Quantitative indicators of environmental sustainability risk for a tropical shelf trawl fishery

Abstract

Indicators of trawl exposure were developed for 837 bycatch and benthos species, assemblages and habitats on the Great Barrier Reef shelf, by analysing their spatial distributions (mapped by a previous study) in relation to management zones, overlap with trawl grounds, and the intensity of trawl effort – and estimating the proportion of their distributions exposed to trawling. Exposure to trawl intensity as a swept-coverage was a more sensitive indicator than exposure to trawled grounds or exposure as permitted by management zones. Few habitats and assemblages were highly exposed. About 33 species had high exposure to trawl effort, whereas approximately 70% of the 837 species had low exposure. The indicators for species were extended, using relative catch rates, to estimate the proportion of populations caught annually (exploitation rate). Five species had high estimated exploitation rates and 28 were intermediate, whereas most (>800) species had low exploitation rates. The productivity potential of species to counter the incidental catch was assessed using recovery scores from life history traits. This qualitative approach indicated species at higher relative risk due to trawling. A quantitative indicator of absolute sustainability was estimated using available natural mortality rates to calculate the proportion of fishing mortality at maximum sustainable yield (FMSY). Three species exceeded a limit reference point (≅1.0×FMSY), one species exceeded a first conservative reference point (≅0.8×FMSY) and two others exceeded a second conservative reference point (≅0.6×FMSY). While few species were assessed at high risk, there were uncertainties in the distributions, relative catch rates, and natural mortality rates that required a precautionary response, including considering additional species with high indicator values. The species identified as high risk by the quantitative sustainability indicator and by the qualitative productivity scores corresponded poorly. This raises concerns about the reliability of qualitative approaches often used to conduct risk assessments for data-poor species.