Spatio-temporal occurrence of different early life stages of Periophthalmus modestus in a tropical estuary

To elucidate the distribution pattern of Periophthalmus modestus, an amphibious fish, at its southernmost habitat, an investigation was conducted monthly at different locations along the Red River Delta area over two years. Spatially, planktonic larvae were caught abundantly at sampling sites along the Ba Lat Estuary and at a single site in the nearby mangroves close to the river’s main tributary. The benthic and amphibious juveniles were caught mainly in the mangroves. This finding suggests that P. modestus larvae may be transported up the river from the spawning ground by the tidal current and may use the main tributary of Ba Lat Estuary as a nursery habitat for the planktonic larval stage and expand their habitat to the mangrove along the estuary during the settling stages. Temporally, planktonic larvae occurred from February to July and peaked in May, suggesting that the main reproductive season of this fish in Vietnam is from February to July, which is earlier than in temperate areas. The abundance of larvae and juveniles of this species over time was positively correlated with water temperature. Nonmetric multidimensional scaling (NMDS) models based on size-class assemblages, months of sampling and samplings sites, together with an overlay of environmental vectors, revealed that water temperature was significantly correlated with fish size. Fish in the smaller size class tended to occur at lower temperatures, and fish in the larger size class tended to occur at higher temperatures. The distribution pattern of this species revealed in this study may partly contribute to its successful adaptation to circumstances in this tropical area.