A green turtle comes up for air. Like many air-breathing marine megafauna, green turtles optimise their swim depth during migration to minimise the cost of transport, travelling at around three body-depths beneath the surface in order to avoid creating waves whilst maximising horizontal distance travelled (Picture © R. D. and B. S. Kirkby).
Researchers from Swansea and Deakin Universities have found that marine animals across mammals, birds and reptiles swim at similar relative depths when travelling and not feeding to save energy.
Dr Kimberley Stokes, Professor Graeme Hays and Dr Nicole Esteban from Swansea and Deakin Universities led research across six institutes in five countries comparing the swim depths of several sea turtle, penguin and whale species. All travelled at around three body depths from the surface in order to swim in the 'sweet spot' that minimises wave formation at the surface and vertical distance travelled.
Some semi-aquatic animals, such as mink, swim at the surface where wave generation is a major source of wasted energy. However for marine birds, mammals and reptiles travelling great distances over their lifetimes, adaptation to minimise the energetic cost of transport is expected, particularly on long journeys.
It has long been known that additional drag from wave creation minimises once a travelling object is at depths greater than three times its diameter, but it was hard to compare with travel depths of wild animals due to tracking limitations.
In this new study published in Proceedings of the National Academy of Sciences (PNAS) near surface swim depths were recorded to within 1.5 centimetres in little penguin and loggerhead turtles, along with motion data and video footage from animal borne cameras. This was compared with satellite tracking data for long-distance migrations in green turtles and data from other studies on penguins and whales. It was found that these animal swim at optimal depths predicted from physics when either 'commuting' to a foraging patch in the wild or migrating over longer distances while not feeding. This adaptation helps reduce the cost of transport in travelling animals and has implications for conservation management through reducing boat strike fatalities and fisheries bycatch.
Swansea University’s Dr Kimberley Stokes, lead author of the study said:
“There are of course examples where animal swim depth is driven by other factors, such as searching for prey, but it was exciting to find that all published examples of non-foraging air-breathing marine animals followed the predicted pattern. This has rarely been recorded because of the difficulty in retrieving depth data from animals that migrate over large distances, so it was great to find enough examples to show a common relationship between swim depth and body size from animals across the size spectrum from 30 cm to about 20 m in length.”
The full study can be read here.
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