Incidental dolphin capture in a Western Australian trawl fishery: Bycatch reduction devices no silver bullet

Abstract

An estimated 350 dolphins were incidentally caught between late 2003 and 2009 in the Pilbara trawl fishery, north-western Australia. Data from skipper logbooks and independent observers were used to assess spatial and temporal patterns of dolphin bycatch in this fishery. Both datasets indicated that dolphins were caught throughout all four of the fishery management areas (total area ca. 13,000 km2), across all depths, and all year-round. However, based on a smaller number of trawls, independent observer records better explained the variation in dolphin bycatch than did the skipper logbooks. The most significant predictors of dolphin bycatch were: (1) which vessel was fishing, with one of the four vessels catching the greatest proportion of dolphins; (2) the time of day of fishing activity (corrected for effort, the lowest dolphin capture rate occurred between 00:00 and 05:59 when the least fishing occurred); and (3) whether nets included bycatch reduction devices (BRDs) (there was a 50% reduction in dolphin bycatch after the introduction of BRDs). Nevertheless, concurrent research using underwater video footage collected within actively fishing trawl nets indicated that BRDs with bottom-opening escape hatches lead to the under-estimation of bycatch as some dead dolphins fall out during the trawl and are thus not recorded on deck. These results suggest that spatial or seasonal adjustments to trawling effort, other than an overall reduction in fishing across the fishery, would be unlikely to reduce dolphin bycatch rates. Trials of BRDs with top-opening escape hatches, from which air-breathing megafauna might escape, have been recommended, with both observers and underwater video camera deployments to validate the design’s efficacy. The vulnerability of this dolphin population to the current level of trawling remains unknown without information on dolphin abundance and the genetic connectivity of this population with adjacent populations