Temporal and spatial variations in the growth of 0 group roach (Rutilus rutilus) in the River Great Ouse, in relation to water temperature and food availability

In the River Great Ouse, a highly flow-regulated lowland river in eastern England, the most common cyprinid fish, roach (Rutilus rutilus), showed marked variations in growth both between years and between sites. Growth rates were highest at the most downstream sites and also in a large marina into which many 0 group roach migrated in midsummer. Between-year differences in growth rates in early summer were not always associated with parallel deviations in water temperature (degree days >12°C). Instead, variations in prey (zooplankton) abundance may also influence the initial growth rates of roach larvae. A switch in the diet of 0 group roach in late June from small invertebrates (Cladocera, Chironomidae larvae, etc.) to one dominated by detrital aufwuchs may explain the discrepancies between observed growth rates and those predicted from a temperature growth model.