Capture Rate As A Function Of School Size In Pantropical Spotted Dolphins, Stenella Attenuata, In The Eastern Tropical Pacific Ocean

The frequency with which dolphins from the northeastern offshore stock of the pantropical spotted dolphin, Stenella attenuata, experience chase and capture by tuna purse-seiners in the eastern topical Pacific Ocean was estimated by comparing dolphin school-size frequencies in sighting data from research vessel observer records, with those recorded ins vessel observers. The objective of the study was to provide a preliminary basis for estimating stock-wide effects of fishery-induced disturbance in these dolphins.Our analyses indicate two major characteristics for this stock: first, capture frequency appears to increase rapidly with increasing school size, and second, approximately half of the stock at any given time occurs in schools smaller than those apparently preferred by purse-seiners. This implies that if individual dolphins have a preference for associating with schools of a particular size, then individuals associating primarily with large schools would be subjected to chase and capture much more frequently than those associating with small schools. However, because the largest schools are relatively rare and account for a small percentage of individuals, the majority of dolphins in the stock would experience relatively few captures per year, although some would experience a high rate. It is not known whether dolphins do indeed exhibit such a preference, or if instead individuals associate with schools from a wide range of sizes at different times.Schools of 1000 or more dolphins are estimated to be set on approximately once a week each on average, but such schools are estimated to represent just under one tenth of the animals in the northeastern offshore stock. Schools set on most often by tuna purse-seiners, containing from about 250 to 500 dolphins, are estimated to be set on between two and eight times each per year and are estimated to include approximately one third of the stock. An estimated one half of the stock occurs in schools smaller than 250 animals; schools of this size are estimated to be set on less than twice per year each.