In early April of this year, Idaho DFG conducted our seventh population estimate to survey the fish populations in the tailwater of the Big Lost River, where we caught 1,012 Rainbow Trout in the survey. We are most interested in gathering data on the Rainbow Trout and Mountain Whitefish populations in this portion of the river. Referring to the figure below, abundances of both species are increasing compared to our last surveys conducted in 2018. Rainbow Trout abundance has increased 26% and Mountain Whitefish abundance has increased 57%.
Although overall abundances are less than historical numbers, there are currently a lot of large Rainbow Trout greater than 16 inches present in the population. We estimated more than 850 Rainbow Trout greater than 16 inches per mile present in the fishery! In addition, we have a very strong juvenile year class in the population, which will contribute to the catchable-fish population in the upcoming years. Our population estimates include fish that are greater than or equal to about 6 inches, so the estimate does not include the juvenile fish that are smaller than 6 inches seen in the figure below.
We observed varying abundances in size classes over time (figure below) and we will be increasing the frequency of our population surveys in the tailwater to collect more up-to-date data that will help us better understand the population dynamics of these wild Rainbow Trout and Mountain Whitefish populations. We anticipate the Rainbow Trout size class shifting to more fish in the 8-16 inch range over the next few years as the older, larger fish die, which will provide more space and opportunity for the juvenile fish to grow and compete for resources.
We tagged a proportion of fish in the population with T-bar anchor tags that include a unique ID number that can be reported to IDFG at tag.idaho.gov. These tags provide fisheries biologists with important information that helps us better manage and understand our fish populations. If you catch a fish with tag, please report it at the web address.