Pinniped blubber supports multiple functions including thermoregulation, reproduction, and buoyancy. Although blubber depth is frequently used as an indicator of health, the effect of sample site and seasonality are rarely taken into account. We monitored blubber depth via imaging ultrasound at 10 sites monthly for 1 year in 2 adult Steller's sea lions (Eumetopias jubatus) and 3 adult harbor seals (Phoca vitulina). Blubber of harbor seals was thicker and more variable than blubber of sea lions, and was thickest in winter. Changes in harbor seal blubber at all sites tracked variation due to season and mass. Sea lion blubber changed with mass only at specific sites, with no seasonal effect. The differing effects of season, mass, and location on the body must be carefully considered in both species before any interpretation of condition.