FISH SPECIES COMPOSITION IN FRESHWATER BODIES AND KARST LAGOONS ON THE ISLANDS OF BAI TU LONG AND HA LONG BAY

Abstract

This study was conducted during 2023 - 2024 to investigate the fish species composition in freshwater bodies, tung, ang (karst sinkhole lakes and lagoons) on the islands of Bai Tu Long Bay and Ha Long Bay, northern Vietnam. A total of 48 fish species, belonging to 38 genera, 31 families, 18 orders, were recorded. Among them, Perciformes was the dominant order with 11 species (22.9%), followed by Gobiiformes with 9 species (18.7%) and Acanthuriformes with 4 species (8.3%). Other orders contained only a few species. By area, 40 species were found in Bai Tu Long Bay, compared to 32 species in Ha Long Bay, while only 8 species were recorded in freshwater streams and lakes on the islands. Of the 48 species, 15 species (31.3%) were identified as economically valuable, contributing significantly to local fisheries and livelihoods. Meanwhile, one endangered species, Evynnis cardinalis, listed in the IUCN Redlist (2025) and Vietnam Red Data Book (2024), was also documented. The fish assemblages reflected transitional ecological characteristics between freshwater, brackish, marine ecosystems, with a high degree of similarity between the two bays (32 species). However, biodiversity is threatened by overfishing, pollution, climate change, and invasive species (Oreochromis niloticus, Gambusia affinis). This research provides baseline data that are crucial for sustainable management, rational exploitation, and conservation of fish resources in Bai Tu Long and Ha Long Bay.