Spatial distribution of commercial dredge fishing effort: application to survey design and the spatial management of a patchily distributed benthic bivalve species

The patchy spatial distribution of many benthic commercial bivalve species plays an important role in determining the fishing and management strategies applied within a fishery. This study used fisher catch-return data and high resolution Vessel Monitoring System ( VMS) data to determine the distribution of fishing effort within a region opened to commercial dredge fishing during the 2003 Tasmanian commercial scallop ( Pecten fumatus) fishery, Australia. Fisher catch return data suggested that most ( 88%) of the open region was fished, however, fine-scale VMS data showed that 50% of the fishing effort occurred within 0.85% of the total area available to fishing, and 95% of the effort occurred within similar to 12% of the open region. The distribution of VMS inferred that the fishing effort was found to be patchy at all measured spatial scales ( 5 x km to 250 x 250 m grid cell sizes); however, the degree of patchiness decreased with lower spatial scale cell sizes. Significant differences in the dredged benthic communities recorded within regions exposed to different concentrations of fishing intensity ( heavy, moderate and low) were observed, with low and moderately fished regions containing high abundances of screwshells and their associated hermit crabs. Water depth was found to explain some of these observed differences, suggesting that the results may be the consequence of fisher behaviour over pre-existing habitat types. The observed distribution of the fishing effort and scallop beds within the Tasmanian commercial scallop, Pecten fumatus, fishery suits a closed area spatial management strategy, with the areas opened to fishing potentially being of the same scale as scallop beds ( km x km).