Upstream Movement of Juvenile Coho Salmon in Relation to Environmental Conditions in a Culvert Test Bed

Abstract

AbstractWe measured the 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‐ to 17‐h test periods). Passage success was higher for large (139 mm fork length [FL]) than small (55 mm FL) fish, was higher at night than during the day, was not affected by shading, decreased as tailwater pool depth increased (22.9–53.3 cm), and did not differ significantly among the fish densities tested in the tailwater pool (35–141 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 (0.028–0.099 m3/s; mean velocities, 0.59–0.98 m/s). The horizontal distribution of fish moving upstream that successfully exited 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—fish accomplish upstream movements in roughened culverts via pathways in the low‐velocity, low‐turbulence boundary layer. 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. The findings from these experiments in the culvert test bed are applicable to hydraulic designs of culverts where upstream passage of juvenile salmon is a concern.