Distribution and diets of larval and juvenile fishes: Influence of salinity gradient and turbidity maximum in a temperate estuary in upper Ariake Bay, Japan

We investigated the fish assemblage and distribution, diversity, and diets in relation to copepod prey communities along the Chikugo estuarine gradient in the Ariake Bay, Japan. Larval and juvenile fish samples, ambient copepod samples were collected and major hydrographic parameters were recorded at seven selected sampling stations (salinity range: 0.4–28.8 psu) during four sampling cruises in spring 2001. A zone of estuarine turbidity maximum (ETM) was identified in the upper part of the estuary which was characterized by low salinity. Two different fish and copepod communities based on the spatial distribution patterns were identified: the oligohaline community in the upper estuary, which was associated with the ETM; and the euryhaline community in the lower estuary, downstream of the ETM. The oligohaline fish community was composed of Acanthogobius flavimanus, Acanthogobius hasta, Coilia nasus, Neosalanx reganius, and Trachidermus fasciatus while the euryhaline community was composed of Engraulis japonicus and Sebastes inermis. Lateolabrax japonicus was distributed over wide spatial areas. Sinocalanus sinensis was the single dominant member of the oligohaline copepod community while the euryhaline community was dominated by Oithona davisae, Acartia omorii and Paracalanus parvus. Strong dietary relationships were identified between fishes and copepods in the same community. ETM appears to have significant influence on the distribution and abundance of the oligohaline copepod S. sinensis and this prey copepod appears to have strong influence on the fishes in the oligohaline regions. Most of the fishes were distributed in the low saline upper estuary where they foraged on the single dominant copepod S. sinensis which contributes the majority of the copepod standing biomass of the estuary and thus appear to support nursery for fishes. It was concluded that the ETM-based copepod S. sinensis plays a key role in survival and distribution of larval and juvenile fishes in Chikugo estuary.