Swimming Performance and Fishway Model Passage Success of Rio Grande Silvery Minnow

We used a swim chamber, flume, and large-scale fishway models to assess the swimming performance, behavior, and passage success of endangered Rio Grande silvery minnow Hybognathus amarus. Field-captured silvery minnow (53-88 mm total length) swam 114-118 cm/s (i.e., up to 20.9 body lengths/s) in a swim chamber in water temperatures of 15, 19, and 23 degrees C. The relationship between time to fatigue and water velocity showed that endurance declined sharply at velocities above 60 cm/s, a threshold that is consistent with critical swimming speed estimates and may represent a transition from aerobic to anaerobic metabolism. Water temperature and fish length were positively correlated with swimming performance. At water velocities of 60 cm/s and less, silvery minnow routinely swam the equivalent of 50 km (125 km maximum) in a swim chamber in less than 72 h. The proportions of silvery minnow that successfully ascended a flume over sand, gravel, or cobble substrate declined as water velocity increased to 53 cm/s. Passage times increased at higher water velocities and at a faster rate over sand substrate because fish were stationary for longer periods over the lower-velocity boundary layers created by gravel and cobble at all velocities. Dual-vertical-slot fishway passage was 52% in a less turbulent flow of 78 cm/s; passage was 8% at a faster, more turbulent flow of 87 cm/s. Conversely, 75% of silvery minnow ascended a rock channel fishway with a holding pool present and a 1% hydraulic gradient in low (58-cm/s) and high (83-cm/s) mean flow velocities. Differences in willingness to swim, longer test duration, and the mosaic of water velocities created by the bed roughness elements may explain the higher silvery minnow passage success in the rock channel. Predictive swimming fatigue relationships, together with fish length and water temperature, may guide decisions regarding fishway lengths and velocities so as to permit passage of Rio Grande silvery minnow.