Are Cormorants Phalacrocorax carbo wintering in Switzerland approaching carrying capacity? An analysis of increase patterns and habitat choice.

The increase rate of Cormorant numbers overwintering in Switzerland between 1970 and 1990 was the same as in the strongly growing breeding population. From 1930 to 1970, however, wintering Cormorants increased while the breeding population remained stable. The reasons are believed to be a better food supply as cyprinid and percid fish biomass increased due to eutrophication of Swiss lakes. Conversely, the increase rate of overwintering birds in Switzerland recently began to slow down while numbers during autumn passage are still following the growth of the breeding population. The resulting sigmoid pattern suggests that Cormorant numbers in Switzerland in winter are approaching carrying capacity. Cormorant density on lakes is strongly correlated with density of Perch and cyprinid fish, especially Roach, which shows signs of decrease as the input rate of nutrients into lakes is being reduced. The stepwise process of filling up different types of water bodies partly supports the Fretwell-Lucas model of habitat occupancy although behavioural responses to reduced persecution may also be involved in habitat choice. Even if increased winter survival were partly responsible for the current strong population increase (which is in fact unknown), the inland waters of western and central Europe hold a too small part of the north-central European Cormorant population (estimated at 300 000 birds in spring 1992) compared to coastal areas, as to play an important role in such a process.

Suter W., ARDEA 83 (1): 255-266.