Observer coverage to monitor seabird captures in pelagic longline fisheries

This review aims to provide guidance on levels of observer coverage appropriate to developing seabird bycatch estimates in pelagic longline fisheries. We review observer coverage in place in pelagic longline fisheries internationally and the extent to which these enable the estimation of seabird bycatch. We identify key factors influencing the levels of observer coverage required to estimate seabird bycatch, and the limitations inherent in suboptimal levels of coverage. We also discuss the extent to which observed bycatch reflects true levels of mortality taking into consideration unobserved, or cryptic, mortality. Whilst this paper is focused on seabirds, similar principles will apply to the consideration of observer coverage required to monitor the bycatch of other species of concern (e.g. marine mammals and reptiles).