Interannual variations in the size structure of swordtip squid Uroteuthis edulis in the southern East China Sea

Identifying the population structure of commercially exploited fish species is essential for effective fisheries management. These insights are particularly important for short-lived squid, which often exhibit considerable variations in life history traits. The swordtip squid (Uroteuthis edulis) inhabits temperate and tropical neritic waters of the Indo-Pacific regions, and is harvested using diverse fishing gears and methods throughout its distribution range. Two seasonal cohort of swordtip squid on the southern East China Sea (ECS) shelf have been identified, each exhibit distinct growth patterns. However, information on the size structures of the seasonal cohorts on the southern ECS shelf remains limited. In this study, we examine the size groups and associated life history traits of swordtip squid in the southern ECS during 2021–2022. Swordtip squid were categorized into large-sized (LS) and small-sized (SS) groups based on natural breaks in the distribution of mantle length for each sex at each sexual maturity stage. The LS group hatched in winter and matured (spawned) in spring-summer, whereas the SS group hatched in summer and matured (spawned) in autumn-winter. The two size groups likely alternated between generations. Notably, some individuals hatched in the same season but developed into different size groups, suggesting the possibility of exposure to varying environmental conditions. These results imply the spawning sites for swordtip squid are widespread in the southern ECS, likely driven by major oceanographic processes that vary seasonally and interannually. Our findings clarify the population structure of swordtip squid on the southern ECS shelf and may facilitate conservation and management measures for the fisheries in the Northwest Pacific region.