Changing water velocities repel migrating salmon

Posted on March 28, 2013

Hydroelectric dams represent formidable barriers for migrating fish, and over the last 40 years much research has been devoted to devising ways of guiding fish away from dam turbines and attracting them to safer alternative routes.  Despite this work, we still have only a poor understanding of the cues that fish use to negotiate dams and other stretches of river with complex hydrodynamics.  However, a key finding has been that migrating juvenile salmon tend to avoid areas of accelerating or decelerating water flow, presumably because under natural conditions, such areas may be associated with physical injury, migration delay or predation risk.  In recent experiments with migrating Chinook salmon (Oncorhynchus tshawytscha) from the Columbia River, fish of length 5-21 cm were placed in flume tanks in which the water flow was either accelerated (by controlling flow through an orifice in a downstream barrier) or decelerated (by installing wall-to wall plates at the entrance).  In decelerating flows the young salmon faced into the current and drifted, but when the rate of velocity change increased to a certain threshold they switched to active swimming into the current.  In both decelerating and accelerating flows, fish showed an avoidance swimming response when the gradient in water velocity along the fish’s body was greater than 1 cm/s per cm of fish length.  This threshold agrees with other observations and modelling work – for example, one study found that Atlantic salmon didn’t pass a modified weir when the velocity gradient exceeded 1 cm/s per cm of fish length.   Interestingly, this threshold seems to be energetically significant, because at this velocity fish are able to maintain position in the flow by swimming at their optimal rate of around one body length per second.

Reference:   Enders, E.C. & Gessel, M.H.  2012.   Effects of decelerating and accelerating flows on juvenile salmonid behaviour.  Transactions of the American Fisheries Society 141, 357–364.