The influence of environmental factors on movements of lowland-river fish in the Yorkshire Ouse system.

The influence of several environmental parameters on movements of fish within the Yorkshire Ouse system, north-east England, is considered. Automated monitoring of fish with passive integrated transponding (PIT) tags, at the entrance to a fish pass on the lower Derwent, was used to examine factors influencing upstream migration of fish between May and August 1998. Overall, 85% of records were from the cyprinids, chub Leuciscus cephalus, dace Leuciscus leuciscus and roach Rutilus rutilus, which are the dominant species in the lower Derwent. Daily numbers of PIT-tagged fish entering the pass in summer were significantly and positively correlated with daylength, but not significantly correlated with mean daily temperature or mean daily flow. There were significant variations in the diel pattern of PIT records between species. Adult chub mostly approached the fish pass at night, while other species entered over a wider range of times, and stocked juvenile cyprinids entered mostly during daytime. Analyses of movements of radio-tracked barbel from the lower Nidd showed that downstream displacements associated with high flow in summer were often followed by homing to the original residence area, but that in autumn displacements were significantly more frequent and homing was significantly less frequent, resulting in a tendency for barbel to move downstream. It is concluded that an appreciation of the effects of environmental parameters on movements by different fish species is important in understanding the causation of spatial variations in distribution of fish in lowland rivers.