Are bycatch rates sufficient as the principal fishery performance measure and method of assessment for seabirds? GEOFFREY NEIL TUCK * CSIRO Marine and Atmospheric Research, Wealth from Oceans Flagship, Castray Esplanade, Hobart, Tasmania, Australia, 7000 ABSTRACT 1. Seabird bycatch in oceanic and coastal fisheries is believed to be the principal cause of the population declines that have been observed for many seabird populations. Pelagic and demersal longline and trawl operations have been implicated in these declines. 2.Many government and national fishery bodies have environmental and fishery legislation that requires fisheries to be managed in a manner that is not harmful to non‐target species, including seabirds. A common tool for measuring the impact of a fishery on incidentally caught seabirds is an estimate of the bycatch rate. Unfortunately, the estimation and interpretation of bycatch rates is not trivial and is complicated by poor observer coverage, under‐reporting of bycatch and data lacking species specificity. 3. In this paper, a stochastic simulation model of two seabird populations affected by a single fishing fleet is presented. The model is used to explore the consequences of applying a bycatch rate management control rule to assess and manage the fishery’s incidental mortality of seabirds. Comparisons are also made with values derived from Potential Biological Removal (PBR) theory. 4.Results conclude that using bycatch rates as a measure to assess performance of the fishery and to reduce bycatch is, under many circumstances, not sufficient to achieve conservation goals. Bycatch rates can be within the limit recommended by management, giving the impression that the fishery has reduced bycatch to sustainable levels, when in fact the low rates are due to the populations having collapsed. 5.The interpretation of bycatch rates, and any subsequent bycatch rate management rules, needs to be considered with respect to changes in fishing effort, to population‐specific impacts, to levels of compliance, and to the robustness of the bycatch rate estimate. Simply applying a bycatch rate control rule without caution can lead to catastrophic results for incidentally caught populations of seabirds. Copyright # 2011 John Wiley & Sons, Ltd. Received 11 January 2011; Revised 21 May 2011; Accepted 24 May 2011 KEY WORDS: ocean; conservation evaluation; endangered species; modelling; birds; albatross; longline; bycatch management INTRODUCTION Background Incidental mortality of oceanic seabirds is recognized as a key issue of conservation concern for many populations and sub‐ populations of seabirds throughout the world. Currently 17 of 22 albatrosses are listed as threatened or endangered, with the principal identified threat being mortality associated with fishing operations (Gales, 1993; Croxall et al., 1998; Baker et al., 2002). Large scale oceanic pelagic longlining targeting tunas and billfish began in the 1950s and rapidly spread from south‐east Asia to equatorial latitudes and then to the Southern Ocean by the late 1960s (Tuck et al., 2003). The widespread foraging nature of seabirds, and albatrosses in particular, and their propensity to be attracted to the baited hooks and offal discarded by vessels on the high seas can lead to fatal interactions (Weimerskirch et al., 1997; Croxall and Gales, 1998). In addition, breeding birds are susceptible to interactions with demersal longline and trawl vessels targeting slope and shelf species such as Patagonian toothfish Dissostichus eleginoides, ling Genypterus blacodes and hake Merluccius sp. (Croxall and Prince, 1996; Favero et al., 2003). It was not until the late 1980s and early 1990s that recognition of incidental mortality of seabirds was highlighted in national and international fora (Weimerskirch and Jouventin, *Correspondence to: G. N. Tuck, CSIRO Marine and Atmospheric Research, Wealth from Oceans Flagship, Castray Esplanade, Hobart, Tasmania, Australia, 7000. E‐mail: Geoff.Tuck@csiro.au Copyright # 2011 John Wiley & Sons, Ltd. AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 21: 412–422 (2011) Published online 13 July 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/aqc.1201