Delineating Ecologically Important Fish Habitats Using Ichthyoplankton Assemblages in the Coastal Waters of Tam Hai, Central Vietnam

Abstract

Fish eggs and larvae provide biological information for identifying areas associated with spawning and early development, yet ichthyoplankton remains poorly documented in tropical estuarine–coastal systems. This study examined ichthyoplankton at 16 stations in the coastal waters of Tam Hai, central Vietnam, during the dry season (April 2022) and rainy season (December 2022) to assess taxonomic composition, abundance, spatial organisation, seasonal variation, and associations with surface salinity. A total of 20,888 fish eggs and 1,812 fish larvae were collected, with 24 larval families identified. Mean fish egg density was substantially higher in the dry season than in the rainy season (634.30 ± 157.10 versus 60.18 ± 25.23 eggs/100 m³), while mean larval density was 50.28 ± 15.03 and 20.20 ± 5.71 ind./100 m³, respectively. Larval assemblage composition showed marked seasonal turnover and spatial differentiation across mangrove, transitional, and coral reef-associated habitats. Salinity was significantly associated with the abundance of four larval families: Gobiidae showed a negative association, whereas Nemipteridae, Dorosomatidae, and Terapontidae showed positive associations. The spatial separation between egg-rich and larva-rich stations indicates that early-life-stage distributions may reflect processes operating after spawning as well as habitat conditions. These findings provide baseline information for identifying areas associated with fish spawning and early development and for supporting fisheries conservation and ecosystem-based coastal management.