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

Abstract

The effects of two types of otter trawl on macrobenthos (mainly sponges, soft corals, and gorgonians) were measured in an experiment involving repeated trawling of a marked area interspersed with video transects to estimate density 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 with other studies indicates that macrobenthos mortality can vary greatly depending on how an otter trawl is rigged. The experimental estimate of one-pass 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 management response to the problem of benthos mortality in the trawl fishery has been 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 effort. (C) 2000 International Council for the Exploration of the Sea.