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Changes in fisheries discard rates and seabird communities

Abstract

It is clear that discards from commercial fisheries are a key food resource for many seabird species around the world1,2,3,4,5,6,7,8. But predicting the response of seabird communities to changes in discard rates is problematic and requires historical data to elucidate the confounding effects of other, more ‘natural’ ecological processes. In the North Sea, declining stocks, changes in technical measures, changes in population structure9 and the establishment of a recovery programme for cod (Gadus morhua10) will alter the amount of fish discarded. This region also supports internationally important populations of seabirds11, some of which feed extensively, but facultatively, on discards, in particular on undersized haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus)1,2,3. Here we use long-term data sets from the northern North Sea to show that there is a direct link between discard availability and discard use by a generalist predator and scavenger—the great skua (Stercorarius skua). Reduced rates of discarding, particularly when coupled with reduced availability of small shoaling pelagic fish such as sandeel (Ammodytes marinus), result in an increase in predation by great skuas on other birds. This switching of prey by a facultative scavenger presents a potentially serious threat to some seabird communities.

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Acknowledgements

We thank S. Humphries, N. Ratcliffe, A. Kelly and S. Waldron for assistance and discussion; and all those who helped over the years to collect prey remains at Foula. We thank the Fishery Research Service, Aberdeen, for providing Shetland sandeel VPA data. The research was supported by the Shetland Oil Terminal Environmental Advisory Group (SOTEAG) and by the European Commission contract ‘Discbird’. S.B. is funded by a Natural Environment Research Council (NERC) postdoctoral fellowship. P.C. is funded by the Fundação para Ciência e a Tecnologia (FCT), Portugal.

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Correspondence to Stephen C. Votier or Robert W. Furness.

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The authors declare that they have no competing financial interests.

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Further reading

Figure 1: Composition of the diet of great skuas tracks the quantity of fishery discards.
Figure 2: Declining discard and sandeel availability results in an increase in seabird predation.

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