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Independent observations of catches and subsurface mitigation efficiencies of modified trawl nets for endangered, threatened and protected megafauna bycatch in the Pilbara Fish Trawl Fishery

Wakefield, C.B., Blight, S., Dorman, S.R., Denham, A., Newman, S.J., Wakeford, J., Molony, B.W., Thomson, A.W., Syers, C. and O'Donoghue, S. (2014) Independent observations of catches and subsurface mitigation efficiencies of modified trawl nets for endangered, threatened and protected megafauna bycatch in the Pilbara Fish Trawl Fishery. Department of Fisheries, Western Australia.

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Mitigation of endangered, threatened and protected (ETP) species is a challenge in many commercial fisheries globally and independent observer programs are often implemented to determine accurate estimates of interaction rates. However, interactions with ETP species may be extremely rare requiring very high and therefore costly levels of observer coverage to provide adequate statistical rigor for such programs. The Pilbara Fish Trawl (Interim Managed) Fishery (PFTF) has a long history of developing and adopting mitigation measures that have resulted in very low capture rates of ETP megafauna, i.e. dolphins, turtles, sea snakes and sawfish. However, there has been uncertainty over the potential for unaccounted mortality of ETP megafauna from subsurface expulsion in poor condition through escape hatches in the PFTF trawl nets (particularly air breathing species). To examine this issue, all trawl vessels in the PFTF (n = 3) were fitted with dual-lens above water and subsurface within-net camera systems from June to December 2012. Above water cameras recorded continuously (except during malfunctions) and all video files were stored in read only folders and encrypted with passwords to prevent tampering. At the end of each trip these secure folders containing the video files were transferred onto external hard drives by Department of Fisheries staff for later analysis. The observer coverage rates of 85.2% of trawl catches above water (n = 1,916 trawls observed), and 71.7% of day-trawls (n = 774 trawls observed) and 53.9% of day-trawl hours (n = 1,013 h observed) below water, far exceeded that stipulated in the Bycatch Action Plan (22%) and levels achieved from previous studies from the PFTF. Captures of ETP megafauna were rare, despite very high levels of attendance in and around trawl nets by bottlenose dolphins (> 75% of trawls). All observed catches of ETP species were reported in statutory logbooks and these catches were consistent with previous data since exclusion grids were mandated in March 2006. Therefore, there was no evidence to suggest that captures of ETP species were being unreported by commercial fishers. About two thirds of all megafauna, including chondrichthyans, were expelled from escape hatches during trawling, with the majority of megafauna expelled relatively quickly (< 10 min). This resulted in more than half of the trawl catches containing no megafauna (51.4%). A total of 705 megafauna individuals were observed to exit the nets through an escape hatch during trawling. Of these megafauna, only one bottlenose dolphin was observed to exit these trawls in poor condition. A large turtle was observed to persist in a trawl for an extended period (60.1 min). However, despite its condition being inconclusive upon exiting, its duration in the net was well within the breath holding capabilities for marine turtles. Thus, the subsurface expulsion of megafauna in poor condition was extremely rare. No megafauna were observed to exit through the top opening escape slit. However, an upward excluding grid with a top opening escape hatch resulted in a higher proportion of escapement for most chondrichthyans. The loss of targeted scalefish through escape hatches occurred during less than 3% of trawls. Extensive subsurface observations determined that current mitigation strategies are highly effective for sea snakes, turtles and chondrichthyans (except sawfish), and that further investigation in the forward sections of trawl nets may provide useful information to improve mitigation strategies for dolphins and sawfish. The very low rates of mortalities of these ETP megafauna by the PFTF were considered to pose a negligible risk to their sustainability based on 1) these rates likely to be less than their natural mortality rates (e.g. at least 371 bottlenose dolphins stranded from 1981-2010), 2) they appear abundant in Western Australian waters despite large scale mortalities from historic foreign fishing (e.g. 13,459 cetacean mortalities from Taiwanese fishing from 1981-86), and 3) they have wide distributions and are highly mobile.

Item Type: Report
Series Name: Fisheries Research Report No. 244, 2014
Publisher: Department of Fisheries
Copyright: © Department of Fisheries, Western Australia.
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