Drag, but not buoyancy, affects swim speed in captive Steller sea lions.
Swimming at an optimal speed is critical for breath-hold divers seeking to maximize the time they can spend foraging underwater. Theoretical studies have predicted that the optimal swim speed for an animal while transiting to and from depth is independent of buoyancy, but is dependent on drag and metabolic rate. However, this prediction has never been experimentally tested. Our study assessed the effects of buoyancy and drag on the swim speed of three captive Steller sea lions (Eumetopias jubatus) that made 186 dives. Our study animals were trained to dive to feed at fixed depths (10-50 m) under artificially controlled buoyancy and drag conditions. Buoyancy and drag were manipulated using a pair of polyvinyl chloride (PVC) tubes attached to harnesses worn by the sea lions, and buoyancy conditions were designed to fall within the natural range of wild animals (∼12-26% subcutaneous fat). Drag conditions were changed with and without the PVC tubes, and swim speeds were recorded and compared during descent and ascent phases using an accelerometer attached to the harnesses. Generalized linear mixed-effect models with the animal as the random variable and five explanatory variables (body mass, buoyancy, dive depth, dive phase, and drag) showed that swim speed was best predicted by two variables, drag and dive phase (AIC = -139). Consistent with a previous theoretical prediction, the results of our study suggest that the optimal swim speed of Steller sea lions is a function of drag, and is independent of dive depth and buoyancy.
Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan Department of Natural Environmental Study, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan isuzuki@aori.u-tokyo.ac.jp.
Ippei
I
Suzuki
Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.
Katsufumi
K
Sato
Department of Life Science, Texas A&M University-Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX 78412, USA Department of Zoology and Marine Mammal Research Unit, Fisheries Center, University of British Columbia, 2204 Main Mall, Vancouver, BC V6T 1Z4, Canada.
Andreas
A
Fahlman
National Institute of Polar Research, 10-3 Midoricho, Tachikawa, Tokyo 190-8518, Japan.
Yasuhiko
Y
Naito
Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.
Nobuyuki
N
Miyazaki
Department of Zoology and Marine Mammal Research Unit, Fisheries Center, University of British Columbia, 2204 Main Mall, Vancouver, BC V6T 1Z4, Canada.
Andrew W
AW
Trites
Cost of transport; Diving; Eumetopias jubatus; Optimal swim speed