UC San Diego

Tuna purse seine vessels fishing for Thunnus albacares was a cause for significant incidental bycatch of dolphins and subsequent depletion of their populations in the Eastern Tropical Pacific (ETP) in the 1960s and 1970s, until the establishment of the United States Marine Mammal Protection Act (US MMPA) in 1972. Some of the most heavily affected species are spotted dolphins (Stenella attenuata), the abundances of which have been estimated to have been reduced by up to one-fifth of their size. Despite a marked decrease in dolphin mortality and incidental take post implementation of the US MMPA, populations have not been recovering at expected rates. One hypothesis for this is that repeated chase, encirclement, and release by purse seiners affects reproduction or population growth rates in ways other than directly observed mortality. Since these effects are related to how often a given dolphin is likely to interact with the fishery and the fishery is more likely to target large schools that carry large amounts of tuna, it is imperative to understand how schools of pelagic dolphins arrange themselves in the ETP. This study uses data from the Stenella Abundance Research Line Transect and Ecosystem (STAR-LITE) cruise from 2007 to determine how schools of different sizes are arrayed in space and time. We then illustrate how these results can improve estimates of indirect effects of purse seine fishing on the recovery of these depleted populations.