Juvenile Coho Salmon Leaping Ability and Behavior in an Experimental Culvert Test Bed

Abstract

AbstractAs part of a research program aimed at identifying the culvert configurations and associated hydraulic conditions that foster the successful upstream movement of juvenile salmon, we 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 outfall drop. Trials were run at a culvert discharge of 0.028 m3/s and a tailwater pool depth of 30 cm. The median success rate was highest (85%) for the 0‐cm drop, followed by 34% at 12 cm, 20% at 20 cm, 2% at 26 cm, and 0% at 32 cm. When overcoming the 0‐cm (streaming flow) and 12‐cm drops, fish predominantly used swimming behavior. For the outfall drops of 20 and 26 cm, leaping behavior predominated. 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 average fork length of fish that were successful at culvert entry was 103 mm (SD, 1.4), compared with 101 mm (1.3) for those that remained in the tailwater pool. Within the size range of the test population, 60‐135 mm (mean, 103 mm), fish size did not affect ability to enter the culvert under the test conditions. 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.