Comparing factors associated with total and dead sooty shearwater bycatch in New Zealand trawl fisheries

Incidental capture of seabirds is a conservation concern because such periodic ‘bycatch’ may cause population declines. Not all birds die upon capture, but distinctions between total and dead captures are rarely done. Thus, it is currently unclear whether using total captures is an adequate simplification, for example when studying factors associated with bycatch. We investigated this question by analysing total and dead procellariiform seabird captures of 663 trawling operations between 1996 and 2008 in the Exclusive Economic Zone of New Zealand. As potential factors associated with bycatch, we considered eleven technical and seven environmental characteristics recorded by onboard observers during commercial trawling. A total of 1 231 procellariids were recovered as bycatch, with sooty shearwaters (Puffinus griseus) comprising 98% of the bycatch sample. Our analyses indicated that specific technical (headline height) and environmental factors (month, daylight, sea state, area code) influenced both total and dead captures of sooty shearwaters, with similar results for both response variables. Using total captures may thus be an adequate simplification in the interpretation of sooty shearwater captures in New Zealand trawl fisheries, and practices that mitigate the overall capture of birds will be most effective.