Indirect Effects of the Fishery

Leaping Dolphins

Eastern Spinner Dolphin with Calf.

Historically, the effect of the eastern tropical Pacific (ETP) tuna purse seine fishery has on dolphins has been measured almost entirely by the number of dolphins observed killed during sets. Findings that depleted stocks of spotted and spinner dolphins are not recovering as fast as we expect has led to concern that the fishery is having effects beyond the observed kill alone. Recent studies have indicated that calves can be permanently separated from their mothers during the set. If these calves are still under their mother’s care and dependent on them for nutrition and survival, they are likely to die when separated.

In order to address the potential effect of this scenario and other related issues, we have developed a multi-faceted, long-term research plan. The plan is designed to examine the broader question of the effect of the fishery on both the survival of young dolphins and on population reproduction. Various studies will look at the population-level effects of the fishery, encompassing such issues as the estimation of demographic parameters, the creation and testing of demographic models, and the creation of an index of fishery effort. A second group of studies will examine individual-level effects, such as the effects of a set on swimming behavior and energetics of mother/calf pairs, quantification of the magnitude of unobserved calf kill in sets, factors of a set that could contribute to unobserved calf kills, and the effects of tuna purse-seine sets on social disruption of dolphin schools and how this may in turn affect individual reproduction.