Study Shows Catch-and-Release Fishing in High Water Temps Doesn’t Significantly Impact Trout Populations

During the hottest parts of the year, fish and wildlife agencies in western trout fishing states often initiate hoot-owl restrictions, closing fishing during the afternoon hours. In some cases, they’ll even close entire streams to fishing temporarily. These rules are enacted to protect trout, which struggle in warm water conditions that are often caused by low flows. However, Idaho doesn’t enact summer fishing restrictions and the Idaho Department of Fish and Game (IDFG) recently released a study demonstrating why.

Last summer, IDFG biologists tagged trout and then released them, and anglers kept track of their catch rates of tagged fish. The biologists did this in four streams with water temperatures ranging from 56 to 78 degrees. Crews returned to the same stretches a few weeks later to see how many tagged trout survived. Mortality did increase by 69 percent for fish caught and released in 73-degree water—but there’s a hitch. At those temperatures, the anglers’ catch rates decreased substantially. At higher water temps, the average catch rate was 77 percent lower than in cool water. So while mortality was higher, the number of trout caught was much lower—effectively negating any difference in the impact of catch-and-release fishing on the overall population. 

In the summer of 2021, the Big Wood River, near Sun Valley, Idaho experienced high temps and low flows, but the river never closed to fishing. Surveys show that of 2,422 trout in one stretch of the river, anglers caught 340 of them, with an estimated 68 dying from catch-and-release fishing. That mortality accounted for less than three percent of the total population, while the overall mortality rate for the river’s trout was actually closer to 50 to 70 percent. This shows that catch-and-release fishing isn’t killing the bulk of the fish.

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Idaho plans not to implement hoot-owl or water temperature-related restrictions this summer, though other states in the West are likely to begin enacting them in the next couple of weeks. “Despite mortality of individual trout, studies show catch-and-release angling works well to recover overfished populations or conserve vulnerable populations. This is why Idaho Fish and Game is hesitant to close fishing in wild trout waters based on warm water temperatures,” write the study’s authors. “On those occasions where trout streams do get above 73 degrees, anglers may want to pause fishing because catch rates are noticeably lower, but not because they are worried about harming the trout population.”