No abstract

Flowing waters may represent a force that structures the locomotor capacity of stream fishes. We used a modified critical swimming speed (Ucrit) 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 (55Ucrit from 26.33 to 69.00 cm·s–1) and was positively correlated (r2 = 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.

Urbanization alters stream and watershed hydrology so that fish from urban stream systems are confronted with extreme flows during storms and runoff events. To test whether residence in urban streams is associated with altered swimming ability, we compared sprint and endurance swimming performances of eight populations of blacknose dace (Rhinichthys atratulus) from different watersheds along an urban/rural gradient. Watershed impervious surface cover, a measure of urbanization, was significantly correlated with sprint performance in dace from all stream types and endurance swimming performance (U(crit)) when only fish from urban streams were analyzed. Three estimators of water flow in a stream system, watershed area, mean annual discharge, and base-flow current speed, were all related to U(crit) in fish from nonurban streams. The U(crit) was significantly repeatable after 6 mo in the laboratory, but dace populations with exceptional U(crit) values lost ability under no-flow, "detraining" conditions. Sprint performance changed substantially in individual dace after 10 wk under no-flow conditions and was a significant function of the animal's original performance. Animals with high sprint performance tended to lose ability, whereas those with poor performance gained ability. Interpopulation differences in both sprint and endurance swimming were robust over multiple years of collection from the same sites.