Until now, the only information on the diet of large baleen whales such as fin, humpback and minke whales in Irish waters, was gathered in 1913 by a Mr Burfield in Belmullet, Co Mayo where two whaling stations were operated by a Norwegian enterprise at the time. These whaling stations caught mostly fin whales as they migrated along the shelf edge close to Co. Mayo. Two new studies published in Marine Ecology Progress Series and Marine Mammal Science used stable isotopes and mixed modelling to reveal the diet of fin and humpback whales in Ireland.
Following regular sightings of fin, humpback and minke whales close to shore over the last 10-15 years there has been a resurgence of interest in whales in Irish waters. From June to February between Slea Head, Co. Kerry and Hook Head, Co. Wexford, fin and humpback whales appear to follow the seasonal movements of sprat and herring as they congregate inshore to spawn. The Irish Whale and Dolphin Group (IWDG), Galway-Mayo Institute of Technology and the Marine Institute collaborated on a project to determine what these whales were feeding on – essential information if we are to adopt a more sustainable 'ecosystem approach' to managing our fisheries. This ecosystem approach recognises that there are predators, other than humans in the marine ecosystem, which are entitled to marine resources and that these are an integral link in a complex food web which needs to be kept intact for the health of our oceans and for the benefit of our coastal communities.
Studying the diet of large whales is challenging. They cover vast distances and dive for up to 15 minutes at a time, and despite their huge size (fin whales up to 21m) they are difficult to follow and observe in the rough seas around Ireland. When they die and strand, it is difficult to obtain stomach samples before the carcase heats up with decay and quickly rots. At any rate, most of the stomach samples from dead whales are empty as these individuals tend to be sick. An alternative approach to investigating their diet was needed and this study used stable isotope analysis. The field of stable isotope analysis is rapidly becoming an essential tool for all scientists to trace food-webs and to follow ecosystem nutrient cycling in marine systems. In a novel way, this enables researchers infer information on their origins and dietary influences.
Skin samples were collected from fin and humpback whales using biopsy darts fired from a cross-bow. For the past decade, the IWDG Large Whale survey team has been conducting this field work during autumn and winter when the whales are close inshore in large numbers. They used local whale-watching boats or the IWDG rigid-hulled inflatable boat Muc Mhara to get close enough to the whales to dart them and photograph unique markings to help identify individual whales. This work was licensed by the National Parks and Wildlife Service. Chemical tracers, in this case stable carbon and nitrogen isotopes, in the skin of the whales were analysed. These same chemical signatures were measured in samples from sprat, herring and various krill species of different age classes, collected during annual fisheries surveys from the Marine Institute's R.V. Celtic Explorer. By matching the best fitting stable isotope signatures, the researchers were able to calculate what proportion of each prey species the whales were consuming.
The results showed that fin whales in Irish waters have a diet comprising 50% krill and 50% young sprat and herring. Throughout most of their range in European waters, fin whales feed almost exclusively on krill, so the Irish whales are unusual in that they rely heavily on small fish. Humpback whales seem to prefer sprat and herring when feeding in Irish waters and their diet comprised less than 30% krill. The research team also examined the long term diet of whales by analysing stable isotope values of baleen plates, which are flat sheets of fingernail-like tissue in the mouths of some large whales, used to filter food from the seawater. By using baleen plates stored in museums in Ireland and the UK the study examined changes in diet over the last 100 years. The main finding was that the long term diets of minke, fin and humpback whales are quite different from each other. This result was somewhat unexpected as these species are often seen feeding in close proximity to each other.
These results which have recently been published in the international science journals: Marine Mammal Science and Marine Ecology Progress Series, arise from a three year PhD project at the Galway-Mayo Institute of Technology (GMIT) and funded by the Irish Research Council.
Dr Conor Ryan said," We now have clear evidence that sprat and herring are important species supporting whales in Irish waters, and we must manage our fisheries with this in mind. We are very worried that sprat is being fished with an open quota, despite a huge gap in our knowledge about the life history of this important species in the Celtic Sea".
Dr Simon Berrow, lecturer in Marine Science at Galway-Mayo Institute of Technology (GMIT), Executive Officer of the IWDG, and member of the Celtic Sea Herring Management Advisory Committee added, "This data will be incorporated into the new Environmental Management Plan commissioned by the Celtic Sea Herring Management Advisory Committee which is essential for the proper management of these fisheries and ensuring this fishery is compliant with Marine stewardship Council requirements. Whale-watching is growing in Ireland and has the potential to expand much more if we manage these fish stocks for both fishers and whales and dolphins, which will bring further benefits to coastal communities".
Dr Brendan McHugh a chemist at the Marine Environment and Food Safety Services in the Marine Institute said, "Collaborative studies like this one between biologists and chemists are key to developing a greater understanding of ecosystem linkages and their impacts."