Clues to conserving our marine mammals are hidden in buckets of seawater

A new study shows how tiny traces of DNA from seawater can provide crucial information about the relationship between marine mammals and key prey species to support their conservation.

A team from the University of Leeds, University of Milano-Bicocca, and the Cetacean Research and Rescue Unit NGO, used the traces of DNA left by animals in seawater (environmental DNA/eDNA) to follow the seasonal movements of minke whales, bottlenose dolphins and harbour porpoises, along with their prey species in the newly created Southern Trench Marine Protected Area off the coast of northeast Scotland.

This is one of the first times the approach has been used successfully to detect patterns between whales, dolphins, and their prey, and shows the potential of the technique to help guide and monitor marine conservation efforts.

The team also found evidence for the presence of Atlantic blue fin tuna and the critically endangered European eel amongst many other marine species of conservation interest.

“Marine protected areas are vital for conservation, but they are often too small and disjointed to provide adequate protection for far-ranging marine mammals with complex seasonal foraging and breeding behaviours, which are sensitive to environmental change,” said Dr Elizabeth Boyse, lead author of this study, who did the work as part of her PhD at the University of Leeds and is now a marine biologist at the British Antarctic Survey.

Improved knowledge about the relationship between marine mammals and their prey species will enable decision makers to better coordinate conversation efforts in response to changing prey distributions due to climate change.

How do marine mammal distributions relate to key prey species?

The Moray Firth, where the study was conducted, is a biologically rich inlet of the North Sea. The Firth attracts high numbers of minke whales between May and October to take advantage of abundant food sources such as sandeels, herring and sprat.

These species make up over 80% of the total fish biomass in the North Sea and are essential prey for marine mammals and seabirds.

The study found that herring and sprat had a peak in abundance in June and again during September-October. Scientists believe that the early peak represents the sprat spawning season, whilst the latter peak represents herring returning to the area as adults.

On the other hand, sandeels feed actively in open water between April and July, but then burrow down into the sand from the end of July onwards to escape predators. eDNA captured in the study confirmed this pattern as sandeel signals decreased during burrowing season.

These patterns help researchers to build a clear picture of minke whale movements within the MPA, with high abundances of minkes inshore in the early summer corresponding to when they are eating sandeels, while the whales shifting offshore in August relate to the arrival of herring.

We suspected our long-term observations of minke whales shifting offshore later in the foraging season was related to prey switching, but it is incredible that we can use this technology to look beneath the surface to confirm it corresponds with the arrival of herring to overwinter.