This study was undertaken to examine some of the interactions between the introduced crayfish PACIFASTACUS LENIUSCULUS and its environment, and so to evaluate its adaptation and impact on the river system. Two methods for marking or tagging crayfish had been developed. one was an improved marking method. The other was the application of Passive Integrated Transponder tag systems.

In the river Great Ouse, P. LENIUSCULUS grew faster than the native crayfish AUSTROPOTAMOBIUS PALLIPES. In 1994, the mean number of pleopodal eggs per female at the end of April was 158. A recruitment of 70 juveniles per square meter was estimated for the original pool (P2) of introduction. By May 1994, 10 years after its first introduction, P. LENIUSCULUS had dispersed 5.5 kilometers (km) up river and 5.9 km down river from the point of introduction.

In 1993, the density and biomass (wet weight) of P. LENIUSCULUS were highest in summer at 4.0 per square meter and 133 grams (g) per square meter respectively, with annual means of 2.2 per square meter and 82 g per square meter for crayfish >30 millimeters (mm) CL in P2. The equivalent figures were 15 per square meter and 78 g per square meter, with annual means of 6.1 per square meter and 33 g per square meter for all sizes of crayfish in an adjacent riffle downstream. The estimated annual production of crayfish >35 mm CL in P2 was 52.58 g per square meter wet weight, 8.55 g per square meter dry weight, or 29.45 kilocalories per square meter, with an annual mean standing crop of 118 g per square meter wet weight and a turnover ratio of 0.44.

The juveniles of P. LENIUSCULUS ingested more benthic invertebrates, while adults fed more on vascular detritus and fish. Cannibalism occurred in all sizes of crayfish. Twenty-two food groups were classified from the gut contents over four seasons. The main diets were vascular detritus, filaments of green alga Cladophora, crayfish fragments, Chironomidae and Ephemeroptera. The estimated daily ration ranged from 0.22-6.02% of body wet weight. The gross efficiency of food conversion decreased with increase in crayfish size.

P. LENIUSCULUS had significant effects on the two main benthic fishes, COTTUS GOBIO L. and NOEMACHEILUS BARBATULUS (L.). In four riffles occupied by the crayfish, benthic fish were least abundant in the riffle where crayfish were most abundant, and increased gradually both up and down the river, as crayfish abundance decreased. In the laboratory experiments, crayfish predated the benthic fish and expelled the fish from shelters which the crayfish then occupied themselves. These interactions, together with the evidences of overlapping diets and the fish remains frequently found in the guts of crayfish from the river, suggested that it was the crayfish that were responsible for the reduction in the abundance of benthic fish in the riffles.

P. LENIUSCULUS was proved to be a burrowing species. It excavates extensive burrows in the mud banks and in the bottom of the river, with the highest mean density of 25.7 per meter of bank. High burrow densities resulted in the collapse of the river banks and can cause considerable soil erosion and river sedimentation.