Juvenile and small fishes in a mangrove estuary in Trang province, Thailand: seasonal and habitat differences

Juvenile and small resident fish assemblages at three sites (beach with mangroves, beach without mangroves and middle reach of mangrove creek), comprising the major habitat types of a mangrove estuary in Trang Province, Thailand, were studied for 3 years. A total of 6616 individuals, comprising 89 species (30 families), were collected using a small seine net. In terms of number of species per family, Gobiidae (18 species) was the most diverse, followed by Engraulidae (nine species) and Leiognathidae (seven species). In terms of individuals, Engraulidae, Leiognathidae, Gerreidae, Atherinidae and Gobiidae were the dominant families. Species richness was significantly greater in the mangrove creek than at the two beach sites, and greater in the wet season than the dry season. Fish abundance (individuals per tow) was also greater in the wet season, with no significant differences among sites. On the other hand, biomass (weight per tow) was greater in the mangrove creek than the mangrove beach, but with no significant difference between seasons. Cluster and ordination analysis based on the number of individuals of each species demonstrated that the fish assemblage structures were markedly different between the beach and creek sites, the assemblages at the two beach sites being similar in the wet season. Most species were specific to a particular habitat type (or types), some occurring exclusively in the creek or beach sites, and some in both the mangrove sites. Such distributional differences were found even between species within a single family. The results indicated that creek and beach habitats function as nursery sites for different fish species, the former being utilized by a greater diversity of species than the latter.