Preliminary estimates of seabird bycatch from tuna longline fisheries for the southern Atlantic and southwestern Indian Oceans, based on three different methods

Population declines of many seabirds, including albatross and petrels, are caused by a range of impacts, notably environmental change and fisheries bycatch. Despite the scale and importance of longline fishing in the southern hemisphere, the impact of this type of fishery on seabird populations is poorly understood. To date, there has been no broad scale fleet-specific assessment of seabird bycatch throughout the southern hemisphere, mainly due to the spatial and temporal limitations in observer data coverage. Here we use three approaches to estimate total bird bycatch across the southern Atlantic and southwestern Indian Oceans: (1) a simple, stratified, ratio based estimator, (2) generalised additive models (GAMs) and (3) the computationally intensive Integrated Nested Laplace Algorithms (INLA). To estimate the total birds captured (N), stratified estimates of Bird catch Per Unit of Effort (BPUE) were multiplied with the total reported pelagic longline effort . A comparison of preliminary estimates of N based on a common data set is presented to illustrate the various methods.