A comparison of catches of fishes and invertebrates by two light trap designs, in tropical NW Australia

Light traps were deployed in two sampling programs. In the first, small and large traps were released to drift with the current at stations along a cr oss-shelf transect on the NW Shelf off the coast of Western Australia. In t he second program, pairs of small and large traps were deployed on moorings 150 m off the coastline. The composition and size-frequency distributions of catches of fishes in small and large traps were similar for both modes o f deployment. In drifting traps, nearly 78% of this catch was composed of r eef fishes, and these were collected in significantly greater numbers by th e small design than by large traps (9.51 vs. 5.84 individuals h(-1), respec tively). Nine taxa (amphipods, mysids, crab megalopae, copepods, cumaceans, isopods, caridean shrimps, polychaetes and the euphausiid, Pseudeuphausia latifrons) accounted for 99% of the total catch of invertebrates by driftin g traps. Of these, catches of amphipods. copepods, cumaceans and P. latifro ns were greater in large traps than in small traps (3,134 vs. 1,687 h(-1), 1,018 vs. 214 h(-1), 551 vs. 165 h(-1) and 74 vs. 9 individuals h(-1), resp ectively). In contrast, crab megalopae were more abundant in catches by sma ll traps than by large traps (3,134 vs. 1,687 individuals h(-1), respective ly). The catch rate of fishes in moored traps was higher than in drifting t raps (105 vs. 20 fishes h(-1)) and was dominated by baitfishes (86% of tota l catch). Reef fishes were also captured in greater numbers by small traps than by the large design (10.17 vs. 4.4 individuals h(-1)) in this mode of deployment. Despite these differences in catch rates, multivariate analysis showed that cross-shelf patterns in catches of fishes and invertebrates we re mapped equally well by both trap designs. Variation in the efficiency of trap designs thus appears to be small when compared to changes in the comp osition and abundance of zooplankton assemblages that occur at scales of te ns of kilometers.