Swimming in two-vector flows: Performance and behavior of juvenile Chinook salmon near a simulated screened water diversion

We used a large, annular flume equipped with a simulated fish screen to assess the swimming and behavioral responses of juvenile Chinook salmon Oncorhynchus tshawytscha to two-vector flows typifying habitats near water diversions, where small fish may become entrained. Groups of 20 fish (4.4-7.9 cm long, at 12degreesC or 19degreesC) were tested for 2 h at one of nine experimental flow regimes derived from combinations of three (approach) velocities perpendicular to the screen and three (sweeping) velocities parallel to the screen and a (0-cm/s) control during daytime (lighted) and nighttime (dark) conditions. In the high-velocity (resultant vector) flow regimes, all fish swam at velocities comparable to the critical swimming velocities measured for similar-sized conspecifics, suggesting that exposure to such flow conditions near a water diversion is energetically expensive. Although most fish exhibited strong positive rheotaxis, older (smolt-size) fish acclimated to the warmer temperature exhibited higher rates of negative rheotaxis (particularly in the intermediate-velocity flow regimes), a behavior consistent with downstream migration. Fish life stage (length), time of day (light level), and water velocity influenced swimming velocity; sweeping velocity, swimming velocity, and rheotaxis influenced screen passage velocity. Regardless of the flow regime, juvenile Chinook salmon contacted the screen most frequently at night, and nighttime contact rates were not affected by the velocity of either flow vector. During the daytime, screen contact rates were inversely related to sweeping velocity and independent of approach velocity. Injury rates were low and unrelated to either flow or screen contact rates, and survival rates were high (>99%) in these predator-free experiments. Fish screen designs that minimize screen exposure duration (e.g., via reduced screen length or increased sweeping velocities) should optimally protect valuable juvenile Chinook salmon, a species that encounters multiple water diversions along many or its migratory paths to the ocean.