Swimming performance of blacknose dace (Rhinichthys atratulus) mirrors home-stream current velocity

Flowing waters may represent a force that structures the locomotor capacity of stream fishes. We used a modified critical swimming speed (U-crit) procedure to investigate the relationship between base-flow conditions and locomotor performance of blacknose dace (Rhinichthys atratulus) from five sites within three watersheds of Baltimore County, Maryland. Our modified test used 5-min intervals between incremental increases of 5 cm.s(-1) in swim-tunnel current velocity. This time increment represented a realistic transit time across riffles found in the home streams of dace. To characterize current velocity conditions of the streams, we measured current velocity at 55 evenly spaced points per site during base-flow conditions. Swimming performance varied greatly among 32 individual fish from the five sites (U-5(5)crit from 26.33 to 69.00 cm.s(-1)) and was positively correlated (r(2)=0.38, p=0.002) with mean base-flow current velocities at the site of collection. Additionally, among fish from the site with the widest and most even distribution of current velocities (from 0 to 54 cm.s(-1)), we observed the largest range of swimming performances. Our results suggest that variation in flow conditions among streams influences swimming ability of blacknose dace and can result in heretofore-unappreciated intraspecific variation in swimming performance.