ESTIMATING THE EFFICIENCY OF A SMALL BEAM TRAWL FOR SAMPLING TIGER PRAWNS PENAEUS-ESCULENTUS AND P-SEMISULCATUS IN SEAGRASS BY REMOVAL EXPERIMENTS

The efficiency with which a small beam trawl (1 x 0.5 m mouth) sampled postlarvae and juveniles of tiger prawns Penaeus esculentus and P, se misulcatus at night was estimated in 3 tropical seagrass communities ( dominated by Thalassia hemprichii, Syringodium isoetifolium and Enhalu s acoroides, respectively) in the shallow waters of the Gulf of Carpen taria in northern Australia. An area of seagrass (40 x 3 m) was enclos ed by a net and the beam trawl was repeatedly hand-hauled over the sub strate. Net efficiency (q) was calculated using 4 methods: the unweigh ted Leslie, weighted Leslie, DeLury and Maximum-likelihood (ML) method s. The Maximum-likelihood is the preferred method for estimating effic iency because it makes the fewest assumptions and is not affected by z ero catches. The major difference in net efficiencies was between post larvae (mean ML q +/- 95% confidence limits = 0.66 +/- 0.16) and juven iles of both species (mean q for juveniles in water less than or equal to 1.0 m deep = 0.47 +/- 0.05), i.e. the beam trawl was more efficien t at capturing postlarvae than juveniles. There was little difference in net efficiency for P, esculentus between seagrass types (T, hempric hii versus S. isoetifolium), even though the biomass and morphologies of seagrass in these communities differed greatly (biomasses were 54 a nd 204 g m(-2), respectively). The efficiency of the net appeared to b e the same for juveniles of the 2 species in shallow water, but was lo wer for juvenile P, semisulcatus at high tide when the water was deepe r (1.6 to 1.9 m) (0.35 +/- 0.08). The lower efficiency near the time o f high tide is possibly because the prawns are more active at high tha n low tide, and can also escape above the net. Factors affecting net e fficiency and alternative methods of estimating net efficiency are dis cussed.