Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns ( Penaeus semisulcatus ): results from field experiments in small enclosures

Abstract

Fine mesh enclosures (0.9 m 2 in basal area, 1 m high, with 100 μm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m -2) on a high biomass seagrass bed (about 70 g m -2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m -2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week -1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week -1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week -1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.