PNNL

Abstract

We measured upstream passage success of juvenile coho salmon (Oncorhynchus kisutch) in relation to select experimental factors in a culvert test bed at a salmon rearing facility on the Skookumchuk River in western Washington State. Passage success, the term used for the response variable, was defined as the number of fish in the headwater tank at the end of the test divided by the number of fish released in the tailwater pool at the beginning of the test (2-17 h test periods). Passage success was higher for large (139 mm FL) than small (55 mm FL) fish, was higher during night than day, was not affected by shading, decreased as tailwater pool depth increased (22.9 to 53.3 cm), and did not differ significantly among the fish densities tested in the tailwater pool (35 to 141 fish/m3). There was a clear, negative exponential trend in the response relationship between transformed (arcsine of the square root) passage success and culvert discharge (0.028 to 0.099 m3/s; mean velocities 0.59 to 0.98 m/s). The horizontal distribution of fish moving upstream successfully exiting the culvert into the headwater tank was skewed to the right side of the inlet where the reduced velocity zone (RVZ) was located. This observation supported the RVZ hypothesis about upstream movement of juvenile salmon in a culvert—the pathway within some culverts with low velocity, low turbulence, and the cues necessary for orientation and entry enhance passage success. To facilitate salmonid passage at road crossings, the preferred resource management alternatives are bridges or stream simulation, but in situations where these approaches are constrained by cost or logistics, the hydraulic design of culverts may be appropriate. Findings from experiments in the culvert test bed are applicable to hydraulic designs of culverts where upstream passage of juvenile salmon is a concern.