Schifferli, L., M. Burkhardt & M. Kestenholz
(* = Kurzbeitrag)
Bestandsentwicklung des Kormorans Phalacrocorax carbo in der Schweiz 1967–2003.
(von 1994 bis 2006 vergeben)
Phalacrocorax carbo, winter population, breeding population, trend, Switzerland
Winterbestand, Winterquartier, Brutbestand, Brutkolonien, Bestandesentwicklung, Verbreitung, Fischerertrag, Nahrungsangebot, Schlafplatz, Störung
Schweiz, Bodensee, Zürichsee, Zugersee, Neuenburgersee, Bielersee, Murtensee, Genfersee, Lago di Lugano, Lago Maggiore, Rhein, Rhone, Reuss, Niederlande, Dänemark, Deutschland
Population of Great Cormorants Phalacrocorax carbo wintering in Switzerland, 1967–2003, and numbers during the breeding season. Based on counts at roosts and on national waterbird counts in mid-January, 1967–2003 and mid-November, 1991–2002, we document the numbers wintering in Switzerland and adjacent waters (Fig. 1). Numbers in January increased exponentially, from 331 in 1967 to the maximum of 8415 in 1992, and in parallel to the growth of the breeding population in The Netherlands, Denmark and Germany (Fig. 3), the major sources of the Swiss wintering population. In subsequent winters, Cormorant numbers fluctuated at a lower level (mean 1993–2003 5686 ± 464), in spite of a continued growth of the breeding population. On 15 large lakes (surface >10 km2), holding three quarters of the national total, fluctuations following the peak run in parallel to the yield of professional fisheries (Fig. 5), which was taken as an index of food supply (Perca fluviatilis, Rutilus rutilus and «other Cyprinid fish»). This confirms the predictions of Suter (1995a), suggesting that food would limit the population in the Swiss winter quarters earlier than on the breeding grounds. In November, 1991–2002, Cormorants were more numerous (mean 8623 ± 2256) than in January in each winter (Fig. 2).
Until 1976, Cormorants were restricted to the Lakes of Constance, Geneva, Zurich and Neuchâtel (Fig. 7). Subsequently, other lakes were colonised. Cormorant numbers on lakes peaked in 1989. The final stage of increase was mainly the result of an expansion to rivers (free-running and dammed parts), which held a third of the January numbers in 1991 (Fig. 6). Coinciding with measures taken to scare Cormorants fishing and roosting on rivers holding important populations of threatened fish species (e.g. Thymallus thymallus), the proportions of Cormorants on these waters declined and stabilised (January mean, 1996–2000: 19.7 ± 5.1 %).
Since the mid-1980s, the number of summer visitors has increased to some 200 individuals (Fig. 9). About half of them used Lake Geneva. However, first breeding was recorded on Lake Neuchâtel in 2001. The number of pairs has been increasing, and in 2004 100 young fledged from 53 broods (Fig. 10).
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