PNNL

Abstract

Under a research program aimed at identifying culvert configurations and associated hydraulic conditions that foster successful upstream movement of juvenile salmon, researchers investigated the ability of hatchery-raised juvenile coho salmon (Oncorhynchus kisutch) to leap into an experimental culvert under varying hydraulic conditions. Five outfall drops ranging from 0 to 32 cm were tested to represent differing degrees of culvert perch height. Trials were run at a flow of 1, 1.5 and 2 cfs and tailwater pool depth of 30 cm. Video cameras monitored and recorded fish leaping and other behaviors in the tailwater pool and culvert inlet. The success rate was highest at the 0-cm drop (71%), followed by (30%) at a 12-cm drop, (18%) at 20-cm drop, (3%) at 26-cm drop, and (0%) at 32-cm drop. When overcoming the 0-cm (streaming flow) and 12-cm outfall drops, fish predominantly used swimming behavior. For the outfall drops of 20 and 26 cm, leaping behavior dominated. In a typical leaping event, fish initiated the leap near the floor of the tailwater tank and used burst swimming to propel their bodies from near the standing wave (produced by the fall of water exiting the culvert) into the culvert entrance. Both high- and low-trajectory leaps were observed; some fish attained a height up to 5.2 times their body length. The rate of successful entry events increased with test duration. Leaping success was greatest for fish that used the forward motion of the standing wave at the edge of the plunge plume. No significant differences were observed in range of fish sizes (60 to 138 mm FL) for fish successful at culvert entry compared to those that remained in tailwater pool. These results indicate that juvenile coho salmon are capable of overcoming some degree of culvert perch to move upstream if tailwater pool water depth and pool size are sufficient and hydraulic passage conditions inside the culvert are favorable. Even though some juvenile coho salmon are capable of overcoming a perched culvert, this condition should be considered a practical barrier to upstream movement.