Why are chinook runs declining in Bristol Bay, while sockeye runs are shattering records?
Chinook salmon numbers have declined significantly in the Nushagak watershed, Bristol Bay’s most abundant commercial fishing district. The runs there have fallen far short of in-river escapement goals in five of the last six years. Yet at the same time, sockeye runs are shattering records.
Tim Sands is the westside area management biologist with the Alaska Department for Fish and Game, where this issue is top of mind.
“That's one of the questions we get: ‘Well, are the sockeye up there competing with the kings? Is that what's going on?’” Sands said. “I don't think so. The sockeye for the most part are rearing in the lake. Kings are in the river. They spawn in different areas.”
Sands says sockeye and chinook lay and bury their eggs in different spawning grounds and at different depths. So most likely the issue is tied to ocean conditions when kings migrate out to sea. But what’s exactly behind this decline remains a mystery.
“There's something else going on,” Sands said.
Last year the Nushagak sockeye run reached a record high with over 22 million fish harvested, that’s almost triple the 20 year average. Meanwhile, chinook numbers have fallen so dramatically it was listed as a state stock of concern by the Board of Fisheries this year.
As a result, state fishery managers enacted a suite of unprecedented restrictions on commercial and sport fishing in an effort to allow more chinook to return to the Nushagak. The district remained closed until 420,000 sockeye passed the Nushagak sonar [June 25], in an effort to allow more salmon the chance to get up the river, without encountering commercial nets.
Katie Howard is an ocean fisheries scientist with the Alaska Department of Fish and Game, researching the decline of chinook across the state.
“They're really different species, we have this tendency to think of all salmon as being kind of the same, but they're not,” she said. “They just lead really different lives.”
Howard says one big clue in the decline of chinook salmon is how widespread the problem is.
“It's not just Alaska. It's everywhere where chinook are,” she said. “The reports we're getting from pretty much everyone is that with a few minor exceptions, chinook have been down everywhere, so that kind of points towards a cause that can affect things on a really large geographic scale. So not local causes.”
The decline of chinook has been well documented all across Alaska, where populations have plummeted in the Yukon, Kuskokwim, and Kenai Rivers, and beyond. These declines have caused widespread concern from communities, and restrictions and closures on fisheries around the state, including in the Nushagak.
“They're using different habitats, but they're all experiencing the same decline. So that kind of leads you to think that maybe it is climate related,” Howard said.
One key factor Howard and her team are looking at is water temperature, which affects fish on the long journey home.
“We're also seeing in a lot of locations, some pretty strong evidence that heat stress is problematic for chinook stocks, especially when they have stocks that are migrating through during particularly hot periods of the year,” she said.
She says chinook swimming through warmer waters become distressed.
“When the water temperatures increase, these fish get stressed out because they're really cold water fish and that makes it even harder for them to swim because it's affecting all of their organ function,” Howard said.
In the open ocean, chinook are deep divers compared to other salmon species, and also have a more fish-focused diet. When ocean temperatures heat up, food availability changes. And chinook salmon get hungrier.
“When temperatures are warmer, it increases their metabolism,” she explained. “So, they burn through their energy reserves more quickly. And, they have to eat more to make up for that.”
This was seen starting in 2013 when a prolonged period of warmer water temperatures sat stagnant over the gulf of Alaska, unsettlingly referred to as “the blob.” Biologists say the unusually hot waters rattled the food chain and dramatically changed what was on the menu for salmon.
“It has really changed the marine landscape of what the ecosystems look like, and what food is available and where it's available,” she said.
While the impacts of warming waters on chinook appear to be widespread, Howard says, different rivers have different problems. Her research team has investigated the devastating decline of chinook on the Yukon River, which closed fishing entirely for communities historically reliant on salmon.
“We're seeing this really high prevalence of Ichthyophonus infections in Yukon chinook,” she said. “That disease affects their heart tissue, so it makes it much harder for them to swim.”
Howard also observed thiamine deficiencies in Yukon chinook, an important nutrient essential for female salmon to lay viable eggs.
She says between nutrient deficiencies, disease, and heat stress, it’s hard for chinook salmon to catch a break.
“It's both freshwater and marine issues acting on the adult spawners that may lead them to either not make it to the spawning ground, to make it and not spawn, or to leave eggs that don't really have the nutrients that they need to be successful and survive,” Howard said.
Bill Templin is the chief fisheries scientist of salmon for the Alaska Department of Fish and Game, looking at chinook trends on a statewide level, and the drivers behind them.
“The way the drivers work on individual salmon stocks can be very different depending on the size of the river, the sources of the water, the places that they go and the time they spend there in the ocean,” he said.
He says that while the symptoms of chinook decline may vary from river to river, the root of the problem is the same.
“So one thing that's a problem on the Yukon may not be a problem in the Nushagak and something that's a problem in the Nushagak may not be the problem on the Kenai right, but they're all potentially driven by these large scale events, like warmer water in the ocean,” Templin said.
On the other hand, Bristol Bay’s sockeye runs are reaching record highs, with a historic 79 million fish returning last year. That’s more than 80 percent above the 20 year average.
Researchers point to differences in habitat and the increased availability of sockeye’s freshwater food source - zooplankton, which fish are chowing down on in the lake systems of Bristol Bay’s watersheds.
Katie Howard, ocean fisheries scientist with Fish and Game, says for these sockeye, warming water temperatures may help, rather than hurt.
“Bristol Bay is just a really unique place,” she said. “There are these really large deep lakes, so even when there's warming temperatures, the fish can move to where the temperature is appropriate for them. The warming temperatures produce a lot of algae, and the algae helps feed the little critters that the smolt are feeding on. And so there seems to be some positive effect of these warming temperatures on sockeye because they live in this really unique habitat and they can take advantage of that.”
Chinook salmon occupy different freshwater habitats, and don’t reap the same benefits of this booming biological buffet.
But the Bristol Bay fishery is not a closed system. After a year or two in lakes and streams, millions of salmon fly the coop and strike out for the ocean, where they are subject to increased competition, changing food sources, rising temperatures, and more.
Nushagak area management biologist Tim Sands says to manage the fishery moving forward, these ongoing changes need to be taken into account.
“The Nushagak salmon fishery has been in existence since 1884, and we still have all those species around,” he said. “So in my mind that shows that the fishery can coexist with all those species, and should continue to coexist if it were just the fishery, if that were the only variable we could manage for everything, and protect everything. These other variables that come in that kind of perturb the balance that we've had, are unknown, and we have to figure it out.”
As of July 6, only 28,422 chinook salmon have made it up the Nushagak River so far, and there’s still a ways to go to meet this year’s minimum escapement goal of 55,000 fish. Moreover, the middle 50 percent of Nushagak chinook runs historically falls between June 19 and July 3, leaving the Nushagak short on fish, and short on time.
Other watersheds of Bristol Bay are also seeing a scarcity of chinook, or counts are unknown. In the Chignik River drainage, king counts are the lowest on record to date, with just 18 counted so far this season.
For now, fisheries managers say the best thing to do is focus on efforts to increase escapement, and to continue research to better understand what’s behind the decline of Alaska’s chinook salmon.
(Correction: This article has been updated to correct a misprinted date of the opening of the Nushagak District to commercial fishing on July 26. The district opened Sunday, June 25, 2023)
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