The fishery biology of the three-spot seahorse (Hippocampus trimaculatus) in the Con Dao Archipelago, Vietnam

The three-spot seahorse (Hippocampus trimaculatus Leach) is among the most heavily harvested seahorse species in Vietnam. Due to overexploitation and habitat degradation, H. trimaculatus is listed in Appendix II of CITES (2014), the IUCN Red List (IUCN, 2025), and the Vietnam Red List of Threatened Species (Vietnam Academy of Science and Technology, 2024). Although H. trimaculatus is harvested throughout Vietnamese waters, its fishery biology data remain limited, particularly for the Con Dao Archipelago, a biodiversity hotspot of high conservation importance. This study investigated the fishery biology of H. trimaculatus in the Con Dao Archipelago and proposes conservation measures. A total of 499 H. trimaculatus specimens were collected from local fishers in April and October 2024 in the Con Dao Archipelago and identified on the basis of molecular analysis and morphology. Interviews were conducted with 16 fishers and seven seafood shops to gather information on exploitation and trade. The results indicated that H. trimaculatus is being exploited unsustainably, as evidenced by the fishing gear used, the male-to-female sex ratio of 1:1.1, the portion of brooding males (21.04%) in the total catch, the capture of immature individuals, and the scarcity of mega-spawners. Sexual dimorphism, allometric growth, and differences in the length-weight relationships of the H. trimaculatus population are described and compared with other seahorse populations. This study proposes management recommendations concerning fishing gear, sex ratios, and size limits of H. trimaculatus to support its conservation in the Con Dao Archipelago.