SMRU > Research > Feeding/Diet
Marine mammals are conspicuous consumers of fish and other marine life, and have long been seen by some as potential competitors for fishery resources and threats to the financial success of fishing interests. By virtue of their consumption they are also potentially important in controlling ecosystem level effects in the sea.
Determining what individual animals have been eating is usually done either by examining faeces or the stomach contents of dead animals. In each case the basic approach is the same, where the material in question is sieved and remaining hard parts of prey items, such as bones, and in particular the very hard and dense otoliths or ear bones of teleost fish are isolated and identified according to their species specific shapes. Measuring the otoliths also enables the size of the fish concerned to be calculated. Where enough a representative samples are available it is possible to estimate what the marine mammal population from which the sample was drawn are likely to have been eating.
Other methods of determining diet include fatty acid and stable isotope analysis, which though less accurate in terms of identifying individual species and sizes of fish being eaten, provide an overview of what sorts of prey individuals may have been eating over a longer time frame, of months to years. Such analyses can be done using biopsies from free swimming animals.
Phil Hammond, Kate Grellier and Rob Harris studied the diet of grey and harbour seals in the UK, examining thousands of faecal samples from around the country and also conducting captive experiments where fish of different species are fed to seals and the resulting faeces collected and examined to identify and quantify biases in the otolith sampling methodology.
Simon Northridge is studying the stomach contents of bycaught and stranded cetaceans from around the UK.
Ian Boyd is developing a research area using stable isotopes in
the annual growth rings of seal teeth to examine gross changes in diet
throughout the life of individuals. This is in collaboration with Michael Bird
in the School of Geography and Geoscience.
Previous studies by SMRU have also included fatty acid analysis of grey seals to explore differences in the overall trophic level of predation by seals in different parts of the UK.