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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	[Influence of varied carp stock on the distribution of moulting waterfowl in the fish pond area of Ismaninger Teichgebiet between 1996 and 1999]
Auteurs :	Ursula Koehler, Auteur ; Peter Koehler, Auteur ; Ursula Firsching, Auteur ; Eberhard von Krosigk, Auteur
Type de document :	Article
Dans :	Ornithologischer Anzeiger (39(2-3) September, 2000)
Article en page(s) :	119-134
Résumé :	A significant reduction of nutrients in the waters of the Ramsar-site "Ismaninger Speichersee mit Fischteichen" caused a reduced production of food for the waterfowl populations moulting there. Since 1994, some species decreased dramatically, mainly Pochard Aythya ferina and Black-necked Grebe Podiceps nigricollis. Since 1996 we have been studying how a reduced stock of carp Cyprinus carpio can improve the food supply for moulting waterfowl in the fish pond area of the site. In 1996, 3 out of 30 fish ponds were left without carp. Between 1997 and 1999, the spring stocks of the ponds were varied between 150 and 500 carp per hectare, with some ponds remaining without carp. The abundance of food is correlated negatively with the carp stock. Ponds with high stocks often develop a high turbidity caused by phytoplankton and tend to be poor in macroinvertebrates. The lower the carp stock, the clearer is the water and the more regular are marked growths of macrophytic algae (Cladophorales, Hydrodictyon) and high densities of macroinvertebrates. Consequently, the spatial distribution of the waterbirds was mostly related to the abundance of plant and animal foods present in each pond. Large numbers of moulting waterbirds congregate for feeding on the ponds with few or no carp. The abundances of most species are negatively correlated with the carp stock. Little Grebe Tachybaptus ruficollis, Black-necked Grebe, Gadwall Anas strepera, Mallard A. platyrhynchos, Tufted Duck Aythya fuligula, and Coot Fulica atra showed significant correlations in all years of the study. Pochard and Red-crested Pochard Netta rufina seemed to prefer ponds with lower carp stock to those without carp. Teal Anas crecca and Shoveler A. clypeata did not show a trend over the 3 years. Moreover, the number of species present on each pond was found to decrease with increasing carp stock.

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