Population Structure of Mangrove-Associated Fish

Abstract

Mangrove forests serve as nursery habitat, feeding ground, and refuge for a highly diverse assemblage of fish species that include both mangrove specialists and transient visitors from adjacent coral reef, seagrass, and estuarine habitats. Despite the well-established ecological importance of mangroves for fisheries productivity, the population genetic structure of mangrove-associated fish -- and thus the connectivity among mangrove patches that determines resilience to habitat loss -- remains poorly characterised for the majority of species. This study combined fish community surveys, acoustic telemetry, and microsatellite genotyping to characterise the population structure of eight commercially and ecologically important mangrove-associated fish species across 24 mangrove sites spanning 1,847 km of Indo-Pacific coastline, encompassing a gradient from intact to severely degraded mangrove cover. Community surveys at 24 sites recorded 247 fish species from 84 families, with species richness declining by 54.7% in severely degraded versus intact mangroves. Acoustic telemetry of 847 tagged individuals confirmed that 74.8% of tagged fish were mangrove-resident (day-range < 500 m from mangrove edge) and 25.2% made regular excursions to adjacent coral reef or seagrass habitats. Microsatellite genotyping of 1,247 individuals from three focal species revealed significant population structure (mean FST = 0.084 +- 0.024) driven primarily by inter-site distance and mangrove patch isolation rather than oceanographic connectivity -- indicating that mangrove connectivity conservation, not current-driven larval dispersal alone, is critical for maintaining population genetic exchange among sites. Sites with < 30% intact mangrove within 10 km showed significantly elevated genetic drift signatures (higher FROH; lower allelic richness; p < 0.001) consistent with reduced effective population size from habitat loss.