Incidental dolphin capture and bycatch mitigation in a Western Australian trawl fishery

The incidental capture of common bottlenose dolphins (Tursiops truncatus) is an ongoing protected species management problem in the Pilbara Fish Trawl Interim Managed Fishery, Western Australia. We investigated this issue using four approaches: the analysis of skippers’ logbook and independent observer data on bycatch; underwater video of dolphins interacting with trawl gear; genetic methods to estimate population structure and connectivity; and a photo-identification study to estimate trawler-associated dolphin community size. Logbooks and observer records were used to assess dolphin bycatch patterns from 2003 to 2009. During this six year period, between 172 and 366 dolphins were reported caught across all management areas, depths and seasons. Dolphin capture rates reported by independent observers varied between 1.6 and 3.8 times higher than those reported by skippers, with observer records also better explaining the variation in dolphin bycatch. Significant predictors of dolphin bycatch were fishing vessels; time of day; and whether nets included bycatch reduction devices. Underwater video footage taken inside trawl nets indicated that even the observer records underestimated bycatch, as some dead dolphins fell out of bottom-opening escape hatches during trawls and were not landed on deck. Genetic evidence suggested one panmictic population, but no connectivity between trawler-associated and adjacent coastal dolphins. Mark-recapture analysis of photoidentified dolphins around one of three trawlers over two oneweek fishing trips yielded a global mean estimate (± 1 SE) of just 183 ± 11 dolphins. These data indicate that a relatively small dolphin community shows fidelity to foraging around trawlers over periods ranging from weeks to years. Potentially, an overall reduction in fishing effort and improved bycatch reduction devices (with top-opening escape hatches from which air-breathing animals might escape) would reduce bycatch. The vulnerability of this dolphin population to depletion from the current and ongoing level of trawling is likely, but currently remains unknown.