Feeding ecology of the signal crayfish Pacifastacus leniusculus in a British lowland river

The feeding ecology of the introduced signal crayfish Pacifastacus leniusculus (Dana) was studied in the River Great Ouse, the major lowland river in mid- to eastern England. A total of 22 food groups were categorized from the gut contents. They showed that P. leniusculus is an omnivorous species. In terms of frequency of occurrence or gravimetric composition, the top five main diets for all sizes of crayfish in four seasons were similar. They were vascular detritus, filamentous green algae Cladophora, crayfish fragments, Chironomidae, and Ephemeroptera. Signal crayfish showed season-related feeding habits. Vascular detritus and Cladophora were ingested most frequently by all sizes of crayfish throughout the year, especially in the autumn and winter. Adult crayfish (> 45 mm in carapace length) consumed more Chironomidae in the spring while consumed more Tricoptera in the summer. Coleoptera were consumed more by the young-of-the-year (YOY) in the spring. Signal crayfish also had size-related feeding habits. The larger the crayfish, the more vascular detritus was ingested. The frequency of occurrence of Cladophora was increased with crayfish size in the winter while that was higher for the median sized crayfish (20-45 mm in carapace length) in the summer. Fish were predated mainly by adult crayfish in the summer and autumn. Cannibalism occurred mainly in the summer and autumn and increased with crayfish size. Epheroptera were increasingly consumed with crayfish size in the summer while Coleoptera were consumed only by the YOY. The rations estimated for three g-h periods of a day showed that P. leniusculus is a typical nocturnal feeder. The estimated daily ration ranged from 0.22 to 6.02% of crayfish body wet weight with higher rations in the adults than juveniles and in the summer and autumn than other seasons. The,gross efficiency of food conversion decreased rapidly with increase in crayfish size. (C) 1998 Elsevier Science B.V. All rights reserved.