Fish, climate and the Gulf Stream: the influence of abiotic factors on the recruitment success of cyprinid fishes in lowland rivers

1. Climatic effects are increasingly being recognised as an important factor causing inter-annual variability in organism abundances in aquatic and terrestrial ecosystems. This study investigated the relationships between water temperature (cumulative degree-days >12 °C), river discharge (cumulative discharge-days above basal discharge rate), the position of the North Wall of the Gulf Stream (NWGS), and the 0+ growth (September mean length) and recruitment success (year-class strength, YCS) of three species of cyprinid fishes in two contrasting English lowland rivers, using a 21-year dataset. 2. Contrary to the majority of studies on 0+ fishes, growth in the Yorkshire Ouse was most significantly correlated with river discharge, with water temperature of less importance. By contrast, temperature was more influential than discharge in the River Trent, possibly because of its regulated hydrological regime, although none of the relationships were statistically significant for this river. 3. Year-class strength of roach (Rutilus rutilus) was positively correlated with the position of the NWGS, and there was evidence of synchrony in recruitment success between rivers, but the relationships were poorer for chub (Leuciscus cephalus) and dace (Leuciscus leuciscus). The strongest relationships between YCS and discharge during specific time periods were for when the fish were in their early (especially larval) developmental stages, although none of the relationships were statistically significant because of inter-annual variations in river discharge relative to the timing of fish hatching. 4. Fishes are key predators in the majority of aquatic ecosystems and, as such, fluctuations in their abundances can have implications for ecosystem functioning as a whole. This study has demonstrated an underlying influence of broad-scale climatic effects on the recruitment of riverine fishes, in spite of local variations in biotic and abiotic conditions. The relative importance of various abiotic factors on the recruitment success of riverine cyprinid populations varies spatially and temporally. For example, river discharge is likely to be of relatively greater importance in poorly-structured rivers or those that are prone to large and rapid fluctuations in flow, while temporal variations occur because of inter-annual differences in river discharge relative to the timing of fish hatching. Biotic factors may also be important determinants of fish recruitment success, especially in