Effects of otter trawling on macrobenthos and management of demersal scalefish fisheries on the continental shelf of north-western Australia

The effects of two types of otter trawl on macrobenthos (mainly sponges, so ft corals, and gorgonians) were measured in an experiment involving repeate d trawling of a marked area interspersed with video transects to estimate d ensity of benthos. The gears tested were a demersal otter trawl and a semi- pelagic trawl fished approximately 15 cm above the seabed. Fishing with the semi-pelagic trawl had no measurable effect, whereas the standard demersal trawl reduced benthos density by 15.5% on each tow through the site. Only 4% of the benthos detached was actually retained in the net. Comparison wit h other studies indicates that macrobenthos mortality can vary greatly depe nding on how an otter trawl is rigged. The experimental estimate of one-pas s mortality was combined with the frequency and distribution of commercial trawling to estimate patterns of annual mortality of macrobenthos in 6-min square blocks throughout the area where the fishery operates. The managemen t response to the problem of benthos mortality in the trawl fishery has bee n to limit trawling for scalefish to a small proportion of the area of the continental shelf and to control the level and distribution of trawling eff ort. (C) 2000 International Council for the Exploration of the Sea.