Determining spatial and temporal overlap of an endangered seabird with a large commercial trawl fishery

Abstract

Spatial and temporal overlap between the short-tailed albatross Phoebastria albatrus, an endangered species, and the Alaskan groundfish trawl fleet is of concern because of the potential for incidental mortality. Due to the small size of this albatross population (ca. 2000 birds) and their wide-ranging foraging behavior, opportunistic sightings contribute the majority of the data on their distribution. In contrast, detailed information exists on fishing effort. In this study, 2 methods were used to predict albatross distributions: (1) bounded interpolation of sightings and (2) distance to the 1000 m isobath. The fishery was grouped into 5 sectors based on fish processing mode and predomi- nant product type. Locations and durations of observed trawls were associated with the predicted albatross densities and overlap was quantified using 2 metrics. Spatial and temporal overlap of alba- trosses and the trawl fleet was influenced by the assumptions used to infer albatross distribution and also differed among trawl sectors depending on the metric used to calculate overlap. The sectors with the highest and lowest overlap changed seasonally. In general, overlap scores were lowest in winter and spring due to both low trawl effort (in winter) and low albatross sightings (winter and spring). Relative effort in trawl sectors, in addition to proximity to high predicted albatross densities, con- tributed to overlap rankings such that some sectors with greater effort in areas with lower predicted albatross density had higher overlap scores than sectors with less effort in areas with high predicted albatross densities.