Otolith microstructure of endangered Colorado squawfish Ptychocheilus lucius was investigated to determine patterns of otolith growth and to validate daily deposition of increments. Sagittae and lapilli formed prior to hatching. After fish hatched, otolith increments were deposited daily whether larvae were reared at a constant 22°C temperature or subjected to fluctuating temperatures (±2.5°/d) centered at 18, 22, or 26°C. Otolith increments were clearer and counts of increments were more accurate for fish reared at fluctuating than at constant temperatures. Otolith growth was lower at 18°C than at 22 or 26°C, but evidence of a direct effect of temperature on otolith growth was inconclusive. Lapillus diameters of slow‐growing Colorado squawfish were larger than those of similar‐sized but fast‐growing fish, indicating that fish and otolith growth rates were not proportional. When larvae were starved, growth in body length generally ceased immediately but otolith growth continued for up to 15 d. Otolith growth was reduced for up to 5 d after starved fish began to feed. Timing of starvation and reduced growth may not be accurately recorded by reduced otolith increment spacing. Low‐contrast otolith increments in wild fish may indicate periods of low food abundance and starvation. Increased otolith growth early in life could reflect the start of exogenous feeding by Colorado squawfish larvae, a habitat shift to warmer water, or both. Otolith analysis will be useful for elucidating age, growth, and recruitment patterns of young Colorado squawfish.

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 238C. 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. Dualvertical-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.

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We estimated exclusion and survival rates of fathead minnow Pimephales promelas exposed to four configurations of a high‐velocity inclined profile‐bar screen. These screens are functionally different from conventional positive‐barrier designs because fish behavior and swimming ability are not design considerations. We tested screens inclined at 45° or 60° that had 1.0‐mm or 0.5‐mm slot widths and used 5–45‐mm total length fathead minnow released high or low in the water column. The exclusion rate for 45.0‐mm and 22.5‐mm fathead minnow was 100%. Survival of 45.0‐mm fish was 88%; latent mortalities were attributable to nonscreen causes. Survival of 22.5‐mm fish was 100%. Exclusion rates for high‐ and low‐release 12.5‐mm fathead minnow were nearly 100%. Survival rates for high‐release 12.5‐mm fathead minnow were 62– 86% and were similar to or higher than those for low‐release fish (15–71%). Exclusion rates of 7.5‐mm and 5.0‐mm fathead minnow in tests with the 0.5‐mm screen were 88–95% regardless of release position. Exclusion rates for 7.5‐mm and 5.0‐mm fish tested with 1.0‐mm screens were mostly lower (2–90%), especially for low‐release fish. Survival rates for 7.5‐mm fathead minnow in high releases were 26–62%, but survival rates for low‐release 7.5‐mm fish (0–9%) and 5.0‐mm fish (28%) were low. The screen angles we tested had little consistent effect on exclusion or survival rates. The successful exclusion and survival we documented for various life history stages of fathead minnow, coupled with the high hydraulic efficiency and self‐cleaning properties of high‐velocity inclined profile‐bar fish screens, indicates that this is a potentially effective tool for managers seeking to reduce entrainment loss of fish in aquatic ecosystems.