An evaluation of the use of critical swimming speed for determination of culvert water velocity criteria for smallmouth bass

Critical swimming speed (U-crit) is a common measure of the relationship between exercise intensity and duration within the prolonged performance envelope. This relationship is often used to establish water velocity criteria for fishways and culverts; however, the technique involves the assumptions that fish will choose to move at (1) a swimming speed equivalent to U-crit and (2) a ground speed that results in a passage time equal to the interval used in the critical speed protocol. The objective of this study was to evaluate the validity of these assumptions and the usefulness of velocity criteria based on critical speed data in predicting passage success. To accomplish this, I measured the critical swimming speed of wild smallmouth bass Micropterus dolomieu in a respirometer and the relationship between the water velocity and swimming speed of fish in a 50-m raceway. The latter was then used in conjunction with the former to calculate water velocity criteria based on ground speeds actually attained by fish, thereby eliminating the second assumption. Critical swimming speeds ranged from 65 to 98 cm/s and were positively correlated to fish length (24-44 cm). Swimming speeds attained by free-swimming fish in the raceway increased significantly with water velocity. Maximum allowable water velocities for raceway fish (adjusted appropriately for expected swimming speed) ranged from 54 to 63 cm/s, depending on fish length. Despite this, the proportion of individuals that made complete ascents against water velocities ranging from 40 to 120 cm/s was high (82-95%), and the probability of a successful ascent did not change significantly with water velocity, fish length, water temperature, exposure time, or time in captivity. As such, it is concluded that critical speed should not be used to set culvert water velocity criteria for this species.