Impacts of otter (Lutra lutra L.) predation on fishponds:: A study of fish remains at ponds in the Czech Republic

The increasing numbers of otters ( Lutra lutra L.), which are protected by the Czech Act of Nature and Landscape Protection, are causing serious problems for fishpond management. The diet of otters on pond farms consists predominantly (80%) of common carp, Cyprinus carpio, and to a lesser extent other pond fish species ( perch, Perca fluviatilis, zander, Stizostedion lucioperca and grass carp, Ctenopharyngodon idella). The size of carp captured by otters ranged between 376 - 683 mm TL ( 500 +/- 88 mm) and 1,049 - 11,768 g ( 3,478 +/- 2,867 g). Reconstructed original weight and length of captured grass carp and perch were 599 and 182 mm TL, and 2,665 and 163 g, respectively. In most of prey fish corpses left by otters, only viscera and associated parts were consumed. The weight of individual common carp corpses was estimated as 73.0 +/- 24.6 (26.3 - 95.9)% of the original reconstructed weight, which means that only 27.0 +/- 17.2 ( 4.1 - 73.7)% of fish body mass was consumed by otters. In perch, 62.8% of fish body mass was left unconsumed. Heavy losses have been reported also on fish stocks in ice-covered ponds during the winter period, when shoals of resting fish have been disturbed and stressed due to otter hunting.