Harbour seals are one of our most recognisable marine mammals, commonly sighted on coastlines throughout northern Europe. Despite their familiarity, scientists knew comparatively little about what they get up to, when they leave our shorelines to find food.

Now a team of researchers from Denmark, the UK and Germany have solved this by attaching miniature computers to animals to monitor their detailed movements during weeklong offshore foraging trips in the German Wadden Sea. In a recent paper in Nature Scientific Reports, they show that harbour seals search for and capture prey continuously throughout offshore trips, even when they are travelling to and from the coast.

Movements in unexpected ways

Lead author on the study Heather Vance from the University of St Andrew's Seal Mammal Research Unit (SMRU) expected seals to make a beeline for good feeding spots, and then return to shore when prey started to run low. However, the research team were surprised to find that harbour seals hunted prey at similar rates while travelling as they did when they got to offshore sites. The tags also revealed that the seals moved in an unexpected way: rather than dividing their time between travelling and hunting, the tagged seals swam continuously with their head and body pointed towards the seabed, searching for food. While a seemingly inefficient way to travel, this strategy maximised the time harbour seals spent foraging.

This then begs the question: why do these seals travel so far from their coastal haul-outs when food is available locally? “The distance these seals travel may be more heavily influenced by competition from other harbour seals than by the location of prey,” Vance suggests.

The tags, designed by Dr. Mark Johnson from the Aarhus Institute of Advanced Studies at Aarhus University in Denmark, record highly-detailed movements and behaviour for up to 30 days. Describing them as "fit-bits for seals", Dr. Johnson said "Being able to detect individual prey captures and swimming strokes throughout these long offshore feeding trips gave us a real insight into how seals spend their time".

Tags – the ‘big brother’ of conservation ecology

The team found that seals rested at sea, either bobbing at the surface or sleeping on the seafloor, for longer and longer each day during trips and the need for a good night's sleep may ultimately drive seals back to the haul-out.

“These high-resolution tags are the ‘big brother’ of conservation ecology, giving us a close-up view of where and how marine predators feed. This is enormously important for effective conservation decision-making such as where to put marine protected areas,” said Professor Sascha Hooker from St Andrews and a co-author on the study.

Professor Ursula Siebert from the University of Veterinary Medicine Hannover adds that “the harbour seal’s habitat is under a lot of pressure. The results of this study, partly funded by German Federal Agency of Nature Conservation, are important to help us understand how these animals use this habitat and how human activities can affect them.“

The scientific research article

"Drivers and constraints on offshore foraging in harbour seals" by H. M. Vance, S. K. Hooker, L. Mikkelsen, A. van Neer, J.Teilmann, U. Siebert & M. Johnson can be found at: https://rdcu.be/chexd.

Contact

Mark Johnson, AIAS-COFUND Fellow, Associate Professor
E-mail: markjohnson@bios.au.dk

Aarhus Institute of Advanced Studies &
Department of Biology – Zoophysiology
Aarhus University