The impact of fish trawls on megabenthos (sponges) on the north-west shelf of Australia

We attempted to resolve widely different accounts of the impacts of fish trawls on large attached seabed fauna. We quantified the catch and damage by fish trawls on sponges and used a video camera in the trawl net to observe the effects of fishing gear on sponges. The depth of the 30 min trawl tows ranged from 25 to 358 m, with an average of 78.3 m. Sponges were caught in 85% of trawl tows (n = 108). The mean catch of sponges was 87.2 kg for half-an-hour (S.D. = 132.9 kg). The largest single catch of sponges was 797 kg. All catches with more than 100 kg of sponge were from waters shallower than 100 m. On the basis of our observations, we divided the sponges into two types (lump = broad base, branched = narrow base) and three height classes (<300, 301-500 and >500 mm). The size composition of sponges on the seabed, as observed on video, differed significantly from the composition of the catch: 80% of lump sponges and 100% of branched sponges less than 300 mm passed under the net; 68% of lump sponges and 80% of branched sponges between 301 and 500 mm in height passed under the net. Fewer than 3% of the intermediate-sized sponges were broken up as they passed under the net. Sponges larger than 500 mm high were impacted the most: only 30% of lump and 60% of branched sponges passed under the net. Of the 70% of lump sponges that passed into the net, at least 20% were broken into pieces. The branched sponges were either torn off by the footrope (20%) or were smashed and passed or rolled under the net (20%). Over 90% of another major megabenthic, group, the gorgonians, passed under the net. Overall, the net removed about 14% of sponges and 3% of gorgonians. The impact of commercial trawling on megabenthos is estimated for an average year. Our results provide a mechanism for the two main effects of otter trawling: (1) reduction in megabenthos and especially understanding the effects of trawling on sponges; (2) changes in species composition in trawled areas. (C) 2002 Elsevier Science B.V. All rights reserved.