Are bycatch rates sufficient as the principal fishery performance measure and method of assessment for seabirds?

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. 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. 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. 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. 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.