Swimming depth and ocean currents affect transit costs in Steller sea lions Eumetopias jubatus

Transit costs associated with commuting between resting sites ashore and foraging areas at sea constitute an appreciable portion of foraging expenditures in pinnipeds. We examined transit swimming in 3 Steller sea lions Eumetopias jubatus trained to follow a moving boat at different speeds and depths. We measured dive behavior (duration) and focused specifically on activity measures (fore-flipper stroking and overall dynamic body acceleration [ODBA], an overall measure of body motion), which may be proxies for metabolic expenditure. Sea lions appeared to increase efficiency while transiting at depths that approached 3 times their body diameters (mean depth = 151 +/- 1 cm SEM, n = 87). Although the response was not uniform for all tested scenarios, all of the observed significant adjustments to dive behavior and swimming mechanics supported an increased efficiency at this depth. An increase in transit speed (4.5 versus 3.5 knots [kn] surface speed) was associated with elevated flipper stroke frequencies (+5%) and stroke output (ODBA stroke super(-1), +48%). Sea lions transiting against the flow of a tidal current had reduced dive durations (-10%), while total ODBA was consistently elevated (+8% overall). This response to tidal flow was accompanied either by elevated ODBA stroke super(-1) (3.5 kn) or a parallel increase in stroking (4.5 kn). Our data demonstrate that small changes in the physical environment affect transiting in Steller sea lions, and imply that altered prey fields or changing ocean conditions can carry energetic consequences.