In what may just be the marine life equivalent to hogging the middle lane, researchers from Swansea and Deakin Universities have found that marine animals across mammals, birds, and reptiles, all swim at similar relative depths of the ocean when travelling and not feeding to save themselves energy.

Research led across six different institutes in five countries, each comparing the swim depths of several sea turtle, penguin, and whale species, has discovered that all travelled at around three body depths from the surface of the ocean to swim in the “sweet spot”, a layer of the ocean’s water that minimises wave formation at the surface and vertical distance travelled.

There are some semi-aquatic animals, such as mink, that swim at the surface where wave generation is a major source of wasted energy. The new study – published in Proceedings of the National Academy of Sciences and led by Dr Kimberley Stokes, Professor Graeme Hays, and Dr Nicole Esteban from Swansea and Deakin Universities – found that marine life known to travel great distances over their lifetimes, however – including birds, mammals, and reptiles – have adapted to minimise the “energetic cost” of transport.

Of course, 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 has until now been difficult to compare with the travel depths of wild animals due to tracking limitations.

In this new study – titled Optimisation of swim depth across diverse taxa during horizontal travel – near surface swim depths were recorded to within 1.5cm in little penguin and loggerhead turtles, along with motion data and video footage from animal borne cameras.

This was then 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 animals 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.

“There are, of course, examples where animal swim depth is driven by other factors, such as searching for prey, but this was exciting to find that all published examples of non-foraging air-breathing marine animals followed the predicted pattern,” said Swansea University’s Dr Kimberley Stokes, lead author of the study.

“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 30cm to about 20 metres in length.”

Such a discovery will of course have implications for the conservation and protection of marine life, including mammals, birds, and reptiles known to travel great distances and the protections that ought to be afforded to the depths of ocean space they occupy when on their travels. 

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