An Alaska king salmon/Craig Medred photo

This story was updated on July 16, 2019

An Alaska Department of Fish and Game biologist on the Kuskokwim River is suggesting water temperatures got so high during the state’s July heat wave that salmon started dying from heatstroke.

Add yet another problem to the list of issues confronting the residents of the economically depressed Yukon-Kuskokwim River Delta who still depend on the surrounding countryside for food.

King salmon runs crashed in the region earlier in the decade and have remained below average every since. The subsistence fishing season this year began this year with stiff fishing restrictions in place to protect those iconic and struggling Chinook.

The biggest and longest-lived of the Pacific salmon, they once grew to almost 100 pounds in size. Statewide, they have been shrinking and slipping in numbers for a decade.

In the big picture, no one knows why the runs have faded. But state fisheries biologist Ben Gray told KYUK in Bethel that he thinks he knows why some salmon on the way to Kusko spawning grounds went belly up in the river this July.

Water temperatures for a time in the Kuskokwim climbed to 70 degrees, and at that point “what could happen is salmon metabolism speeds up to the point that they’re having heart attacks and going belly up and floating downriver,” he said.

Charles Brazil, the regional fisheries management coordinator, was a little more circumspect on Tuesday, noting that while the water in the river has been warm there is no real evidence to indicate the heat alone killed fish.

“Heat stress-induced mortality can be difficult to conclude,” he emailed. “There are tests for specific enzymes that are indicators of heat stress, but the overall condition of fish is also an important variable (diseases, parasites, or other conditions may additionally affect fish health).”

Among the other fishery problems along the Kusko this year, KYUK reported, fishermen have been dealing with what appears to be an outbreak of the parasite ichthyophonus or henneguya, which cause salmon flesh to appear milky.

Ichthyophonus can be picked up by adult fish while in the marine environment. Henneguya infects them while juveniles and can then multiply at sea.

Neither of those parasites have been confirmed by lab test, but Theodore Meyers, the state’s principal fish pathologist, emailed that “salmon in these watersheds have been examined off and on by the fish pathology program for over 30 years and the parasites mentioned in the (KYUK)news article have always been common and easily recognizable by their clinical signs of infection.”

The parasites are not harmful to humans, and Meyers said, and “neither parasite, nor any parasite for that matter currently known in Alaskan salmon, is capable of causing fish die-offs in large numbers, although the flesh quality may be degraded.”

Meyers was of the opinion that fish dying in the Kusko, and some reported elsewhere in Western Alaska, most likely suffocated. Fish suffocating from a lack of oxygen in warm water, especially if large numbers of fish school in a small area, is well documented.

“The larger fish mortalities recently reported in several Alaskan watersheds is very likely caused directly or indirectly by the higher water temperatures that result in lower dissolved oxygen and sometimes algal blooms that further deplete the oxygen,” Meyers said.

Though both Chinook and chums have been reported dead in along the Kusko, Brazil said state biologists have found only chums in the Kusko and pink salmon around Norton Sound where an unusually high number of pink salmon are returning to streams.

Indications are a warmer Bering Sea has caused a pink boom.

Across the Bering Sea from Norton Sound, Russian biologists are forecasting a record, odd-year harvest of 293,000 metric tons of pink salmon, according to TradeEx Foods, a global seafood provider. The Alaska forecast is for almost 139 million pinks, or about 158,000 metric tons. The state record harvest of 272 million pinks in 2013 weighed in at 309,800 metric tons.

“…Pink salmon are plugging rivers for the third year in a row,” state officials reported from Norton Sound. “The (fish)buyer has little interest in pink salmon this season.”

Worries everywhere

While Gray believes 70-degree water temperatures could have killed Kusko salmon, the fish appear to have endured water temperatures that climbed to a record 82 degrees in the Deshka River 400 miles to the east at about the same time.

A tributary to the Susitna River near of former Gov. Sarah Palin’s hometown of Wasilla, the Deshka is historically one of the most productive king salmon streams in the Susitna drainage.

Born in the foothills of the Alaska Range and fed by snowmelt, the river meanders its way south through the warmest part of the state’s Southcentral region and for that reason has always been subject to large temperature swings.

The salmon appear to have adapted.

When water temperatures skyrocketed on the Deshka in late June and early July of this year, the salmon stopped migrating and held up in the glacial Susitna. On paper, it looked like a disaster might be brewing when an already weak run of fish came near an end for 12 days at the end of June and into July.

From June 29 to July 10, only 43 kings – an average of less than four per day – passed through a fish-counting weir upstream from the confluence of the Deshka and the Susitna.

Historically, that is a time period when biologists would have expected to see hundreds of fish moving upstream every day. In one of the rivers big years back in 2006, more than 5,000 kings went through the weir during the same time period.

This year, however, the fish clearly held in the cool water of the glacially fed Susitna until the heat wave broke, some rain fell and the Deshka cooled, said area sportfish biologist Sam Ivey.

When that happened, they took off for spawning ground far up the Deshka.

More than 350 fish swarmed through the weir on Thursday at the front of a parade of kings. More than 850 arrived the next day, and the fish were still coming on Sunday when 328 passed upstream.

“Those high temperatures held up the end of the run for some 18 days,” Ivey messaged Monday. “That happened back in 2013, but not to the same extreme.  We expected to see more fish move at some point, but there was no way to predict how many there would be.  It’s nice to see those numbers.”

Upstream, he added, the 7,000-plus fish that made it into the river before the heat wave began seemed to be making do as well.

“Fortunately, we haven’t observed any adult mortality,” Ivey reported. ” The kings that were already inriver when the temperatures started rising must have found suitable cold water refugia to wait it out.  We had a crew up minnow trapping juveniles upstream of the weir, and they did note that all the kings they saw were nosed into cold water feeder tributaries.”

Climate change

With climate-change fears steadily growing and global-warming debates sometimes sizzling, both the tolerance of salmon to warm water and their adaptability to changing ocean conditions have been the subject of increasing scientific scrutiny.

The good news for Alaskan Chinook is probably written in the extent of the southern range of the fish which stretches to Monterey Bay and the Central Valley of California where summer temperatures regularly hit 100 degrees. The temperature there on Monday was 96.

How hot the water they can tolerate is not clear. Studies of various species have led to mixed findings, and there have been differences even within the same species.

The lethal limit for kings in the lower 48 has been thought to be somewhere around 25 degrees Centigrade (77 degrees Fahrenheit), but….

One study reported “a lethal limit of 25ºC for acclimation at 20ºC,” Pacific Northwest scientists Ann Richter and Steven Kolmes reported in a 2005 analysis of available data. Another study, however, “found high mortalities with daily maximums in the range of 23.8º – 25.5ºC (75º-78º),” they added.

“Records indicate that spring-run Chinook salmon in the Sacramento-San Joaquin River system spend the summer holding in large pools where summer temperatures are usually below 69.8°F–77°F,” fisheries biologist in California reported. “(But) sustained water temperatures above 80.6°F  are lethal to adult spring Chinook salmon.”

Heat tolerance among Chinooks appears to vary watershed to watershed, differ between spring-run fish and fall-run fish, and shift due to acclimation. Scientists with the U.S. Geological Survey (USGS) are now in the midst of studying the effects of heat on the Chinook in the Yukon River of Interior Alaska where summer temperatures regularly climb into the 80s and sometimes get hotter.

Scientists in California trying to determine the “aerobic scope” –  the range of temperatures high and low – that salmon can tolerate put young Chinook in swim tunnels and cranked up the heat.

“All fish swam at a test temperature of 23°C (73ºF)regardless of acclimation temperature, but some mortality occurred at 25°C (77ºF)” they reported in Conservation Physiology in 2017. The study was titled “Unusual aerobic performance at high temperatures in juvenile Chinook salmon.”

“While providing valuable new information about the thermal performance of Chinook salmon, caution is always needed when applying data from hatchery fish tested in the laboratory to their wild counterparts, although some evidence suggests that these physiological capabilities can be similar for fish tested in the field and the laboratory,” the warned. “Nevertheless, the capability of juvenile Chinook salmon from both acclimation groups to perform with acute warming up to 23°C (73ºF) was unexpected.”

Whether Alaska fish harbor the genetic capability to adapt to the temperatures to which California kings have adjusted is unknown, but salmon have been found to be highly resilient and capable of rapid evolution.

Oregon State scientists Mark Christie and colleagues caused a stir in 2012 when they presented evidence that salmon raised in a hatchery could in a single generation evolve to maximize their odds of survival in the alien environment.

But the only truly surprising thing about the study was the speed of change. Other studies had previoulsy concluded salmon could transform themselves within decades.

“A run of salmon facing new environmental conditions diverged into two populations in as few as 13 generations — a time span of only about 60 years,” University of Washington scientists reported in 2000. 

“The work revealed how long it took a run of sockeye salmon with common ancestry to diverge into two populations genetically different enough that they could no longer spawn with each other as successfully,” Sandra Hines wrote for the UW News in 2000.”When that happens the groups are becoming ‘reproductively isolated.’ Being reproductively isolated is one of the most important benchmarks used to decide if a single species has diverged into two.”

Not only did the sockeye originally transplanted from Baker Lake lose some of their reproductive commonality, they also changed their body types. Deep-bodied male sockeye – males that are wider than average from their top fin to their bellies – disappeared from the population transplanted in the Cedar River but remained in the population transplanted to Lake Washington.

The hulking males “would be more successful mating in the waters off the beach than in the Cedar River, where deep-bodied fish are more likely to be stranded in shallow water, eaten by predators or be less maneuverable in fast water,” Hines wrote.

European scientists studying similar changes in Atlantic salmon have concluded this ability to genetically shape-shift is generally good news even if the changes among those salmon appear to be a shift toward earlier maturation and smaller fish on the spawning grounds.

“This is another example de-bunking the myth that evolution takes millions of years” is how Yann Czorlich from the Natural Resources Institute Finland and the University of Turku explained the study to the University of Helsinki’s Sustainability News. 

“On the one hand, this can be considered a good thing seeing it means there is hope for salmon to adapt to their changed conditions. But on the other, it’s bad news for anglers who want to catch big salmon and join the ’20 kilogram club’ as there may be fewer big salmon in the future unless we can identify and halt the factors causing their decline.”

Little big fish

Alaska fisheries biologists have been witnessing a similar phenomenon in the Pacific. Kings spawning in the Kenai, Copper and other well-known Chinook rivers are returning earlier than in the past and are of smaller size, they reported in PLOS One in 2015.

Size selective-fisheries – be they prosecuted by commercial fishermen with mesh-selective gillnets or anglers catching and releasing smaller kings while searching for trophy – have long been suspected of playing some role in this size shift.

But the Alaska team of scientists led by Bert Lewis with the Alaska Department of Fish and Game concluded “that  similar patterns of length and age across (10) fisheries and exploitation rates suggest(ed) that size-selective fisheries may not be the primary mechanism driving the declines in size and age at maturity documented here.”

They suspect the answer to what is happening is more likely found in conditions at sea, be they related to water temperatures or the availability of food on the ocean pasture.

“The number of salmon in the Pacific Ocean is at an all-time high, in part, because of large-scale hatchery production across the North Pacific,” Lewis wrote. “Inter- and intra-specific salmon competition can lead to slower growth rates and to reductions in the mean sizes of returning fish. Beyond correlations, it has proven difficult to directly link specific biotic and environmental mechanisms to the changes observed here, because of the ocean-wide scale of these interactions and the many confounding mechanisms.”

Bill Templin, Fish and Games chief fisheries scientist, has repeatedly warned that correlation is not causation, but there has been a lot of speculation that the big kings, along with prizes sockeyes and coho around the rim of the Gulf of Alaska, have been losing out in a competition with increasingly plentiful pink salmon.

Unprecedented state salmon harvests this decade have been driven by huge returns of pinks. Of a projected harvest of 213.2 million salmon for Alaska this year, Fish and Game expects 137.8 million – about 65 percent – will be pinks. 

For most of Alaska history, a 137.8 million harvest of all species of salmon would have been a good catch. But this is a new age. The state’s three biggest salmon harvest have come in the last six years. 

Pinks are a small, low-value salmon with a two-year lifecycle. They are spawned in the late summer, go to sea the next spring, and then become little eating-machines in order to bulk up to return to Alaska as 3- to 5-pound adults the next year.

And although scientists have their suspicions competition with pinks could be affecting kings, proving that is all but impossible at this time. The North Pacific remains a big ocean into which Alaska’s salmon basically disappear to, hopefully, prosper and grow fat.

That doesn’t always happen, however. Scientists aren’t sure when – or if – the shrinking size of the kings will reverse itself. The Kenai was once famous for its monster kings. In the years immediately after the late Les Anderson pulled a 97-pound, 4-ounce world record from the Kenai in May 1985, a lot of anglers went on the hunt for the fabled 100 pounder.

They never found it, and in the past decade even 70-pound fish – once common – have become rare.

“It is unclear if the mechanisms responsible for selecting smaller, younger fish are likely to change in the near future so that we will again see large Chinook salmon as a significant portion of Alaskan populations,” Lewis wrote. “Chinook salmon returns throughout Alaska have declined in recent years, with consistent declines in run size beginning about 2007.”

Climate change might have played a role. So, too, competition with pinks. And now, for at least one summer, heat has joined the list of potential problems. No one knows how much that hot Deshka water will affect the spawning success of Deshka kings, but most agree such water makes spawning conditions less than optimal.

CORRECTION: This story was edited after publication to reflect that biologist Ben Gray’s observation on dead Kusko kings was a suggestion as to what might have killed them and not a claim as to what actually killed them.