Low water levels shouldn’t affect fish and critters | AspenTimes.com
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Low water levels shouldn’t affect fish and critters

Jeremy Heiman

Though water levels are getting lower and lower in local rivers, the waterways’ ecosystems are likely not facing any danger, a scientist says.

Low water, brought on by low winter snowfall and hot summer weather, makes life tough for trout and the aquatic bugs they feed on. But the critters can survive even if the rivers stop flowing and break up into pools, said Barry Nehring, aquatic wildlife researcher for the Colorado Division of Wildlife in Montrose.

In 1977, Colorado had the lowest water levels of any year on record, Nehring said, even lower than in the drought years of the 1930s. He was working in the San Luis Valley at the time, he said, and though the situation got pretty bad, the trout survived.

“There was grass growing in the riffles,” Nehring said, “but you could go into the pools and find good-sized native trout.” Trout have the ability to survive stressful conditions such as that for as much as six to eight weeks, he said.

Trout are sensitive to water temperature, Nehring said, and are more and more stressed as temperatures rise above 68 degrees Fahrenheit. But if cool nights cool the water temporarily, they can survive daytime water temperatures up to the 80s.

Nehring said he’s found 15-inch rainbow trout surviving in pools where the temperature reached 84 degrees during the day.

The aquatic insect larvae the trout dine on, sometimes referred to as macroinvertebrates, can adapt to low water conditions, too, he said. The creatures can burrow into wet sand or gravel on the river bottom to survive, if the river bed isn’t completely baked dry.

“They’re actually more resilient than the fish,” Nehring said.

This ability to survive, in both the fish and the insects, is built in by time and evolution, he said.

“In a geological or evolutionary time frame, fish and aquatic insects probably went through more stressful times than they are now,” he said. “And they made it through.”

Sediment is more of a threat to fish, though, in dry times. With water levels low, silty irrigation water returning to rivers can muddy the main stream. Water can also be clouded by runoff from overgrazed pasturelands.

“Sediment to a fish is like smoke to humans,” Nehring said. “Fish can’t respire with mud on their gills.” Fish already stressed are most susceptible to suffocation.

“Elevated water temperatures plus sediment is probably a pretty serious condition,” he said.

The plants in the aquatic ecosystem can also withstand low water, and even no water at all. In the mountain rivers, algae and filamentous aquatic plants are most common, Nehring said. Both can survive being dried out, and both have several ways of reproducing once the water comes back.

Brown trout, brook trout and whitefish all spawn in the fall, and their reproduction can be affected by low water, Nehring said, because spawning females prefer a water speed of about one foot per second and a gravel bottom of the right size stones. It’s harder to find just the right place to lay eggs with low water, he said, but not usually impossible.

“The fish can usually find the right combination to get the job done,” he said.


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