Implications of the effects of trawling on sessile megazoobenthos on a tropical shelf in northeastern Australia

We estimate the possible overall status of populations of attached seabed fauna after 20 years of trawling in Australia's Great Barrier Reef(GBR), based on the key results of a five-year experimental study that provided an understanding of faunal resilience, in terms of removal rates per trawl. The removal rates of most seabed fauna were between 5 and 20% per trawl (range 0-40%). In attempting to estimate population status, it was also necessary to review patterns of trawl effort intensity and add a simple model for possible recovery dynamics of fauna. Large areas of the GBR are subject to trawling. In 1996, effort was recorded in 1300 statistical grids, each 6 x 6 minutes, an area equivalent to similar to 153 000 km(2). Effort was highly aggregated among the grids, with about 20%;, concentrated into <5% of trawled grounds (intensive); at the other extreme, about 20% of the effort was spread over about 60% of the trawled grounds (extensive). Trawling was also highly aggregated at fine scales within grids; consequently a smaller area is actually trawled than is indicated by summing up 6 min grids. The amount of fauna removed each year is related to the resilience of the fauna to removal, the intensity of trawling, and its degree of aggregation. In lightly trawled grids, the annual removal may have been only a low percentage, but in the most intensively trawled grids, more than 80% of the least resilient fauna may be removed each year. In high-effort grids, aggregated trawling removes smaller amounts of benthos than if effort were distributed randomly or uniformly. The average annual removal of fauna over all trawled grids differs for different fauna. A total of about 4% of high-resilience fauna may be removed, 8% of medium-resilience fauna, and similar to 15% of low-resilience fauna. The overall vulnerability of fauna is a combination of resilience and recovery rates. Fauna with no capacity for recovery will eventually be completely removed From all trawled areas. All fauna with a capacity for recovery have the potential For sustaining a population level in balance with the amount removed by trawling, up to certain limits. The most vulnerable fauna may be completely removed From the 5-10% of grids that are trawled with >2000-3000 h of effort. More fauna will be removed from grids with higher effort. Though 50-70% of trawled grids have been trawled only lightly (<700-1000 h) each year, over the last 20 years there has been a cumulative effect. A generalized depiction across all trawled grids is likely, but fauna with low vulnerability may be depleted by only 3% overall; medium vulnerability Fauna may be depleted by about 20%; and highly vulnerable populations may be depleted by about 55% overall. Because of differential vulnerability, the composition of the faunal community will be substantially altered in most grids, with a shift to less vulnerable species. (C) 2000 International Council for the Exploration of the Sea.