An assessment of dolphin behaviour and bycatch mitigation techniques in the Pilbara Fish Trawl Fishery, Western Australia

The incidental capture of unwanted species along with target species is a common and wasteful aspect of many fisheries. Trawl fisheries in particular have a high rate of incidental capture, or bycatch, due to the low selectivity of their nets. The bycatch of protected, endangered and threatened species is of particular concern and poses significant challenges for the ecologically sustainable management of wild capture fisheries. In the Pilbara Fish Trawl Interim Managed Fishery (PFTIMF), approximately 50 common bottlenose dolphins (Tursiops truncatus) are reportedly caught incidentally every year. Failure to adequately address the bycatch of this protected species has prevented the PFTIMF from attaining ‘Managed Fishery’ status. In this study, I analysed 195 hours of underwater video footage recorded inside active trawl nets in the PFTIMF to address the following aims: Chapter 2: a) examine and describe fine-scale dolphin behaviour in and around the trawl nets, and b) discuss the implications of these interactions for the reduction of dolphin bycatch in the fishery; and Chapter 3: a) assess the efficacy of two bycatch reduction devices (BRDs) at reducing the catch of dolphins and five other taxonomic groups of concern, and b) provide an estimate of the number of wildlife species caught annually in the PFTIMF. Dolphins frequently interacted with active trawl nets, and were recorded inside and outside the net for up to 98% and 99% of the trawl duration, respectively. Inside the net, dolphins exhibited a significantly higher proportion of foraging behaviours than outside the net, which may indicate that dolphins enter trawl nets because they provide a concentrated food source. Outside the net, dolphins most frequently engaged in ‘trampolining’, whereby they bounced on the upper surface of the net, often rubbing different body parts on the mesh. It thus appears that dolphins are motivated by several factors to approach and interact with active trawl nets. Most of the 29