Pacifastacus leniusculus has established hundreds of breeding populations in Britain since its introduction in 1976. One such population was established in the River Great Ouse, a major lowland river in mid-to eastern England, from 140 crayfish introduced in 1984. Estimates of growth, density and biomass of the population in 1992 are presented. Under laboratory conditions, the per moult increment (MI) and percentage per moult increment (PCMI) of crayfish were binomially correlated to their carapace length (CL), that is, the MI increased with CL and then decreased as crayfish grew old, and the PCMI decreased with increase in CL. Based on length frequency distribution data collected from the river in October and November and estimated by Bhattacharya's method, seven male cohorts while only four young female cohorts were identified because of insufficient data for old cohorts. The mean CL of each crayfish cohort was 16.6, 31.2, 41.6, 52.0, 57.8, 64.2, and 69.2 mm for 0+ to 6+ year old males respectively, and 15.4, 28.5, 39.7, and 49.8 mm for 0+ to 3+ females. There were no significant differences between growth rates of either CL or WW of males and females at age for 0+ to 3+ (no data of females > 3+ were available for comparison).

In a riffle, the densities and biomass of crayfish were significantly different between four domains with different bottom substrats and highest in the habitat with the highest density of cobbles and thus, greatest density of potential shelters. The mean densities and biomass of crayfish of all sizes in this riffle were between 10.m2 and 61 g.m−2 respectively in July and 15.m−2 and 53 g.m−2 in November. In a pool, mean density and biomass of crayfish > 30 mm CL, estimated by Fish-Ford multiple capture-mark-recapture methods, were 0.8 m−2 and 37 g.m−2 between May and August.