Patterns of Dolphin Bycatch in a North-Western Australian Trawl Fishery

Simon J Allen,Halina T Kobryn,Neil R Loneragan,Julian A Tyne,Kenneth H Pollock,Lars Bejder,Konstantinos I Stergiou

doi:10.1371/journal.pone.0093178

Simon J Allen, Halina T Kobryn + Show 5 more

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https://doi.org/10.1371/journal.pone.0093178

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Journal: PloS one	Publication Date: Apr 2, 2014
Citations: 60	License type: CC BY 4.0

Affiliation: Murdoch University, North Carolina State University

Abstract

The bycatch of small cetaceans in commercial fisheries is a global wildlife management problem. We used data from skippers' logbooks and independent observers to assess common bottlenose dolphin (Tursiops truncatus) bycatch patterns between 2003 and 2009 in the Pilbara Trawl Fishery, Western Australia. Both datasets indicated that dolphins were caught in all fishery areas, across all depths and throughout the year. Over the entire datasets, observer reported bycatch rates (n = 52 dolphins in 4,124 trawls, or 12.6 dolphins/1,000 trawls) were ca. double those reported by skippers (n = 180 dolphins in 27,904 trawls, or 6.5 dolphins/1,000 trawls). Generalised Linear Models based on observer data, which better explained the variation in dolphin bycatch, indicated that the most significant predictors of dolphin catch were: (1) vessel - one trawl vessel caught significantly more dolphins than three others assessed; (2) time of day – the lowest dolphin bycatch rates were between 00:00 and 05:59; and (3) whether nets included bycatch reduction devices (BRDs) - the rate was reduced by ca. 45%, from 18.8 to 10.3 dolphins/1,000 trawls, after their introduction. These results indicated that differences among vessels (or skippers' trawling techniques) and dolphin behavior (a diurnal pattern) influenced the rates of dolphin capture; and that spatial or seasonal adjustments to trawling effort would be unlikely to significantly reduce dolphin bycatch. Recent skipper's logbook data show that dolphin bycatch rates have not declined since those reported in 2006, when BRDs were introduced across the fishery. Modified BRDs, with top-opening escape hatches from which dolphins might escape to the surface, may be a more effective means of further reducing dolphin bycatch. The vulnerability of this dolphin population to trawling-related mortality cannot be assessed in the absence of an ongoing observer program and without information on trawler-associated dolphin community size, broader dolphin population size and connectivity with adjacent populations.

Highlights

Demersal trawl fishing for crustaceans, cephalopods and fish impacts benthic habitats and results in large quantities of incidental catch, or bycatch, of non-targeted species [1,2,3]
We evaluated detailed, long-term catch and effort data from the Pilbara Trawl Fishery to assess patterns of dolphin bycatch and determine whether the introduction of spatial and/or temporal fishery management measures might contribute to mitigating dolphin bycatch
A minimum of ca. 500 bottlenose dolphins were incidentally caught in the Pilbara Trawl Fishery in the last decade; 2

Summary

Introduction

Demersal trawl fishing for crustaceans, cephalopods and fish impacts benthic habitats and results in large quantities of incidental catch, or bycatch, of non-targeted species [1,2,3]. Gill netting and purse seining are the three largest causes of fisheries-related small cetacean mortalities worldwide [4,5,6,7]. Species of small cetaceans, such as the Maui’s dolphin (Cephalorhynchus hectori maui) of New Zealand’s North Island and the vaquita (Phocoena sinus) of the Sea of Cortez, are at risk of extinction from the cumulative impacts of fishing related mortality and disturbance from gill netting and trawl fisheries [9,10,11,12]. Massive reductions in dolphin capture rates were achieved, the impacted populations have not recovered [15][16]

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