A novel presence-only validation technique for improved Steller sea lion Eumetopias jubatus critical habitat descriptions

Abstract

We used published information about foraging behaviour, terrestrial resting sites, bathymetry and seasonal ocean climate to develop hypotheses relating life-history traits and physical variables to the at-sea habitat of a wide-ranging marine predator, the Steller sea lion Eumetopias jubatus. We used these hypotheses to develop a series of habitat models predicting the probability of sea lions occurring within a 3 × 3 km 2 grid in the Gulf of Alaska and the Bering Sea. We compared these deductive model predictions with opportunistic at-sea observations of sea lions (presence-only data) using (1) a likelihood approach in a small area where effort was assumed to be uniformly distributed and (2) an adjusted skewness (Skadj) test that evaluated the distribution of the predicted values associated with true presence observations. We found that the Skadj statistic was comparable to the likelihood test when using pseudo-absence data, but it was more powerful for assessing the relative performance of the different predictive spatial models across the entire study area. The habi- tat maps we produced for adult female sea lions using the deductive modelling approach captured a higher proportion of presence observations than the current habitat model (critical habitat) used by fisheries managers since 1993 to manage Steller sea lions. Such improved predictions of habitat are necessary to effectively design, implement and evaluate fishery mitigation measures. The deductive approach we propose is suitable for modelling the habitat use of other age and sex classes, and for integrating these age/sex-class-specific models into a revised definition of critical habitat for Steller sea lions. The skewness test provides a means of comparing the relative performance of such models, using presence-only data. The approach can be readily applied to other central-place foragers.