Spatial variation in feeding, prey distribution and food limitation of juvenile flounder Rhombosolea tapirina Günther

Abstract

The feeding and prey distribution of juvenile greenback flounder Rhombosolea tapirina Günther was compared between Swan Bay, a sheltered bay with muddy sediments and a well-developed seagrass-detrital system, and an adjacent area of Port Phillip Bay, a more exposed area with coarser sandy sediments. Meiofauna was dominated by benthic harpacticoid copepods in both bays. Harpacticoids in Port Phillip Bay, however, were mainly interstitial while epibenthic species were dominant in Swan Bay. The diet of juvenile flounder in Swan Bay was almost entirely composed of epibenthic harpacticoids. Juvenile flounder in Port Phillip Bay ate epibenthic harpacticoids, harpacticoid nauplii and gammaridean amphipods; interstitial harpacticoids were apparently not susceptible to flatfish predation. In Swan Bay, there was a rapid increase in the number of prey consumed with fish size but the mean size of prey was constant, reflecting the size distribution of harpacticoids in the environment. At one site in Port Phillip Bay, the pattern was similar except that the increase in prey number with growth was much lower. At a second site, prey number decreased while prey size increased with increasing fish size, due to a replacement of harpacticoid nauplii with much lower numbers of gammaridean amphipods. Consumption of prey in terms of weight increased allometrically with fish size at all sites. The rate was significantly higher (80%) in Swan Bay, suggesting that fish in Port Phillip Bay were food-limited. Gut clearance times for juvenile flounder feeding on epibenthic harpacticoids was estimated to be ≈ 13 h. Feeding rate, in terms of prey number, was approximately three to four times higher in Swan Bay relative to Port Phillip Bay. Although abundance of juvenile flounder in Swan Bay was higher than in Port Phillip Bay, prey abundances were also much higher, and predatory impact was estimated to be insignificant. Impact of predation also appeared insignificant in Port Phillip Bay although more data are needed.