What happened to the salmon during their journey in the mysterious 'black box'?

Warming sea levels due to the effects of climate change have made models to predict salmon production in the North Pacific no longer accurate.

During a normal autumn in the north Pacific, about one million salmon would flood the mouth of the Yukon River in Alaska to breed. This line of natural produce has sustained the local economy and culture for centuries.

But, last year, to the surprise of both locals and scientists, salmon production back into the Yukon River dropped to just 10 percent of what it was every year. The event was described as a disaster and the Alaska state government even had to use planes to transport salmon from other areas to ensure supplies.

Salmon hunting bears return to spawn in Katmai National Park, Alaska.

Worrying changes

However, nearly 650 km to the south, in Alaska's Bristol Bay, the situation is completely opposite. The region recorded record production of sockeye salmon last year, with an estimated 66 million returning to its rivers. This number is expected to increase even higher this year.

Salmon in the Pacific are facing opposite fates in different river systems. As the Earth heats up due to the excessive use of fossil fuels, scientists think that will lead to radical changes in the oceans, and the end result will be massive changes in the world. nature, especially the collapse of important fishery resources.

The northern Pacific Ocean accounts for the majority of the world's wild-caught salmon production, and the decline in fish stocks poses a serious threat to communities in the Pacific Rim.

During her three decades as a government scientist, Laurie Weitkamp has witnessed first-hand the consequences of climate change.

Her work includes tracking and predicting salmon population fluctuations, and ocean warming, she says, makes the models now less accurate.

"Sometimes the salmon start going out into the ocean when we thought the conditions were right, but then they disappear. They don't come back," said Ms. Weitkamp, ​​who is a fisheries biologist at National Oceanic and Atmospheric Administration (NOAA), shared.

Much research has been done on the impact of habitat on salmon in rivers and coastal waters, but little is known about this aspect as they swim out into the open ocean.

Usually, salmon will live in the sea for several years before returning to rivers to spawn. Ms. Weitkamp said heat waves caused by climate change are altering the ecology of salmon habitats, threatening the species' survival.

"When the salmon leave the coastal waters, that's when they go into a black box," Ms Weitkamp said, implying that then no one knows what will happen to them.

To solve this mystery, scientists from the United States, Canada and Russia have collaborated to conduct the largest-ever research project into the habitat of salmon in the northern Pacific Ocean. Five research vessels will collect salmon samples at sea and assess environmental conditions over an area of ​​sea more than one million square miles.

The Bell M. Shimada research vessel of the National Oceanic and Atmospheric Administration (NOAA) is on a mission to learn about the living conditions of salmon in the Pacific Ocean.

The scientists hope to map where salmon from different rivers live during the winter months, when food is scarce and they are particularly vulnerable.

They also want to look for signs of competition between different salmon species, following several extreme heat events at sea in recent years.

"We've seen a drastic drop in salmon stocks. They have a high mortality rate in the ocean and that's one of the hardest things to estimate," said Jackie King, scientist in charge of research on the Sir John Franklin ship on the Canadian side, shared.

Finding the solution to the disappearance of salmon

In Canada, about 10 million red salmon return each year to the Fraser River to spawn. But in 2020, this number suddenly dropped to just 293,000.

In Russia's Far East, locals have even gone on to protest to demand action from the government over the severe decline in salmon production. Since this is an issue affecting all three countries, a collaborative research project has been implemented.

However, when the project was about to be carried out, Russia opened a "special military operation" in Ukraine. The US side immediately withdrew 4 of their scientists from Tinro - the Russian research ship. The US government also did not allow the ship to conduct activities in the US exclusive economic zone.

Therefore, instead of the original 80 planned sites, the Russian Tinro ship will only take research samples in about half of these.

"Not taking enough samples is never a good thing. It certainly affects our results," said Alexei Pinchuk, professor of oceanography at the University of Alaska Fairbanks.

Mr. Pinchuk also participates in the project and is a member of the Bell M. Shimada ship on the US side.

Ms. Weitkamp is the head of the research team on this ship. The Bell M. Shimada is NOAA's special-purpose research vessel, and its voyage will last 38 days, with 40 crew members on board, including 14 scientists.

For researchers like Ms. Weitkamp or Mr. Pinchuk, the trip this time was not pleasant at all, as the ship had to face waves up to 4.5 m high, along with constant winds at a high level. more than 90 km/h.

"I'm still shaking," Ms. Weitkamp said after landing at NOAA's Pacific Operations Center in Newport, Oregon.

At each study site, the vessel will cast nets approximately 30 m wide for one hour before pulling up the nets. In the ocean, salmon are spread out more widely than when they swim in the sea or in the river, so each time the net is pulled, there will only be about 30 salmon.

The net is also fitted with a device called a CTD sensor that measures the temperature, salinity and other indicators of the water area. The ship will then proceed to the next study site, approximately 60 nautical miles away, and continue the same work.

From these salmon, scientists on board will take samples of muscle, blood, scales, gonads, liver and ear bones. Salmon ear bones have lines that help determine their age - similar to the rings on a tree trunk.

They also examined the fish's stomachs to see what they were eating, and collected seawater samples to see if there was enough food for the salmon in that area.

Sockeye in the Bristol Bay area - a rare species that has grown in number in recent years.

Many of the pike caught by the Shimada appear to be malnourished. In the coming months, research will be carried out in the laboratory, and scientists will try to determine what difficulties these salmon are facing.

The aim of this project is also to predict how salmon will survive in the north Pacific as oceans warm. In 2013, a sea on the US West Coast was unusually warm. This phenomenon lasts about 3 years over an area of ​​2,500 square kilometers, and is known as "the blob".

Another "color streak" also appeared in the Pacific in 2019.

When the food chain is broken

In theory, proving a link between warming seas and declining salmon production is not easy. Salmon are species in which, at every stage of their life cycle, they face other predators.

Climate change is changing the habitats of the streams where they are born. This is not limited to changes in water temperature, but also takes into account changes in flows due to droughts and wildfires that destroy coastal habitats and cause river flooding.

In addition, changing environmental conditions are also affecting plankton, krill, squid and other fish species, which are the main food sources of salmon in the ocean.

Salmon are also cold-blooded, so an increase in ambient temperature leads to an accelerated metabolism inside their body. Simply put, hot sea water causes salmon to eat more, even when there is little food.

Ed Farley is a researcher from the Alaska Fisheries Science Center on duty aboard the Shimada. "We believe that the heat of the sea water has really disrupted the food chain. And that's one of the reasons we're seeing a drastic drop in salmon production, especially around the bay," he said. Alaska after the first sea heat wave".

This is a serious problem for pink salmon and dog trout, two species that habitually return to southeastern Alaska, which has been affected by the heat wave.

However, for the red salmon in Bristol Bay, the fate of this species is reversed. In the last two years, red salmon production in this region has broken records, expected to exceed 70 million this year.

And no one else, this growth is also attributed to climate change.

Bristol Bay red trout live in inland lakes for some time before swimming out to sea, instead of swimming directly into the sea like other salmon species.

According to biologists, the warming of the water in these lakes will help create more food for young red trout, giving them a stronger foundation when heading out to sea.

The recent dramatic decline of dog trout in western Alaska, particularly those in the Yukon and Kuskokwim rivers, raises a pressing question that scientists want to answer.

Alaskan fishermen attribute overfishing, which includes salmon caught by vessels that are not authorized to fish for salmon, as well as competition from salmon farmed in hatcheries, as the cause of the decline. .

Researcher Alexei Pinchuk in his study aboard the Shimada.

The warmer waters of the Pacific Ocean are slowly moving northward towards the Bering Sea, according to researcher Ed Farley. He thinks this phenomenon has affected canine salmon, especially during their adult stages, and is probably the cause of their mass deaths.

"If you're a growing trout, and you have to spend your first year at sea, what you want is enough food to grow," Mr Farley said.

"And we can see that these young salmon don't get enough fat before winter," added the researcher.

Climate prediction models show that marine heatwaves will become more frequent as the atmosphere and oceans continue to warm.

The dramatic shift in salmon populations is a cause for concern, said Mr Farley, and climate change "is behind most of the mortality we're seeing".

"If we really want to understand the future, we need to understand how climate change will impact these fish," Mr Farley said.