s, with Loliginidae squid emerging as the main commercial target species. However, climate change and overfishing have led to a dramatic reduction in Loliginidae squid resources, highlighting the need to improve monitoring, protection, and management of this species. The Loligo beka and Uroteuthis duvaucelii are widely distributed along the coastal areas of the China Seas, and have commercial and ecological importance. Despite having overlapping distributions, similar life histories, and a strong dependence on the marine environment, there is limited knowledge about their growth and responses to environmental changes, hindering the effective management of their resources. In this study, we investigated the interspecies and intra-species differences in condition factor and their responses to temperature changes by analyzing data collected from wide coastal areas of the China Seas from June 2019 to November 2020. The findings showed that both species exhibited allometric growth and reproduced throughout the year, with two main breeding peaks. There were significant monthly variations and latitude differences in the intra-species growth, with a higher proportion of small-sized individuals (between 5 and 10 g for L. beka and between 10 and 20 g for U. duvaucelii) in low-latitude waters. The latitudinal differences in body weight and distribution between and within the two species were mainly due to natural habitats, especially temperature. Our mixed effect model results demonstrated that both species’ body weight increased with increasing temperature, suggesting that Loliginidae squid have significant environmental adaptability and can be used as an indicator species for studying environmental changes in the China Seas. These findings have significant implications for understanding the population dynamics, species development, and regionally specific management of Loliginidae squid fisheries in the China Seas.