Estimation of relative exposure of dolphins to fishery activity

For the past half-century, the purse seine fishery for yellowfin tuna has been a significant factor in the lives of dolphins in the eastern tropical Pacific (ETP). However, little is known about how frequently an individual dolphin is exposed to the fishery, and there are no methods available for accurately assessing the prior exposure of dolphins encountered at sea. Here we present a method to estimate an index of exposure based on a model of dolphin movement derived from data collected from multiple tracking studies. Based on this movement model, the method weights purse seine sets given their distance from a particular school of dolphins sighted at sea and how long ago they occurred. The method also takes into account the species composition and school size in the set. As a demonstration, we use the method to examine the spatial and temporal distribution of this index over an 11 yr period for which we have detailed data on purse seine sets. While the method has been designed for examining exposure to the ETP purse seine fishery, it is also applicable to studies of other anthropogenic effects where there is concern about exposure rates, such as underwater sound, pollution, or ship strikes.