Spatially explicit detection of predation on individual pinnipeds from implanted post-mortem satellite data transmitters

Abstract

To directly determine mortality and predation in the endangered western Steller sea lion Eumetopias jubatus, we deployed implanted, satellite-linked post-mortem data transmitters in 21 juveniles. Data recovered from 4 of 5 detected mortalities exhibited precipitous drops in ambient temperatures followed by immediate onset of transmissions (N = 3), or gradual cooling and delayed transmissions (N = 1). Precipitous drop data sets were classified as acute death at sea by trauma. A model to estimate algor mortis (body cooling) as a function of mass and ambient conditions was validated through simulations on 4 carcasses. Model outputs suggest that cooling rate masses can be qualitatively distinguished if well outside the prediction uncertainties. The observed gradual cooling rate was best described by a modeled mass one-sixth the animal's mass at release, supporting the classification of the fourth event as acute death at sea by trauma. This suggests that at least 4 in 5 detected mortalities likely represent acute deaths at sea, probably due to predation. We conclude that precipitous drop events with immediate transmissions can be classified as acute death likely by predation, but gradual cooling events with delayed transmission should be reviewed on a case-by-case basis. In 3 cases that provided both ante-mortem and post-mortem locations from external and implanted transmitters, respectively, these differed by less than 17 km, illustrating that this technique provides spatially explicit data of predation on individual sea lions. © Inter-Research 2009.