South Canal of the Gunnison River
- To monitor fish population trends in the canal and evaluate the effectiveness of an electric fish barrier in reducing fish entrainment through a marked fish study
There are over 105,000 irrigation structures on rivers and streams across Colorado, most in fish-bearing waters. Fish entrainment in irrigation canals is known to be a large problem in the western U.S. but its impact on fish populations in Colorado is unknown. The South Canal, an irrigation ditch in southwest Colorado, diverts about 360,000 acre feet of water each year (around 1,000 cfs per day) from the Gunnison River into the Uncompahgre Valley for agricultural irrigation use. With the construction of several new hydropower plants on the canal in 2012, CPW expected to see an increase in fish mortality and impacts to recreational fishing in the South Canal and Uncompahgre River.
In order to reduce impacts on the Gunnison River's Gold Medal trout fishery, an electric fish barrier was installed at the east portal of the canal. Consisting of a series of vertically suspended electrodes, the system aims to discourage brood stock fish from entering the canal and potentially dying in the hydropower turbines.
CPW initiated a study to compare fish populations in the canal before and after the barrier was built and to document fish movement across the barrier with tagged fish.
Study results revealed that some fish passed through the barrier and survived the turbines, with more small fish successfully passing than large fish. Fish population estimates in the canal have declined after the electric barrier, but not significantly at the 95% level. A total of 288 tagged fish less than 12 inches and four fish greater than 12 inches were recovered below the barrier. No fish greater than 16 inches have been documented crossing the barrier. Only 1.3% of all tagged fish were documented to have been entrained in the canal while the barrier was operating.
The electric barrier appears to successfully exclude larger fish from the study reach, but not age 1 and age 0 trout. The electric fish barrier is successful in its stated objective, but not in reducing overall fish densities in the canal because the growth and survival of smaller fish maintains a stable population of fish in the canal. Reducing the entrainment of large fish is likely a benefit to the trout population of the Gunnison River. Further study is needed to evaluate if smaller trout can be successfully excluded by the electric barrier.