GROWTH, DENSITY AND BIOMASS OF CRAYFISH, PACIFASTACUS-LENIUSCULUS, INA BRITISH LOWLAND RIVER

Pacifastacus leniusculus has established hundreds of breeding populati ons 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 gr owth, density and biomass of the population in 1992 are presented. Und er laboratory conditions, the per moult increment (MI) and percentage per moult increment (PCMI) of crayfish were binomially correlated to t heir carapace length (CL), that is, the MI increased with CL and then decreased as crayfish grew old, and the PCMI decreased with increase i n CL. Based on length frequency distribution data collected from the r iver in October and November and estimated by Bhattacharya's method, s even male cohorts while only four young female cohorts were identified because of insufficient data for old cohorts. The mean CL of each cra yfish 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 g rowth 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 be tween four domains with different bottom substrata 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.m(2) and 61 g.m(-2) respectiv ely in July and 15.m(-2) and 53 g.m(-2) in November. In a pool, mean d ensity and biomass of crayfish > 30 mm CL, estimated by Fish-Ford mult iple capture-mark-recapture methods, were 0.8m(-2) and 37 g.m(-2) betw een May and August.