What is it about?

Some semi-aquatic animals, such as mink, swim at the surface where wave generation is a major source of wasted energy. However for marine mammals, reptiles and birds 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 has been difficult to compare this with travel depths of wild animals due to limitations in tracking technology. We combine high-resolution depth tracking with satellite data from long distance migrations and previously published examples to find that several species of sea turtle, penguin and whales travel at depths around three times their body diameter, the 'sweet spot' that minimises both wave drag and vertical distance travelled.

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Why is it important?

The findings are of broad interest because they apply across a wide variety of air-breathing marine animals, with implications for conservation management through reduction of boat-strike fatalities (a significant source of mortality for whales) and fisheries bycatch. Tracking animals to find out where they live and travel has become a key part of designing effective conservation measures such as protected areas and fisheries regulations, and with depth tracking in the marine realm this also applies in three dimensions.

Perspectives

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 almost 20 m in length.

Kimberley Stokes
Swansea University

Read the Original

This page is a summary of: Optimization of swim depth across diverse taxa during horizontal travel, Proceedings of the National Academy of Sciences, December 2024, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2413768121.
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