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

Fine mesh enclosures (0.9 m(2) in basal area, I in high, with 100 im mesh) and a jet-net retrieval system were developed to test the influence of juve nile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds i n the Embley River estuary of tropical Australia. Small juvenile tiger praw ns (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 seag rass bed (about 70 g m(-2) of mostly Enhalus aeoroides) and one with low bi omass (about 10 g m(-2) of mostly Halodule uninervis). After 2-3 weeks in t he enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However. not al l 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 Halo dule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential fo r interference competition between prawns, compared with the low biomass Ha lodule. Growth rates on Enhalus were significantly faster at a stocking den sity of 4 prawns per enclosure (1.3 mm CL week(-1)) than at a stocking dens ity of 16 and 32 prawns per enclosure (both 0.8 mm CL week(-1)), and did no t differ significantly between April and October/November (temperatures wer e about 30 degreesC at both times). The mean growth rate at 8 prawns per en closure (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 sugges t that the carrying capacity for juvenile tiger prawns was greater in the h igh biomass Enhalus than the low biomass Halodule bed.