Abstract: Strong alteration of waterfowl habitat may derive from fish-farming intensification in fishpond systems. The specific effects of fertilization on duck breeding were described in fishponds of the Brenne region (Central France) where fish stock density is usually lower than in fishpond systems of Central Europe. The study was carried out in 1999, 2000 or 2001 in a sample of 75 ponds, either unfertilized (F0) or fertilized with organic or inorganic manure (F1), or with organic and inorganic manure (F2). Fish-stock mean density doubled from F0 (215.9 kg.ha-1) to F2 (441.9 kg.ha-1). Biomass density in macrophytes of potential invertebrate prey for ducks was higher in F2 than in F0 and macrophyte abundance did not differ across fertilization categories. Duck brood density (number/square root of pond surface area) in F2 was higher than in F0 and in F1. Moreover, 5-week old pochard Aythya ferina brood size was higher in F2 than in F1, probably as a result of a higher persistence of broods with 4 ducklings. We conclude that manure application and correlative increase in fish density is not necessarily antagonistic with duck breeding provided that fish stock is below 500 kg.ha-1, water turbidity does not hinder macrophyte development and large enough helophyte belts are available for nesting.
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