An angler releases a rainbow trout in the Gray Reef section of the North Platte River while keeping the trout in the water as much as possible. For those who fish in the heat of summer, biologists recommend quickly landing the fish before releasing. (Christine Peterson)

In the last 60 days, temperatures in portions of northern and southwestern Wyoming have run up to 8 degrees above normal. And that isn’t just highs. 

If a town is used to, say, nightly lows in the mid-50s, it’s been experiencing lows that never drop below 60 and highs cresting well into the 90s, said Chris Jones, meteorologist with the National Weather Service in Riverton. 

“We don’t see the relief we’re used to getting at night, and that is what a lot of times becomes more noticeable to people,” Jones said. “If it doesn’t get down to 66, for example, and it’s 71, it gives a boost to the next day, and dries out soil more, and then it will keep allowing the temperature to get a little higher every day.”

This summer’s record or near-record temperatures, combined with low winter snowpack and exceptionally low river flows, are causing fisheries- and land-managers across the state to issue grave warnings to anglers. Fish are dying, and fishing in the heat of the day will make it worse, they say. 

Some of those warnings come in the form of recommendations, and others entail changes to fishing regulations. In Yellowstone National Park, for example, fishing is now prohibited after 2 p.m. The park also has a warning for those unwilling to adhere.

“Your cooperation will protect the park’s fisheries and may preclude the need to prohibit fishing at all times of the day on some rivers and streams if conditions worsen,” the park said in a news release.

Hot water, dead fish

Warm river temperatures aren’t unusual for Wyoming in late summer. Fisheries biologists often recommend in mid- or late-August that anglers be more careful catch-and-release fishing or only fish in the early hours of the day. 

But this summer’s conditions have been more severe than usual.

An angler casts for fish below a rock wall in a pool of the Clarks Fork River. (Daniel Kenah/WyoFile)

Not only have temperatures been well above average in many places, but precipitation has also been scarce, particularly in the north and western portions of the state, Jones said. 

By early July, places like Rock Springs were registering less than a fifth of normal rainfall since April 1. The ground is so dry that even when rain did fall recently, very little absorbed into the earth, Jones said. In Rock Springs, for example, when over an inch of rain fell in late July, the dangerous combination of downpour and sun-baked soils created urban flooding

“The ground isn’t receptive,” Jones said. “It’s like concrete, you get a lot more run-off.”

The situation could have serious implications for a number of wildlife species including deer and pronghorn if the drought persists. Shrubs and plants don’t grow, making it difficult for some animals to get enough nutrition heading into winter. 

But it’s fish that concern biologists most right now.

Trout are cold-water animals that require high oxygen levels. The warmer the water, the lower the oxygen. 

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Prolonged water temperatures higher than 75 degrees Fahrenheit are dangerous to trout, and anything over 80 degrees is deadly, said Hilda Sexauer, Wyoming Game and Fish Department’s Pinedale fisheries supervisor. 

Add in the stress of being caught, fought and released, and many fish won’t have a chance, she said. 

That’s why Yellowstone placed restrictions on fishing and Game and Fish has sent out multiple email blasts asking anglers to think about trout before heading to the water. 

And based on the forecast, the conditions aren’t likely to improve. 

Temperatures are likely to be hotter than normal with lower than average rainfall well into October, Jones said. 

“Can you get a week out of the ordinary? Absolutely,” Jones said, but added that the overall outlook is hot and dry. 

Consider alternatives

For many anglers, the question for the rest of the summer becomes: Should they fish?

The answer is nuanced, fish biologists say. 

Game and Fish isn’t prohibiting fishing, even in the afternoons, but it is recommending anglers don’t catch and release fish after 2 p.m. in hotter, lower-elevation parts of the state. Department biologists also ask that anglers land fish as fast as possible and keep them in the water. 

Cleaned rainbow and brook trout, ready for the pan. “If a fish is exhausted and cannot hold itself upright, and if regulations allow, consider having it for supper because the fish has a poor chance of surviving,” Game and Fish recommendations state. (Matthew Copeland / WyoFile)

Other recommendations include avoiding squeezing fish or putting fingers under their gills, and considering using barbless hooks for quicker release, and artificial flies or lures if you plan to release the fish.

Biologists also encourage anglers to assess the health of a fish before releasing it. 

“If a fish is exhausted and cannot hold itself upright, and if regulations allow, consider having it for supper because the fish has a poor chance of surviving,” Game and Fish recommendations state.

Water is hottest in the afternoon and evening. Fishing at sunset is far more dangerous for trout than sunrise, even if the air temperature feels cooler. 

Anyone fishing in the heat of summer should also consider bringing a thermometer along with them to measure water temperature, said Anna Le, an aquatic biologist and educator in Yellowstone. 

There is no single magic “safe” temperature for trout fishing. Species, current speed, river structure, water quality and myriad other factors contribute to survivability. Many experts and anglers, however, advise calling it a day when water temps reach 68 degrees.

Le has shown up to the banks of rivers in Yellowstone ready to fish and turned around because the water temperature was just too high to ethically catch and release fish, she said. 

“For me, the concern is stressing them out, especially if you’re entering their habitat or hooking them when there’s low levels of oxygen in the water,” she said. She also worries about introducing bacteria more capable of flourishing in warmer temperatures.

Anglers can also consider fishing for warmer-water fish like sunfish, bass, catfish or carp. 

In Casper, Game and Fish announced fish kills at Yesness Pond. The heat may well stress and kill trout at small streams and lakes at low elevation around the region including Deer, Boxelder and LaPrele creeks, said biologist Matt Hahn in a yet another heat-related news release. 

Green River angler and Trout Unlimited Project Manager Nick Walrath has switched to targeting carp in places like Flaming Gorge Reservoir or even in the Green River itself because the water is too hot to catch and release trout, he said.

Heading to colder waters in high mountain streams and lakes is also safe. Some Wyoming mountain ranges, including the Snowy Range in southeast Wyoming, still have snow melting and feeding lakes, keeping temperatures plenty cold to responsibly catch and release. 

This winter may bring relief, Jones said. It is a La Niña year, which typically brings above-normal precipitation to the northwest portion of the state in the winter and an equal chance of above, average or below for the rest of the state.

With water temperatures in the 70s in many lower-elevation rivers and small lakes, fisheries biologists are encouraging people to find other destinations to fish, such as high-mountain lakes that are still cold. (Christine Peterson)

Christine Peterson

Christine Peterson has covered science, the environment and outdoor recreation in Wyoming for more than a decade for various publications including the Casper Star-Tribune, National Geographic and Outdoor...

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  1. A decent piece on which the reporter/writer worked hard only to have some tone-deaf photo editor illustrate the piece with a pile of gutted trout.

    1. Well at least they weren’t native trout – just introduced brookies and rainbows.
      Yep, I’m a snooty native fish guy.