An Analysis of Otter Lutra lutra Predation on Introduced American Crayfish Procambarus clarkii in Iberian Streams

Abstract

1. Patterns of otter, Lutra lutra, predation on introduced American crayfish, Pro-cambarus clarkii, were examined in an Iberian intermittent stream over a 2-year period, in order to assess the relative importance to otters of this new food resource. Diet was described from spraints collected monthly, and the importance of different prey types was determined in terms of their relative numeric and energetic contributions to the diet. Monthly fluctuations in diet and abundance of principal prey species were compared. 2. Energetically, the crayfish ranked second after eels, accounting for 18% of the estimated annual intake of energy. Crayfish and eels were particularly important in the diet from April to October. For the rest of the year, crayfish accounted for < 10% of the monthly energetic intake, and cyprinids and toads were the most important prey. The seasonality of crayfish consumption, as well as that of cyprinids, toads and frogs, was highly correlated with fluctuations in their availability. However, the consumption of eels during their period of maximum availability was unexpectedly small, probably because this was also when crayfish were most abundant. 3. In spite of this, crayfish and eels co-occurred more frequently in spraints from the same months than could be expected from chance, whereas crayfish and fish other than eels tended to occur separately. It is suggested that when feeding on crayfish, otters also have a high likelihood of catching eels (a preferred prey for otters). 4. Although the introduction of the crayfish apparently increased the availability of prey to otters from spring to early autumn, there were no perceptible increases in food resources for the remainder of the year. Otter populations inhabiting Iberian streams may still be limited by indigenous prey species, such as cyprinids, which make up the bulk of the diet in late autumn and winter. The maintenance of indigenous prey should thus be considered a major issue in otter conservation strategies, even in areas holding abundant populations of introduced American crayfish.