The effects of cessation of fish farming on duck breeding in French fishpond systems

Fishponds are man-made aquatic systems subjected to more or less active management by fish farmers. They also are an important habitat category for waterbirds. In this study, we attempted to assess the consequences for duck breeding of fish farming interruption in a typical French fishpond region, the Forez, where fish production has declined strongly since the 1990s. Duck pair density, a brood: pair ratio and mean brood size were studied in two successive periods, 2010-2012 and 2013-2015, in 19 ponds abandoned for fish farming between 2008 and 2010, and in 34 ponds still stocked with fish among which 17 were harvested every 2 or 3 years and 17 harvested annually. Between the two study periods, a decrease in abandoned ponds was observed in diving duck (Aythya species) and red-crested pochard Netta rufina pair density and for dabbling duck (Anas species) and red-crested pochard brood: pair ratio. A possible explanation is that fish influenced the nutrient level within the aquatic ecosystem through bioresuspension of phosphorus when stirring the sediment for feeding and through nitrogen production by excretions. Decreased brood sizes in ponds still used for traditional fish farming are suggested to be the result of density-dependent predation rates.