Last month, The Fisheries Blog ran a very interesting piece on flow management and trout populations. The author discuses the impact increased flow has had on population in a section of the Colorado River:
“A thriving tailwater rainbow trout fishery exists below Glen Canyon Dam in Arizona. The 15 miles between the dam and Lees Ferry is managed for a Blue-Ribbon trout fishery which attracts anglers from around the world. What we do know is that dam operations play a major role in influencing the trout population. Prior to 1991, the flow regime was characterized by extreme fluctuation, which prevented or limited natural fish reproduction, and thus the fishery was supplemented through stocking. Subsequently, the dam has produced modified low fluctuating flows intended to reduce flow variation, and the fishery responded by sustaining itself through natural reproduction. Other flows have been tried, most notably the high-flow experiments (HFE) conducted in 1996, 2004, and 2008. …
The 2008 high-flow experiment caused managers to scratch their heads. The rainbow trout population increased to nearly three times the size it was the previous year, a response that was not seen during the previous two high-flow experiments. It turns out that the high flow did a good job at cleaning the gravels which provided quality habitat and hiding spaces for trout fry. The food base in the river was also enhanced because there was an increase in food items that were highly palatable for trout (i.e., black flies and midges).”
It goes on to discuss a sustained higher flow in 2011 that led to even higher population numbers and the best catch rates on the river to date.
The full article is at: http://thefisheriesblog.blogspot.gr/2012/11/dam-trout-how-do-trout-populations.html