Applying novel approaches to old datasets : utilizing opportunistic observations and Bayesian estimation to describe spatial use patterns for Steller sea lions

Abstract

Despite two decades of satellite telemetry studies conducted on Steller sea lions, scientists still lack basic spatially-explicit knowledge about Steller sea lion habitat use. The Platforms of Opportunity data collected by the National Marine Fisheries Service contain Steller sea lion sighting records throughout the species' entire range and have the potential to fill the critical gap in knowledge about what areas Steller sea lions are using. The Platforms of Opportunity data have not previously been used to identify marine mammal habitat because they contain sightings without associated effort records (e.g. time spent surveying or area sampled). In this study a novel approach was used to overcome this issue through development of an effort index that allowed for calculation of effort-corrected Steller sea lion encounter rates. A Bayesian negative binomial model was used to quantify both the encounter rate and the uncertainty surrounding that rate within 15 km² grid cells across the species' entire range. Year-round encounter rate estimates were derived in addition to breeding and non-breeding season encounter rates. Although the results of this analysis confirmed many of the areas known to be important Steller sea lion habitat, several previously unrecognized high-use areas were identified. Current critical habitat designated areas only encompass about 37% of high use areas estimated using this methodology.