Beneath the surface: profiling blubber depth in pinnipeds with infrared imaging

Infrared thermography (IRT) was assessed as a non-invasive tool to evaluate body condition in juvenile female harbor seals (Phoca vitulina), (n=6) and adult female Steller sea lions (Eumetopias jubatus), (n=2). Surface temperature determined by IRT and blubber depth assessed with portable imaging ultrasound were monitored concurrently at eight body sites over the course of a year in long-term captive individuals under controlled conditions. Site-specific differences in surface temperature were noted between winter and summer in both species. Overall, surface temperature was slightly higher and more variable in harbor seals (9.8±0.6°C) than Steller sea lions (9.1±0.5°C). Limited site-specific relationships were found between surface temperature and blubber thickness, however, insulation level alone explained a very small portion of the variance. Therefore, while validated IRT data collection can potentially provide valuable information on the health, condition and metabolic state of an animal, it cannot provide a generalized proxy for blubber depth.