Multi-habitat utilization and diverse life history of the endangered Sea Carp (Cyprinus acutidorsalis) in the Wanquan Estuary, Hainan island, China: Evidence from otolith microchemistry

The Sea Carp Cyprinus acutidorsalis (Wang) is an endemic species limited to a few estuaries in China, uniquely adapted to long-term residence in brackish water environments. This species is endangered due to anthropogenic disturbances, and its life history and habitat use remain poorly understood. This study is the first to investigate the life history and habitat utilization of Sea Carp in the Wanquan Estuary, Hainan Province, China, using otolith microchemistry. We analyzed the otolith samples from wild-captured Sea Carp and freshwater-reared juvenile fish. Significant differences in ratios of ⁷Li, ²⁴Mg, and ⁸⁸Sr between the two groups demonstrated that the otolith microchemistry can effectively reflect habitat conditions. We identified three distinct spawning areas, ranging from freshwater areas to nearshore habitats. Six life history contingents were identified, including four migratory and two resident types, with a preference for estuarine habitats. Our findings highlight the importance of multiple spawning grounds and diverse life histories in enhancing the adaptability and resilience of Sea Carp populations. We recommended that conservation efforts focus on maintaining habitat connectivity between estuarine and marine environments, protecting mangrove areas, and regulating fishing activities. This study provides valuable insights into the habitat utilization and life history of rare and endangered estuarine fish species, contributing to the development of effective management plans for their preservation worldwide.