Idaho Study Shows Catch-Release Trout Fishing During Low Water, High Temperatures Does Not Harm Trout Population Numbers

By Martin Koenig, Idaho Fish and Game Natural Resource Program Coordinator

Last summer was particularly dry and hot in much of Idaho, renewing concerns among some trout anglers about fishing when water temperatures climb. Anglers are often concerned that maybe they shouldn’t be fishing on the hottest days for fear that they might be reducing trout populations, but a recent study on Idaho rivers during a hot summer showed catch-and-release angling did not harm the trout population.

Higher stream temperatures have become more common in recent years, leading anglers to ask two common questions. First, is catch-and-release fishing actually causing harm to trout populations by fishing on summer days when water temperatures get warm? Secondly, should Fish and Game use “hoot owl” fishing closures to prohibit even catch-and-release fishing when water temperatures get too warm? Last summer, Idaho Fish and Game biologists performed a study to help answer these questions. Biologists also provide a real-world example to put this in perspective of actual trout populations.


The study included four streams in eastern Idaho, where anglers caught dozens of trout ranging from 7 to 15 inches, on days where water temperatures ranged from 56 to 78°F. Biologists marked each landed trout with a small external tag, recorded the water temperature when it was caught, and then released it back into the stream. 

Anglers kept careful track of how long they fished, and how many trout they caught, so they could estimate a “catch rate” (or, how good the fishing is). A few weeks later, biologists returned and surveyed the trout population with electrofishing gear to look for the tagged trout. Using the tags from recaptured trout, biologists could see if water temperature at the time the fish were landed had affected which trout survived and which ones didn’t.

The study found that mortality was 69% higher for trout landed at 73°F water temperatures than for those landed when waters were less than 66°F. These results suggest that higher water temperatures were indeed decreasing the survival of caught-and-released trout. However, catch rates were much lower (77% lower!) at the higher water temperatures above 73°F, and much better when temperatures were below 66°F.

So while mortality was higher at the hottest temperatures, the number of trout caught was much lower because it was much harder to catch fish at those warmer temperatures. This phenomenon is well known to trout anglers, who often stop fishing in the heat of the day because catch rates are poor compared to cooler times of the day.


Conditions for trout in the Big Wood River were tough during the summer of 2021. It started off with a low snowpack, and the weather got hot quickly in the spring, and stayed that way all summer. River flows peaked early and stayed low throughout much of the year. Fish and Game monitors the trout population here on a 3-year rotation in the fall. There were no “hoot owl” fishing restrictions in place during 2021, so the population surveys that fall offered a chance to see how the trout population was looking after a season of fishing during the long, hot summer. In addition, information reported by anglers catching tagged fish could be helpful to know how many fish in the population are typically caught.

In fall 2021, Fish and Game staff surveyed the trout (and whitefish) population of the Big Wood River. This was done using electrofishing gear and mark-recapture calculations to estimate the total number of trout in the river.

One way biologists estimate the number of trout caught by anglers is to tag them, then release them back into the river. In 2014, Fish and Game biologists tagged 159 trout throughout the Big Wood River. In the three years after being tagged, a total of 14% of those trout had been caught (and none were reported as being kept). Dividing that up evenly across the 3 years, the average annual encounter rate (amount of fish caught by anglers) is only about 5% per year.

Now let’s do a little basic math to estimate the impact of catch-and-release fishing on the population. The most recent population survey estimated the Hailey section of the Big Wood River had approximately 2,422 trout. While the tags reported 14% of trout were caught over the three years (encounter rate), let’s just assume they were all caught in the first year just to be cautious.

If anglers caught and released 14% of the Hailey reach trout population that would be about 340 trout.

(2,422 trout) x (14% encounter rate) = 340 trout caught

Of those caught and released, other studies typically suggest a 5% mortality rate. But let’s assume conditions were stressful and that 20% of the released trout died, which would be 68 trout that did not survive.

(340 trout caught) x (20% mortality) = 68 dead trout

Those 68 trout that died from catch-and-release fishing would make up about 2.8% of the total population.

(68 trout) ÷ (2,422 total Hailey reach trout) = 2.8% of the population

In the Big Wood River, the normal yearly mortality of trout is between 50-70%! So when we put that into perspective, catch-and-release fishing is having very little impact, even if we assume 20% of those caught trout died after being released, which biologists estimate is more like 5% actually. Even if the encounter rate was 50%, we still would only impact 10% of the trout population.

Despite the hot, dry summer, the fall 2021 population survey on the Big Wood River found the trout population is healthy. It doesn’t look like normal fishing activity during summer of 2021 had any impact on the overall trout population.

Most of the time, conditions during the winter are the real drivers of trout population changes. Low water flows and extremely cold temperatures can combine to make tough conditions for trout to survive in winters. Last year’s drought conditions meant low winter flows, so despite the stability of trout populations leading up to winter, we could see lower trout numbers coming into the 2022 fishing season, but time will tell.


Trout anglers cause some low amount of fishing mortality regardless of the water temperature. However, while mortality of individual trout goes up at higher water temperatures, angler catch success goes down at the same time. Because catch rates drop as the water temperatures climb, anglers are causing as much fishing mortality at lower water temperatures as they are at higher water temperatures because they just catch fewer trout.

It goes without saying that warm water and wild trout do not go well together. Trout need cold, clean, well-oxygenated water to thrive. Droughts and hot summers have become more common lately, and biologists also consider the added stress trout face by living in warmer water. Stress is nothing new to trout, which are adapted to deal with stresses like being chased by predators like cormorants and river otters, or dealing with extreme flood waters in the spring. However, we need to keep in mind that trout populations are very resilient. After stressful events such as floods or drought, trout populations can rebound quickly when conditions are good.

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. Keep in mind, water temperatures in trout streams above 73°F are rare.

We studied summer water temperature data for Idaho’s most prominent wild trout fisheries. In the last several years, temperatures exceeding 73°F are extremely rare, occurring in only about 1% of the 300,000+ hours of data we gathered. So in the vast majority of Idaho’s wild trout stream fisheries, they rarely see extremely warm water temperatures in the first place. On those occasions where trout streams do get above 73°F, anglers may want to pause fishing because catch rates are noticeably lower, but not because they are worried about harming the trout population.

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