Seasonal occurrence and size structure of juvenile sicklefin lemon sharks (Negaprion acutidens) at Kuroshima Island, Yaeyama Islands, Japan

Context In many coastal shark species, habitat use shifts across life stages. Therefore, examining size-specific patterns of occurrence is essential for understanding their spatial and temporal ecology. Aims This study aimed to describe the seasonal occurrence patterns and size distribution of the sicklefin lemon shark (Negaprion acutidens) in a northern part of its distributional range. Methods Capture surveys were conducted in the coastal waters around Kuroshima Island, Yaeyama Islands, Ryukyu Archipelago, Japan from 2017 to 2020. To evaluate seasonal and spatial occurrence patterns, the catch per unit effort was estimated using zero-inflated negative binomial models. In addition, ages of individuals in the population were estimated using size-frequency analysis with a normal mixture distribution model. Key results In total, 199 sicklefin lemon sharks were measured, of which 157 were tagged and released. Their total lengths ranged from 61.2 cm to 135.0 cm, and the size distribution was strongly skewed toward smaller size classes. Sicklefin lemon shark occurrence exhibited clear seasonality, with a pronounced peak during the high-temperature period. The normal mixture distribution model identified three cohorts in the size structure around Kuroshima Island. Conclusions This study provides baseline ecological information on sicklefin lemon sharks at the northern edge of their distribution, and suggests that most recruited individuals may move to other areas within approximately 2 years. Implications This study suggests that broader regional investigations will be necessary to develop effective conservation plans for this species.