Researchers at PML and the University of Exeter have combined two innovative technologies to probe the mystery of how seabirds locate food hotspots across vast tracts of ocean.
The way in which marine predators find prey in what to us seems a largely featureless landscape, often tens or hundreds of kilometres from a ‘home base’, has long puzzled biologists.
For seabirds, such as gannets, the distance of journeys to and from foraging areas and hunting success is a finely balanced matter of life and death. Knowing where they find food is also crucial for their conservation. The advent of tiny data loggers carried by birds has provided important insights into where they feed, but less is known about the conditions they experience there.
A second, satellite-mounted, technology enables scientists to spot areas of the sea where two currents meet or upwelling occurs, commonly known as ocean fronts. They are often rich in nutrients, which encourages plankton growth, and are magnets for marine life, including fish which then become the prey of larger animals such as the Northern Gannet (Morus bassanus).
By bringing the two technologies together, the scientists have shown that the birds search out regions where fronts form most frequently, so making their foraging more efficient and less energetically costly.
Dr Steve Votier from the University of Exeter who led the gannet research programme commented: “We have been studying gannet behaviour for nearly 10 years and have learnt much about what they do while at sea, but the key to this research was working with oceanographers. Using state-of-the-art satellite imagery they have enabled us to investigate how gannets forage at fronts – discontinuities in temperature and phytoplankton”.
During the study, 66 gannets were tracked over two breeding seasons, revealing that the average foraging trip was around 178kms – with some travelling as far as 430kms in a single round trip.
The scientists, who brought together satellite observations of plankton abundance and sea surface temperature, were able to produce composite maps of front occurrence. Among other things, composite maps show those fronts that are more persistent, these being of much greater value to the predators.
Such established fronts are far more likely to have developed the complex food webs necessary to support top predators; temporary fronts may not last long enough to attract the fish which form the food of predators, such as gannets. So tracking showed where the birds visited, whilst the presence of the detected fronts provided a major part of the answer of what it was that drew them there.
The detection of the fronts and their associated richness, explain why the gannets fly considerable distance to find food, but how does a bird flying over a large area of seemingly featureless sea detect a front? Kylie Scales, from PML, who co-led the study as part of her PhD studies explained:
“The exact mechanism is still not clear but the tracking shows that the birds repeatedly return to the general area where fronts are persisting – so they may be learning from other colony members, and then remembering. But when they arrive in the vicinity it may be visual cues that guide them to the best hunting sites, which are where the fronts are located. They may be spotting other foraging birds, foam and debris, which often accumulate at frontal sites, or they could be seeing how surface currents are flowing against each other, while sense of smell may also be important for some predators in detecting the presence of dimethyl sulphide which is produced by plankton.”
“Our study”, Scales continued, “has demonstrated the importance of ocean fronts to flying predators. This technique, which we used in the Celtic Sea, can be used anywhere across the global ocean. It has great potential for identifying key feeding grounds for a wide range of predators – and that means it should become an indispensable tool in identifying conservation hotspots.”