This summer’s hot sun has pushed some anglers off the water before lunch, but it’s only the bad luck of a short-term weather pattern. Evidence continues to grow, however, that long-term climate change is impacting global water temperatures.

Rising water temperature changes the way fish live, and sensitive trout are among the first game fish to be impacted

Modern humans and many plant and wildlife species evolved to their present forms directly because of the long-term warming and cooling of the planet. This time we know enough to recognize how the changes occur and the degree to which human activity contributes to the change.

Rising lake temperatures force most fish to deeper, cooler water. In rivers and freestone streams, fish move to comfortable temperatures. When they move a long way, we call it a migration. But in limestone streams, fish can follow water temperatures only as far as the creek’s origin. Trout live near the headwaters of limestone streams.

Where water pools underground on top of impervious beds of limestone, it quickly settles at 52-54 degrees, the temperature of the rocks surrounding it. And whether it’s winter, summer or during a global temperature upheaval, the water leaks out of the ground at that temperature. As the spring water flows farther from its ground source, its temperature adjusts toward the ambient air temperature, surface waters first.

In 2015, fisheries researchers at Penn State University’s College of Agricultural Sciences published a method to rate the long-term effects of changes in the climate. Their scale measures how much farther anglers would have to drive to find native brook trout.

To identify streams likely to support wild brook trout in current and future climates, researchers combined models predicting stream temperatures and locations where they believed self-sustaining populations of brook trout might occur. They calculated the driving distance from 23 cities within the brook trout’s range to the 10 nearest stream segments likely to have wild brook trout under current and future scenarios. The research was published in the journal Fisheries.

But the study didn’t address how limestone streams that emerge from the ground at a constant temperature were expected to change. And the estimation of wild trout populations did not consider future anglers’ access to stocked trout.

For the trout, temperature tolerance is about more than comfort. Colder water holds more oxygen, needed to make the energy necessary to hold their bodies in place in the current. As the water warms, dissolved oxygen dissipates causing trout to feel stressed well before depleting oxygen levels threaten to kill them.

Trout temperature tolerances vary by species. Lake trout, which have lived in Lake Erie since the last Ice Age, require the coolest water. The prime temperature range is 42-44 degrees. They can live for 40 years, and many are caught at sizes between 25 and 30 inches. The state record — 31 pounds, 13 ounces and 40 inches — was set in 2019.

Lake trout reproduce in lakes Superior, Michigan, Ontario and parts of Lake Huron. In 2021 New York researchers found lake trout fry in waters near the Pennsylvania-New York border. The species was identified through genetic bar-coding, confirming for the first time since the 1960s the presence of naturally reproducing lake trout in Lake Erie. Still, in the shallowest and warmest of the Great Lakes, the autumn spawn is stymied by invasive zebra and quagga mussels, and sea lampreys literally suck the fluids from their bodies. The U.S. Fish and Wildlife Service stocks lake trout. Commercial exploitation is banned, and aquatic wildlife managers say the minimal recreational harvest has little impact at the population level. An estimated 70,000 lake trout live in Lake Erie.

Brown trout, originally imported from Europe in the late 1800s, and rainbows, brought in from the Pacific Northwest, are generally found in water that is 44-75 degrees whether they’re stocked or wild. Both begin to experience stress when the temperature reaches about 68 degrees. Temperature tolerance can vary by age. Brown trout adults and juveniles are more tolerant of higher temperatures than rainbow adults and juveniles. Because the browns generally live longer and grow bigger, only 650,000 browns are stocked in Pennsylvania, which has an annual trout stocking of 3.2 million.

“Rainbow trout usually exhibit higher survival to desirable size, support a fishery of longer duration and provide more fish of larger size in the second and third year after stocking than brook trout,” stated a 2007 Fish and Boat study of stocked trout movement patterns. “Rainbow trout are adaptable to a wide variety of conditions and efficiently use available forage for excellent growth.”

For that reason, and because they’re generally easier for anglers to catch, two-thirds of the trout stocked in Pennsylvania, some 2.1 million, are rainbows.

Brook trout, Pennsylvania’s state fish, are native to Appalachian waters. The Pennsylvania Fish and Boat Commission stocks an additional 450,000 per year. Stocked at 9-11 inches, they are bigger than most of their hand-size wild cousins. All are genetically predisposed for a comfort range of 44-68 degrees with an optimal temperature of about 58.

“Wild fish are smart enough that as water warms, they move up to the headwaters,” said Dave Nihart, Fish and Boat coldwater unit leader. “Stocked trout may be left behind. The [water] temperature in the hatcheries is in the 50s, but they’re not experienced enough to anticipate that they’re going to have to move.”

Water temperature impacts prey species, as well. Sculpin generally prefer cold headwaters. Chubs, suckers and other forage fish are likely to remain in warming waters longer than trout. Fly hatches occur at specific water temperatures, so long-term changes in water temperature would likely impact insects first.

The presence of naturally reproducing wild trout is evidence of clean water. Sixty-three of Pennsylvania’s 67 counties are known to contain organic populations of wild trout. Allegheny County has two designated wild trout streams totaling nearly 30 miles. No reproducing wild trout have been found in Beaver, Greene, Washington and Philadelphia counties.

In particularly warm summers like the current one, even trout that find cool-water relief can be at risk when they meet up with anglers.

“In many cases during very hot and dry conditions, trout will seek out the closest source of cold water to provide thermal relief,” stated cautionary warnings issued by the state Fish and Boat Commission. “This often results in many trout congregating at the mouths of cool-water tributaries or spring seeps.”

“The commission asks anglers to consider that while crowded and thermally stressed trout in a pool of water may look like an easy target, these fish are typically in poor condition and difficult to catch. Anglers should avoid fishing for trout during these conditions, as it can have lasting impacts on the population.”

John Hayes: [email protected]

CREDIT: Maryland Department of Natural Resources