The Ted Trueblood Chapter has devoted much time and resources to improving habitat along the lower Boise River with a goal of enhancing the wild trout fishery. Our activities have included work on several habitat projects, and in addition we have participated in a limited manner with Idaho Fish and Game’s periodic monitoring of the fish population.
The last population monitoring work occurred in early November 2010. We recently reviewed the report from IDFG and found some interesting information to share.
First, the report shows 2010 population estimates compared to previous years, and for one section along the Boise River we now have four different surveys for comparison.
The chart below (which we created from one of the data tables in the report) shows the estimates for the number of fish per square meter of water surface, comparing 2010 information with 2007, 2004 and 1994. We also added some historical context to each year of observation, such as whether there has been recent drought, special regulations to enhance the fishery, and recent habitat improvement projects.
Fish densities in the past few years are much better than that found in 1994. The early 1990s were drought years and winter flows were meager, in particular 1992-1993 the releases from Lucky Peak were only 80 cfs (and to their credit some of that water was a “loan” from irrigation districts). The other thing back then was in east Boise there were no quality fishing regulations as there are now.
So by 2004 fish densities were much better and the quality regulations have been in place for a few years. TU volunteers helped with the survey in 2004 and had first hand observation of the brown trout spawners.
Since 2004 the densities of brown trout have dropped but in 2010 there is a big jump in rainbow trout numbers. Since 2004 TU put in place some spawning habitat enhancement projects and so we will pat ourselves on the back since the rainbow trout numbers went up after the habitat projects were implemented. Seriously, we know the cause and effect process is much more complicated, but it would be nice to think the side channel work at River Run, Heron Creek and Warm Springs Golf Course helped in some way.
Just as important — maybe more important — is water: the winter time flow conditions. Here’s a chart showing what has happened over the past 20 years that helps put the fish density information in larger context:
The Boise River through town has experienced more favorable water flows in winter months in recent years. Drought years made winter 2001-2002 a tough year but since then water managers have been able to have relatively better flows, and much better than the early 1990s. (We scaled the y-axis to maximum 800 cfs to emphasize the lower flow years. 1997 winter flows averages 3,400 cfs and 2006 flows were 1,200 cfs.)
One more item from the IDFG report noted that “due to inexperience of volunteer netters used during the marking run, we captured more fish during the recapture run which resulted in wide confidence intervals around population estimates.” While we have helped some with these field surveys it is worth considering whether the fish netting should be left to the fisheries technicians who have more experience handling the nets and walking backwards downstream. It is not easy work, and having the best set of fish collectors as part of these surveys may result in improved data collection and more precise population estimates. We middle aged fish enthusiasts can cheer from the sidelines, or in this case, the river bank.
