Abstract

Annual roach recruitment success was variable in the rivers of England between 1969 and 2001. Spatial and temporal patterns revealed common phases of weak and strong recruitment. Strong recruitment periods were 1975/77, 1983/85, 1989/90 and 1995/96. The recruitment mechanism appeared to be climate driven, with temperature and discharge the major variables in determining the recruitment strength of a cohort. The key phase appeared to be during the first year of life of the cohort and in particular during critical periods in their growth year when susceptibility to displacement by increased discharge was determined by their actual body length. The recruitment pattern produced a similar numerical population abundance trend, with increased abundance following the successful recruitment of a strong year class. This was in contrast to biomass, which was relatively stable.