A comparison of demersal communities in an area closed to trawling with those in adjacent areas open to trawling: A study in the Great Barrier Reef Marine Park, Australia

Abstract

The faunal composition of an extensive area (10,000 km 2) closed to trawling in the Great Barrier Reef Marine Park, Australia was compared with that in two adjacent areas open to fishing. Sampling was undertaken over 2 years using a dredge and a prawn trawl. For analysis, we split the study area into a northern and a southern section, and compared adjacent pairs of zones open and closed to trawling, after adjusting for a strong cross-shelf longitudinal gradient using sediment and depth data. The median coefficient of variation for the ratio of biomass in adjacent pairs of open and closed zones was around 15% for both sampling devices. However, the difference between open and closed pairs of zones was not consistent either within species or across species. Instead, these differences were consistent with the moderate difference between the northern and southern halves of the study area. Three reasons are proposed for the lack of measurable difference that could be attributed to trawling. Firstly, the level of trawling in the open area has been relatively low and, because of targeting of concentrations of prawns, it has been unevenly distributed leaving extensive areas unfished. This suggests that trawling has not had a major impact on the demersal fauna of the region. Secondly, illegal trawling in the closed area could also have confounded the results of the study, though logbook records suggest the level of this fishing was low and its extent restricted. Thirdly, the complexity of the region as shown by a north–south difference in fauna of the closed area might obscure any effects of fishing. We also suggest that areas with high habitat diversity ranging from complex structured seabed to relatively clear open muddy or sandy plains do not lend themselves to open versus closed studies because of the differential effect of fishing on the various habitats.