Drag, but not buoyancy, affects swim speed in captive Steller sea lions

Abstract

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.

Hydrostatic and Hydrodynamic Characteristics of Swimming Animals- an Inspiration for Hybrid Buoyant Aircraft

“…As far as buoyant lift is concerned, it is mainly linked with the criterion of good performance for an animal that takes into account that how much energy it can process and store (guts, so to speak) and how many offspring it can produce (gonads, loosely put). In the case of Sealion, the maximum available buoyant lift is 50% of the gross weight (Suzuki et al, 2014). This digit is consistent with the recent findings related to the optimum buoyancy ratio for hybrid airship (Raymer, 2006).…”

“…Interestingly, as per the findings of Suzuki et al (2014) this range of fineness ratio is found comparable to the California sea lion. This marine animal has a fineness ratio of 5.55 and location of maximum thickness of outer profile is at 34% of the overall length (Cheneval, 2005).…”

“…This buoyant lift is independent of the drag force exerted on the body of a sea lion (Suzuki et al, 2014). Buoyant lift is independent of the surface/wetted area of the marine animal.…”

“…Complete geometric details of the outer contour of the California sea lion are not yet available. Perhaps, due to the flexible body of sea lions, it is nearly impossible to measure the same; the only information available is the location of maximum thickness, which is about 34 % of body length (Suzuki et al, 2014). In one of our studies related to the hybrid lifting hull, we selected Eppler-1200 airfoil for the hybrid hull, based on the visual judgment of geometric profile of California sea lion and requirement of takeoff ground roll angle to fulfill future certification requirement (Haque et al, 2014).…”

Effects of added drag on cetaceans : fishing gear entanglement and external tag attachment

“…Drag harnesses (Cornick et al, 2006), collars (Lang and Daybell, 1963), pipes (Aoki et al, 2011;Suzuki et al, 2014), wooden blocks (Boyd et al, 1997) have been attached to marine mammals while measuring their long-term energy consumption with radio-isotope labeled water (Costa and Gentry, 1986), heat flux (Willis et al, 2005) their diving and swimming behaviors (Boyd et al, 1997;Aoki et al, 2011), or their colony return rates (Costa and Gentry, 1986;Boyd et al, 1997).…”