No harm to salmon reported

Temperatures high enough, in theory, to harm fish were measured in 2019 and again last year at the Chilkat Lake weir, along a major thoroughfare for the Chilkat Valley’s sockeye salmon, according to Alaska Department of Fish and Game and Takshanuk Watershed Council data.

No obvious damage to salmon was reported, but the data are of interest and concern to local scientists since warm freshwater has caused die-offs elsewhere.

The maximum temperature recorded by Takshanuk at the weir two days last summer exceeded the state’s upper limit for the survival of aquatic life — 20 degrees Celsius, or 68 degrees Fahrenheit. On 82 days, maximum daily water temperatures at the weir rose above 55.4 degrees Fahrenheit (13 degrees Celsius), which can disrupt salmon spawning and egg and fry incubation.

In 2019, Fish and Game recorded 21 degrees Celsius at the weir, the highest since the department began measuring temperatures there in 1967.

“It’s very concerning. We don’t want to be seeing those high temperatures in one of the most important migratory corridors in the Valley,” said Stacie Evans, science director of Takshanuk Watershed Council, which has overseen a freshwater temperature-monitoring project in salmon streams and rivers since 2016. “As soon as I plotted the data out, I called Fish and Game,” Evans said.

The weir is in a slough that links Chilkat Lake with the Tsirku and Chilkat rivers and the ocean, making it a key passage for fish that spawn in the lake or streams that feed into it. The majority of Chilkat River sockeye spawn in the lake. Coho and chum salmon and Dolly Varden also migrate up the slough.

Zeiser said her team didn’t observe any negative effects on salmon last summer. Chilkat sockeye failed to meet the state’s escapement goal in 2021 for the second year in a row, and only the second time since 2007, but Zeiser said heat likely wasn’t to blame.

“We certainly didn’t see any dead fish, and (the warm water) didn’t appear to prevent fish from going through the weir up to the lake to spawn,” Zeiser said. “It all depends on if they were hanging out in the slough for long periods of time.”

Takshanuk has monitored a variety of salmon streams in the Valley for six years, but last year was the first time Takshanuk Watershed Council collected temperature data at the weir. None of the other 18 monitoring sites, from Jones Point to the Lower Chilkoot to the Tahini River, have registered temperatures above 20 degrees Celsius.

In 2019, high freshwater temperatures caused salmon die-offs in Southcentral and Southwest Alaska. In 2015, a heat wave killed 250,000 sockeye migrating up the Columbia and Snake rivers in Idaho. But beyond temperature, factors like how long fish spend in warm water contribute to survival.

“It’s a little bit more complicated than just the water’s too hot for them,” Evans said. “If there were many salmon there during those hot temperatures, they would be using up the oxygen in the water and could experience hypoxia,” which can lead to death. “But it’s also possible they would just turn around.”

Even if the heat doesn’t kill salmon, forcing them to turn back downstream could lead to reproduction and population declines, Evans said.

Based on observations and sampling at the weir last year, Zeiser said there was no evidence of heat-related stress or exhaustion in the fish and that it’s hard to know what future effect heat will have in the short slough, where the fish don’t spend much time.

Evans said this summer Takshanuk plans to expand data collection in the slough and to put sensors at different depths in the lake to gain a broader and better sense of the distribution of temperatures there.

“Right now we just have this one point in the slough. We’d like to see if these temperatures are occuring in the whole slough. The thing that concerns us the most about this is that we recorded these temperatures right near the weir, so the fish can’t rapidly migrate through this warm spot,” Evans said.

Fish and Game keeps the weir open much of the day, and Takshanuk’s data loggers will be linked this summer to bluetooth so weir workers can track temperature in real time and leave the weir open, if needed, to let fish move quickly to cooler water.

That last summer, which was relatively cool and wet, produced such warm temperatures at the weir surprised both Evans and Zeiser. But Evans said the water in the slough is relatively still, and the area just upstream is shallow, broad, and unshaded —  “the perfect example of habitat that is extremely sensitive to warming.”

In addition to measuring temperature trends and noting areas of concern, Takshanuk aims to identify “climate refugia”—salmon streams and spawning grounds that are likely to be resilient to climate change and rising air temperatures.

Based on Takshanuk’s data, several streams in the Valley, like Clear and Herman creeks, could be refugia. Clear Creek, which feeds into the Tsirku, hasn’t eclipsed 45 degrees Fahrenheit since Takshanuk began collecting data there in 2016.

Takshanuk plans to conduct areawide thermal imaging from a drone to detect cool streams that could provide salmon habitat in a warming world.

Generally speaking, Evans said she expects the Valley to be more resilient to climate change than other regions of the world thanks to its biodiversity, dynamic hydrology and variety of ecosystems. “We’re not sure yet (of its resilience), but this research will help us figure that out,” she said.