They’ve been on the brink of extinction due to dams, drought, extreme heat and even the flare-up of wildfires, but now the endangered California Chinook salmon seems to be facing a whole new threat. : his own voracious hunger for anchovies.
After the worst spawning season on record in 2022, scientists now suspect that the species’ precipitous decline is being driven by its oceanic diet.
The researchers hypothesize that the salmon are feeding too much on anchovies, a fish that now swarms the California coast in record numbers. Unfortunately for salmon, anchovies contain an enzyme called thiaminase, which breaks down thiamine, a vitamin that is essential for cell function in all living things.
“These are fish that returned to the river earlier this year and then spawned in the spring and early summer. They had very low thiamine levels,” said Nate Mantua, a fisheries researcher with the National Marine Fisheries Service in Santa Cruz. The concentrations were “worse than last year”.
In humans, a critical deficiency of thiamine or vitamin B1 can lead to heart failure and nerve damage. In female salmon that return to rivers and streams to spawn, thiamine deficiency can be passed on to their many offspring, who suffer from swimming problems and experience high death rates, the researchers say.
Now, with government agencies and Native American tribes fearing the collapse of winter-running Chinooks, scientists are embarking on a campaign to determine why the anchovy population has soared off the California coast, and why. why winter-running Chinooks apparently ignore all other prey. .
“What is very unusual about their diet is that it has been so focused on anchovies and so lacking in other things that they have historically been found eating,” Mantua said. “It’s something we don’t have a lot of information about.”
Researchers from the National Oceanic and Atmospheric Administration, the California Department of Fish and Wildlife, and UC Davis are employing new technologies, such as environmental DNA sampling and isotopic analysis of fisheye lenses, along with older methods, such as sampling of plankton and fish ear bone studies. — to better understand how and why the diet of salmon in the ocean has changed.
Scientists first discovered that salmon suffered from a vitamin deficiency in 2020, after hatchery workers noticed salmon fry behaving strangely: They repeatedly swam in tight, corkscrew-like patterns before dying in spiral at the bottom of the tanks. They learned that a similar situation had occurred in the Great Lakes in the 1960s, when lake trout had exhibited similar behaviors after gorging themselves on alewives, another thiaminase-packed fish.
State, federal, and UC Davis researchers quickly treated salmon fingerlings with thiamine, infusing the vitamin into their tank water; the salmon soon recovered.
But over the past three years, thiamine concentrations in salmon eggs have continued to fall.
“Initially we thought it was just a one-year thing, maybe the way we think about COVID,” said Rachel Johnson, a fisheries biologist with the National Oceanic and Atmospheric Administration and UC Davis. “I was cautiously optimistic that the ocean would reorganize itself back to normal. And we just haven’t seen that.”
Chinook salmon begin their lives in the rivers of central and northern California, before migrating downstream to the Pacific Ocean. There, they typically spend the next two to three years feeding on a variety of fish and invertebrates, such as squid, off the coast.
But since anchovy numbers began to skyrocket in 2016, it has sparked feeding frenzies among salmon and other predators. Humpback and gray whales have been seen in record numbers feeding on the forage fish, and last summer San Franciscans complained that fish were falling from the sky, likely a result of the birds dropping fish from their talons or beaks too much. fillers.
Mantua and Johnson are investigating whether there is a seasonal component to the winter Chinook’s taste for anchovies.
“Some of the diet data we have from the 1950s, 1970s, and 1980s show that salmon caught in central California typically had herring, crab, and krill in the winter and early spring diets. Then the juvenile rockfish would become a larger component in the spring and early summer. And it wasn’t until August and September that anchovies and sardines were the dominant prey,” Mantua said.
Johnson’s lab is trying to find out by examining the lenses in the fish’s eyes.
Like an onion, the lenses build up layer upon layer over the life of a salmon. By examining the chemical isotopes in each layer, Johnson and his colleagues can get an idea of what kinds of food the salmon ate and when.
“It’s like a diet diary … that allows us to monitor the life of a salmon,” he said.
Meanwhile, she and her colleagues at the hatcheries continue to treat the fry with thiamine and inject the females with the vitamin with eggs.
Winter-running Chinooks are one of four distinct seasonal runs of salmon that populate the Sacramento River and its tributaries, but they are the only ones that have been declared endangered by the state and federal government. The name “winter run” refers to the season when sea salmon return to San Francisco Bay to make their long journey to spawn in the ancestral headwaters.
However, those frigid headwaters have long since been blocked by dams, and the fish have been forced to lay their eggs in the Central Valley waters in the heat of summer, killing many eggs. Today, wintering Chinooks survive only thanks to the intervention of government hatcheries and periodic releases of cold water from the very dams that block their passage upstream.
In recent years, drought, extreme heat, and debris flows from wildfire burn scars have taken a heavy toll on their numbers, along with thiamine deficiency.
According to federal data, the total number of juvenile winter Chinooks counted swimming downstream past the Red Bluff Diversion Dam in 2022 was 181,000, the lowest on record. In 2021, the number was 558,000, and in 2020, it was just over 2 million.
Egg survival to frying was also poor, said Michael Milstein, a spokesman for the federal agency. Despite river temperatures staying cooler in 2022 and most eggs surviving, the young salmon struggled after hatching. A recently published preliminary survival rate was 1.94%, again the lowest ever recorded. In 2021, the percentage survival from egg to fry was 2.56%. In 2020, it was 11.46%
To give endangered fish a better chance of survival, state and federal officials have been looking at ways to restore salmon to their traditional cold-water habitats upstream of dams, such as the McCloud River, upstream of Shasta Lake.
From last September through early December, biologists and members of the Winnemem Wintu Tribe worked together on a pilot project on the McCloud River, releasing thousands of juvenile winter salmon and then retrieve some of them.
By mid-December, more than 1,600 of the fish had been recaptured, loaded into aerated coolers, and trucked downstream of the dam, where they were released to continue their journey.
“They looked great,” said Matt Johnson, a senior environmental scientist with the California Department of Fish and Wildlife. The fish, he said, looked bigger than hatchery-raised salmon. “It was strong evidence that McCloud provides great habitat for juvenile Chinooks, which was not surprising to us, given the quality and quantity of habitat in that river system.”
He called the project a success.
Jason Roberts, environmental programs manager for the Department of Fish and Wildlife, said the participation of the Winnemem Wintu Tribe in the project was vital. He said department officials want to repeat the project next year and are talking with tribal leaders and federal officials about co-managing the effort.
“In the face of climate change, we have to return the overwintering animals of the valley floor to their historic habitat if we are to have a chance of survival,” Roberts said.
For Winnemem Wintu, salmon is central to cultural and spiritual traditions, and leaders have long sought to return salmon to the river where their ancestors lived.
Caleen Sisk, the tribe’s chief and spiritual leader, said this year’s effort was a good step.
“I think it has the potential to achieve salmon restoration in rivers above dams, not just McCloud, but this is a great example of what could happen and what would be good for the fish,” Sisk said.
For years, the Winnemem Wintu tribe has advocated an approach to reintroducing salmon that would involve developing a “swimming lane” so the fish can travel up and down the river on their own around the Shasta Dam. The tribe also wants to use salmon that once lived in the Sacramento River but were transplanted to New Zealand more than a century ago. Salmon have thrived in New Zealand’s mountain streams, and tribal leaders say those eggs should be returned.
“We believe that whatever happens to salmon happens to us,” Sisk said. “Maybe this is a step we have to go back to the river as well.”