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Drag and energetics of swimming in Steller sea lions (Eumetopias jubatus) Stelle, Lei Lani


This thesis presents the first hydrodynamic investigation of swimming in Steller sea lions (Eumetopias jubatus). Passive drag was calculated from "deceleration during glide" measurements. A total of sixty-six glides from six juvenile sea lions yielded an average drag coefficient (referenced to total wetted surface area) of 0.0056 at an average Reynolds number of 5.5 x 10⁶. The drag values indicate that the boundary layer is largely turbulent at these Reynolds numbers, which are past the point of transition from laminar to turbulent flow. The position of maximum thickness (average = 0.344) was lower than for a "laminar" profile, and supports the idea that there is little laminar flow. Steller sea lions in this study were characterized by an average fineness ratio of 5.55; the streamlined shape helps to delay flow separation, which reduces total drag. In addition, turbulent boundary layers are more stable than laminar, thus separating further back on the animal. The average glide velocity of the individual sea lions ranged from 2.9-3.4 m/s, or 1.2-1.5 body lengths/s. These speeds are very close to the predicted swim velocity of 1.4 body lengths/s, based on the minimum cost of transport for California sea lions (Zalophus californianus). Efficiencies of swimming were calculated based on drag data and preliminary metabolic measurements. Aerobic efficiencies were calculated to reach 13-17% at a swimming velocity of 3.6 m/s. Metabolic costs of swimming were also predicted based on the power required to overcome the measured drag, with assumed efficiencies from literature values. Both approaches yielded very similar metabolic rates and aerobic efficiencies. The metabolic rate was determined to be approximately 29 ml O₂min kg for a Steller sea lion swimming at 2.0 m/s, and 45 ml 0₂/min kg at a velocity of 3.5 m/s. Drag measurements provide a means to estimate the energetic costs of swimming over a range of natural velocities. This approach has applications for the modeling of pinniped energy budgets which are a necessary component of research projects aimed at investigating the decline of the endangered Steller sea lions.

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