DISTURBANCE OF THE MARINE BENTHIC HABITAT BY COMMERCIAL FISHING - IMPACTS AT THE SCALE OF THE FISHERY

Commercial fishing is one of the most important human impacts on the m arine benthic environment. One such impact is through disturbance to b enthic habitats as fishing gear (trawls and dredges) are dragged acros s the seafloor. While the direct effects of such an impact on benthic communities appear obvious, the magnitude of the effects has been very difficult to evaluate. Experimental fishing-disturbance studies have demonstrated changes in small areas; however, the broader scale implic ations attributing these changes to fishing impacts are based on long- term data and have been considered equivocal. By testing a series of a priori predictions derived from the literature (mainly results of sma ll-scale experiments), we attempted to identify changes in benthic com munities at the regional scale that could be attributed to commercial fishing. Samples along a putative gradient of fishing pressure were co llected from 18 sites in the Hauraki Gulf, New Zealand. These sites va ried in water depth from similar to 17 to 35 m and in sediment charact eristics from similar to 1 to 48% mud and from 3 to 8.5 mu g chlorophy ll alpha/cm(3). Video transects were used for counting large epifauna and grab/suction dredge and core sampling were used for collecting mac rofauna. After accounting for the effects of location and sediment cha racteristics, 15-20% of the variability in the macrofauna community co mposition sampled in the cores and grab/suction dredge samples was att ributed to fishing. With decreasing fishing pressure we observed incre ases in the density of echinoderms, long-lived surface dwellers, total number of species and individuals, and the Shannon-Weiner diversity i ndex. In addition, there were decreases in the density of deposit feed ers, small opportunists, and the ratio of small to large individuals o f the infaunal heart urchin, Echinocardium australe. The effects of fi shing on the larger macrofauna collected from the grab/suction dredge samples were not as clear. However, changes in the predicted direction in epifaunal density and the total number of individuals were demonst rated. As predicted, decreased fishing pressure significantly increase d the density of large epifauna observed in video transects. Our data provide evidence of broad-scale changes in benthic communities that ca n be directly related to fishing. As these changes were identifiable o ver broad spatial scales they are likely to have important ramificatio ns for ecosystem management and the development of sustainable fisheri es.