Effects of crayfish introduction on otter Lutra lutra food in the Donana National Park, SW Spain

American red swamp crayfish Procambarus (mainly P. clarki) were introduced into the Guadalquivir marismas in 1974, and are now thriving. Fish were the staple prey of otters in 1976, with amphibians and insects as secondary foods. In 1979 crayfish were the 2nd most important food and the frequency of fish in the spraints decreased significantly. Later the role of fish did not change, but crayfish became the most common prey category in 1981 and 1984. Occurrence of amphibians decreased statistically from 1979 (probably because of their scarcity in the habitat, due to high mortality in fyke nets) and that of insects increased (probably because they are found more often by the predator when searching for crayfish).