Quantifying seabird bycatch: where do we go from here?

Abstract

Bycatch of seabirds in offshore commercial fisheries has long been recognized as a serious conservation issue (e.g. Brothers, 1991; Brothers, Cooper & Lokkeborg, 1999), but until recently the problem was principally associated with industrial longline gear. In fact, pelagic seabird bycatch is very much a multi-gear problem. Watkins, Petersen & Ryan (2008) have presented the latest in a string of recent case studies documenting geographically widespread and sometimes severe levels of seabird bycatch in industrial trawl fisheries throughout the Southern Hemisphere (Weimerskirch, Capdeville & Duhamel, 2000; Sullivan, Reid & Bugoni, 2006a; Baker et al., 2007; Gonzales-Zevallos, Yorio & Caille, 2007). Substantial seabird bycatch occurs in trawl fleets of the Northern Hemisphere as well (e.g. in Alaskan trawl fisheries for groundfishes; NMFS, 2006). The issue is one of global importance that requires urgent international attention. National and international fisheries management bodies face several challenges as they move forward to address seabird bycatch within a multi-gear context. One fundamental question is what is the relative and cumulative threat to seabird populations from bycatch in trawl and longline gear? Answering this question depends on having reasonable estimates of mortality from these two gear types across the multitude of international fishing fleets with which particular seabird populations interact (e.g. Baker et al., 2007). This will require that new observer programs be established for national trawl fleets that interact with seabirds in their waters, and that international programs currently working to quantify seabird bycatch in high-seas longline fisheries (see Gilman & Moth-Poulsen, 2007) expand to quantify threats posed by trawl fisheries as well. Trawl gear poses a particularly difficult challenge for seabird bycatch estimation because a large but often unquantified proportion of birds killed by interactions with trawl cables (the dominant cause of mortality in trawl gears) is not hauled onto the vessel and therefore goes unobserved by traditional observer-data collection protocols (e.g. Gonzales-Zevallos & Yorio, 2006; Sullivan et al., 2006a). Watkins et al. (2008) observed that only two out of the 30 known seabird fatalities from cable interaction were eventually hauled aboard, corresponding at best to an overall 0.067 ( 0.046 binomial SE) detection rate for this type of mortality if only haul data had been used to estimated bycatch. This indicates the need to update observer protocols of seabird bycatch monitoring in trawl fisheries, to ensure proper accounting of bird mortalities resulting from cable collisions. Alternative methods for monitoring seabird bycatch in trawl fisheries should also be explored, as observer programs appropriate for monitoring seabird–trawl cable interactions are labor intensive and therefore expensive. Watkins et al. (2008) were only able to observe 0.5% of annual trawl fishing effort for this reason, and even some of the most well-funded fishery observer programs in the world (e.g. several in the United States) are too overstretched financially to meet observer coverage goals (Rossman, 2007). One promising approach is video monitoring (McElderry et al., 2004; Ames, Williams & Fitzgerald, 2005), which Watkins et al. (2008) used with apparent success during the first part of their study. Another strategy may be to augment traditional observer-data collection protocols, which are based on sampling hauled-in bycatch, over the long term with shorter intensive studies aimed at estimating species composition and non-detection rates for seabirds killed by cable interactions that are not retrieved during hauling of gear. These might then be applied as correction factors to statistically estimate actual seabird bycatch in trawls from data collected using conventional observer protocols. To our knowledge, Watkins et al. (2008) were the first to provide some empirical inference about the rate of bycatch non-detection in trawl fisheries and we encourage future studies to report similar data and estimates. As research and management programs respond to increase collection of trawl bycatch data, it would be helpful to adopt common metrics for reporting trawl bycatch. Watkins et al. (2008) reported bycatch per trawl hour, which