Creating fast flow refuges in streams

Posted on October 15, 2015

Hydroelectric power plants create frequent, intense pulses in downstream river flow (hydropeaking) that are typically accompanied by increased sediment movement, high turbidity and habitat scouring. These changes can affect several aspects of the biology of fishes, including migration, spawning, early development and growth. In Switzerland, a 60% decline in catches of brown trout since 1980 has been linked to hydropeaking and the channelization of streams. To develop designs for artificial refuges that can be used by fish under hydropeaking conditions, researchers recorded the behaviour of juvenile trout (Salmo trutta) in a laboratory flume fitted with a rectangular side chamber. The main channel was 12 m long and 1.2 m wide and the side channel was 2 m long and 1.2 m wide. The effects of 11 different deflection structures, which diverted water from the main channel and through the side refuge, on flow patterns and fish behaviour were compared. Batches of test fish had mean lengths of 12.5-16.5 cm. After fish had been placed in the flume, hydropeaking conditions were simulated by raising the velocity from 0.1 to 0.8 m/s over a 15 min period. Each trial lasted three hours. In the absence of any deflecting structure there was almost no exchange of water between the channel and the refuge, where fish spent a third of their time. In contrast, when deflecting structures were added the rate of water exchange increased to 11-22% and refuge use rose to 57-87%. Fish located the refuge quickly, usually by approaching along the flume wall from downstream and entering the refuge in the shear layer between inflowing and outflowing water where velocities were close to zero. The most successful design was a triangular island structure that pointed into the mouth of the refuge. As well as having the highest residence rate for the test fish, this was a stable, erosion-resistant refuge structure. It also created a well-defined flow path and once fish had entered it, they were relatively loathe to leave. In applying these findings to field conditions, the researchers recommend that refuges be 10-15 m long, with the island separated from the bank by at least 3 m to prevent the shelter being clogged by driftwood. The bottom of the refuge should be 1 m above the riverbed to stop bed sediment from washing in, and the water should always be at least 0.5 m deep to avoid fish stranding. This type of refuge is relatively efficient at concentrating drifting food organisms as well as fish.

Reference: Ribi, J.-M. et al. 2014. Attractiveness of a lateral shelter in a channel as a refuge for juvenile brown trout during hydropeaking. Aquatic Sciences 76, 527–541.