CSIRO Marine Research, CLEVELAND, AUSTRALIA
The seabed habitats in the northern Great Barrier Reef (GBR) study area have high diversity. The biota in areas open and closed to trawling showed few significant differences, though this was not a direct comparison of trawled vs untrawled areas. In an experiment where treatment plots were trawled entirely once-over and compared with control plots, ~38t of demersal biota was removed, but despite this impact, no differences between the trawled and control plots could be detected. In a second experiment, the effect of intensive trawling was measured — each repeated-trawl removed 5-20% of the biota and 13 trawls removed 70–90% of the initial biomass. Thus, though the impacts of trawling may not be detectable in areas trawled sparsely or infrequently, the cumulative effect of intensive trawling on vulnerable fauna is substantial. Consequently, the overall effects of trawling are dependent on the patchy distribution and intensity of effort. The experimental results were extended to estimate the annual removal of attached epifauna on trawl grounds in the GBR as well as their relative population status after 20 years of trawling. This suggested that vulnerable fauna have been depleted, changing the composition of the faunal community. The absolute status of the fauna depends on at least two unknowns — their original pattern of distribution relative to trawling, and their ability to recover. Studies of recovery rates are continuing.