Riding on the crest of the wave: Possibilities and limitations for a thriving population of migratory Cormorants Phalacrocorax carbo in man-dominated wetlands

This paper summarises the general findings of the scientific contributions to this special issue of Ardea on Cormorant biology in Europe. After a brief historical introduction setting the main traditional questions that arise whenever man and a species of a free-living animal compete for the same resources in the same habitat, an overview is given of how Cormorant populations have recovered after protective measures were taken over most of Europe. This recovery, particularly spectacular in the continental population, has brought the species once again into direct conflict with human interests. Research has been aimed at unravelling the principal natural factors limiting population growth. Surface area, productivity and accessability of fishing grounds for breeding birds seem to set the extent to which the population may increase but the end of the population growth seems not yet to be at hand. Because of their migratory behaviour birds spread all over western Europe outside the breeding season, showing often site tenacity to stop-over sites as well as predictable migration patterns. Factors influencing the non-breeding distribution include age, sex and probably individual experience of the birds concerned. Local conflicts between human interests and breeding as well as non-breeding Cormorants seem impossible to cope with satisfactorily on a local level, without consideration of the population as a whole. Not every possible impact of Cormorants on fish stocks is bound to be treated as negative: recent studies tentatively indicate that the birds may serve as an 'aid' to fisheries management, keeping eutrophic waters free from dense Bream stocks. Finally, the position of top-predator that Cormorants occupy in aquatic ecosystems, makes them vulnerable to the accumulation of micropollutants. These may hamper breeding success in various ways and, in larger concentrations, may even cause death. Monitoring breeding success as well as pollutant contents in adult and juvenile Cormorants as well as in their eggs may serve as a means to check on trends in environmental quality of aquatic ecosystems.