Depletion of spotted and spinner dolphins in the eastern tropical Pacific: modeling hypotheses for their lack of recovery

We assess the status of 2 dolphin stocks affected by purse-seine fishing in the eastern topical Pacific and evaluate hypotheses for their lack of recovery. We use Bayesian methods and fit generalized models of logistic population growth to abundance estimates for northeastern offshore spotted dolphins Stenella attenuata attenuata and eastern spinner dolphins Stenella longirostris orientalis. In 2002, using the definition of depletion stipulated in the USA Marine Mammal Protection Act, northeastern offshore spotted dolphins were almost certainly ‘depleted’. There is, however, uncertainty in the degree to which the stock was depleted. Eastern spinner dolphins were most likely depleted in 2002, but there is a small probability that this was not the case. Uncertainty in the degree to which both stocks were depleted stems from uncertainties in maximum net productivity levels and carrying capacities. Based on abundance data from 1979 to 2000, both stocks were estimated to have had maximum growth rates of <3% yr–1 with >77% probability, lower than the accepted minimum default value for dolphin populations with reproductive parameters (e.g. calving intervals) like those considered here (Reilly & Barlow 1986, Fish Bull 84:527–533; Wade 1998, Mar Mamm Sci 14:1–37). We fit models that are intended to be indicative of hypotheses that explain why neither dolphin stock has recovered. Our data and prior information provide equal posterior support to hypotheses which attribute the lack of recovery to the fishery and changes in the ecosystem. We conclude that (1) the purse-seine fishery can impact dolphin stocks beyond the impacts of observed fishery mortality, (2) there is uncertainty about the degrees to which such cryptic impacts have population-level consequences, and (3) the existing dolphin-safe labeling standard is, from a conservation perspective, robust to this uncertainty.