AN ANALYSIS OF OTTER LUTRA-LUTRA PREDATION ON INTRODUCED AMERICAN CRAYFISH PROCAMBARUS-CLARKII IN IBERIAN STREAMS

1. Patterns of otter, Lutra lutra, predation on introduced American cr ayfish, Procambarus clarkii, were examined in an Iberian intermittent stream over a 2-year period. in order to assess the relative importanc e to otters of this new food resource. Diet was described from spraint s collected monthly. and the importance of different prey types was de termined in terms of their relative numeric and energetic contribution s to the diet. Monthly fluctuations in diet and abundance of principal prey species were compared. 2. Energetically, the crayfish ranked sec ond after eels, accounting for 18% of the estimated annual intake of e nergy. Crayfish and eels were particularly important in the diet from April to October. For the rest of the year. crayfish accounted for < 1 0% of the monthly energetic intake, and cyprinids and toads were the m ost important prey. The seasonality of crayfish consumption, as well a s that of cyprinids, toads and frogs, was highly correlated with fluct uations 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 o f 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 t hat when feeding on crayfish, otters also have a high likelihood of ca tching eels (a preferred prey for otters). 4. Although the introductio n of the crayfish apparently increased the availability of prey to ott ers from spring to early autumn? there were no perceptible increases i n food resources for the remainder of the year. Otter populations inha biting 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 conside red a major issue in otter conservation strategies, even in areas hold ing abundant populations of introduced American crayfish.