1. BACKGROUND AND AIMS
This project, running from December 1996 to November 1997 followed from a previous EC-funded project (The impact of marine mammals on Northeast Atlantic Fisheries) and had the following aims:
2.SUMMARY OF RESULTS ARISING FROM THE RESEARCH
The diet composition of several small cetacean species, also sperm whales, has been described from stomach contents of stranded animals. Some further work of seal diets has also been completed.
Diets of harbour porpoise in Scotland, Denmark and Holland consist primarily of inshore demersal fish. There are significant differences between seasons and areas. Whiting, sandeel, cod and eelpout are among the most important prey. Common dolphins on the Galician coast take a wide range of inshore and oceanic fish and cephalopods. There are some seasonal differences in diet, but blue whiting, scad and sardine tend to predominate. Sperm whales in the North Sea had been feeding almost exclusively on the oceanic squid Gonatus fabricii, although other oceanic squid species, such us Teuthowenia megalops, Histioteuthis bonnellii, Todarodes sagittatus and the octopus Haliphron atlanticus were also eaten.
Simulation studies were used to estimate confidence limits for diet composition. The precision of diet composition estimates increases with sample size. Sample sizes of less than 10 lead to a marked loss of precision and, potentially, accuracy. Weighting samples equally (rather than allowing heavier samples to contribute more) results in substantially altered estimates but is rejected on theoretical grounds. Sampling error and errors in estimating fish size due to uncertainty in regression parameters both lead to wide confidence limits around estimated importance of prey categories, but the errors were not additive. "Subsampling" error, due to measuring only a subsample of otoliths, has relatively little impact on precision.
Because the different errors are not additive, approximate confidence limits were calculated for porpoise and dolphin diets based on resampling error only. The simulatiion results demonstrate that high precision is not achievable: confidence limits for importance of the main prey species were as much as ± 100% of the median value.
The weight of prey eaten by sperm whale populations in the NE Atlantic was estimated from the dietary data and published estimates of population size and individual food requirements. Assuming that the diet of animals stranded in the North Sea was typical, more than 1,000,000 tonnes of the oceanic squid Gonatus fabricii could be consumed annually. Similar calculations were made for harbour porpoise diets in the North Sea and adjacent waters. For example, it is estimated that porpoises could take approximately 45,000 tonnes of whiting annually. Confidence limits were estimated for amounts consumed in Scottish waters. The estimated annual average consumption of whiting is around 14,000 tonnes, with 95% confidence limits as wide as 5,000 to 29,000 tonnes, depending on the assumptions made. This reflects the high level of uncertainty about population size and structure.
Many of the prey species eaten by dolphins and porpoises are also the target of commercial fisheries. An analysis of the sizes of fish eaten by dolphins shows a substantial overlap with the size taken by fisheries. Porpoises are shown to have similar feeding "niches" in all three main areas studied (Scotland, Holland, Denmark), taking a range of coastal fish species. The diet of the common dolphin in Galicia is dissimilar (low niche overlap), with a higher prevalence of oceanic prey species.
3. LIST OF PUBLICATIONS RELATED TO THIS RESEARCH
Santos, M.B., Pierce, G.J., González, A.F. & López, A., 1997. Dieta do arroás (Tursiops truncatus) en Galicia. Eubalaena 10, 30-39 (in Galician with an English abstract).
Tollit, D.J., Steward, M., Thompson, P.M., Pierce, G.J., Santos, M.B. & Hughes, S., 1997. Species and size differences in the digestion of otoliths and beaks; implications for estimates of pinniped diet composition. Canadian Journal of Fisheries and Aquatic Science 54, 105-119.
Santos, M.B., Pierce, G.J., Boyle P.R., Reid, R.J., Ross, H.M., Patterson, I.A.P., Kinze, C.C., Tougaard, S., Lick, R., Piatkowski, U. & Hernández-García, V., 1999. Stomach contents of sperm whales Physeter macrocephalus stranded in the North Sea 1990-1996. Marine Ecology Progress Series 183, 281-294.
Wijnsma, G., Pierce, G.J. & Santos, M.B., 1999. Assessment of errors in cetacean diet analysis: in vitro digestion of otoliths. Journal of the Marine Biological Association of the United Kingdom 79, 573-575.
Santos, M.B., Clarke, M.R. & Pierce, G.J., 2001. Assessing the importance of cephalopods in the diets of marine mammals and other top predators: problems and solutions. Fisheries Research 52, 121-139.
Santos, M.B., Pierce, G.J., Herman, J., López,
A., Guerra, A., Mente, E. & Clarke, M.R., 2001. Feeding ecology of Cuvier's
beaked whale (Ziphius cavirostris): a review with new information
on the diet of this species. Journal of the Marine Biological Association
of the United Kingdom 81, 687-694.
Santos, M.B., Pierce, G.J., Reid, R.J., Patterson, I.A.P., Ross, H.M. & Mente, E. 2001. Stomach contents of bottlenose dolphins (Tursiops truncatus) in Scottish waters. Journal of the Marine Biological Association of the United Kingdom 81, 873-878.
Santos, M.B., Pierce G.J., Smeenk, C., Addink,
M.J., Kinze, C.C., Tougaard, S. & Herman, J., 2001. Stomach contents
of northern bottlenose whales Hyperoodon ampullatus stranded in the
North Sea. Journal of the Marine Biological Association of the United Kingdom
For further information contact Graham Pierce or Begoña Santos.
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