Abundance, Diet and predators of juvenile banana prawns, Penaeus merguiensis, in a tropical mangrove estuary

Abstract

Sampling of Penaeus merguiensis de Man with a variety of nets in several microhabitats of a small mangrove-lined estuary near Townsville on seven occasions during 1985 and 1986 captured only one adult prawn. Juveniles of 1-21 mm carapace length (CL) entered the mangrove forest at high tide throughout the year in concentrations up to 0.45 m^-3. The mean density of juveniles (3-23 mm CL) in small creeks, which drain into the mainstream of the estuary, was highest in April (41 m^-2) and lowest in August (0.3 m^-2). The diet of prawns varied with size and microhabitat within the estuary. In creeks, flocculent detrital material (FD) composed of organic aggregates, but without recognizable mangrove detritus, was the most important food item for three size groups of prawns. However, the importance of FD decreased with prawn size as did the proportions of small prey such as diatoms, ostracods and harpacticoid copepods in the diet. The importance of recognizable mangrove detritus increased with prawn size, but never made up more than 15% of the volume of the diet. Insects, mainly dipteran larvae and ants, were 15% of the dietary volume of medium sized prawns (11-15 mm CL) in creeks. On shallow banks on the edge of the maintstream of the estuary, prawns consumed more animal tissue than detritus, and polychaetes and fragments of large crustaceans were important items in the diet. Seven of the 122 species of fish captured in the estuary consumed P. merguiensis. Young barramundi, Lates calcarifer (Bloch), (30-50 cm total length) were the most important predators on juvenile P. merguiensis, with prawns making up 22% by volume of their diets. The possible causes of the link between juvenile P. merguiensis and mangroves are discussed.