Thermal shock caused July trout deaths in Anthony Lake

Nov. 11—The approximately 200 hatchery rainbow trout that died in late July after being released in Anthony Lake succumbed to thermal shock when they splashed into the comparatively warmer surface water in the alpine lake, a state fish biologist said.

"It was a temperature difference between the transport and the water body they're released in," said Joe Lemanski, district fish biologist at the Oregon Department of Fish and Wildlife's (ODFW) La Grande office.

Although the water is generally cool in Anthony Lake, at elevation 7,100 feet, direct midsummer sunlight can warm the top layer to as much as 60 to 70 degrees, Lemanski said.

Water in the hatchery transport truck that delivered the trout to the 22-acre lake on July 26, by contrast, was likely in the 50s or below, he said.

"With a good amount of sunlight and ambient heat, it can heat up really quickly," Lemanski said. "By July we'd already had a few 100 degree days, and by the time they were released the temperature difference was too great."

An automated weather station just east of Anthony Lake, and at nearly the same elevation, recorded a high temperature of 75 degrees on July 26, the day 2,000 rainbow trout were released in the lake.

High temperatures over the previous two weeks ranged from 64 degrees to 79 degrees.

Lemanski said ODFW began to get reports from anglers soon after the July 26 release, including one report from a Forest Service employee who took photos of dead fish.

The dead trout were concentrated near the boat ramp at the southeast corner of the lake — which is where the hatchery truck disgorges its load of rainbow trout.

Lemanski said it doesn't appear that a large number of fish were dead before they were released, however, since there were no reports of dead fish floating near the boat ramp during or immediately after the release.

He suspects most of the fish died within a day or less, based on the timing of the reports.

This summer, prior to the lab test results, Lemanski said thermal shock was a possible cause for the fish deaths.

He noted that there were no reports of dead trout after 2,000 rainbows were released in Anthony Lake, at the same site, three weeks earlier, on July 5.

Temperatures were much cooler during the two weeks prior to the July 5 release, with the high below 60 degrees on several days.

Indeed, lingering snowdrifts, the result of an abnormally cool, wet spring that brought snow to the area as late as mid June, delayed the second trout release until July 26.

"They were hoping to get those fish in a few weeks earlier, but the snow accumulated prevented an earlier release," Lemanski said.

After reports started coming in about dead trout near the boat ramp, ODFW officials collected some carcasses.

"A handful of specimens were taken to our fish health lab at EOU, they evaluated them for everything — blood, tissue, bacteria, virology," Lemanski said.

The tests confirmed thermal shock as the cause, and ruled out other, potentially more troubling, problems such as toxins in the water, an infection in the trout, or failures with the hatchery truck or release procedures.