Anglers, commercial fishing crews and fish biologists in the Western U.S. have known for years that Chinook salmon—the Pacific's largest—are getting smaller.
According to a 2018 study published in Fish and Fisheries, the average length of Chinook salmon has declined by 8 percent. The change means that female fish are producing 16 percent fewer eggs. It's also resulting in 28 percent less nutrients being transported to freshwater, and making the fish 21 percent less valuable to people who catch them (CU No. 1968 ).
Other species of salmon are also shrinking. It's happening in hatchery and wild chum, coho, sockeye and steelhead. Studies point to a variety of reasons, including climate change, competition, fishing and predators. Many are smaller because they're returning to their natal streams at a younger age. Regardless of the cause, everyone agrees that the trend is alarming.
Over the past several years, NOAA scientists have been studying one aspect of this phenomenon—the high incidence of Chinook mini-jacks being released from Columbia Basin hatcheries. For more than a decade, and with funding from BPA, scientists at NOAA's Northwest Fisheries Science Center have been looking into why some of the basin's hatcheries are producing high numbers of mini-jacks. And while their work is hatchery-specific, their findings can be used at hatcheries across the region.
Don Larsen, physiologist at NOAA's Northwest Fisheries Science Center, told Clearing Up surveys of hatcheries throughout the Snake and Columbia rivers found that mini-jacks make up between 8 and 71 percent of a population's males prior to release.
Larsen said it's hard to estimate levels of mini-jacks produced in the wild, but indirect comparisons of wild and hatchery fish migrating down through the hydrosystem have shown that less than 5 percent of the outmigrating wild fish are mini-jacks, while as many as 20 percent of the hatchery males were mini-jacks.
Integrated hatcheries—those that get broodstock from wild runs to raise in a hatchery—often have higher mini-jack rates even when they release smaller sized juvenile fish. And since about two-thirds of the Chinook returning to the Columbia River originated from hatcheries, it's having a big impact.
It's one way that hatcheries are contributing to the Chinook's decline in size and age at maturity, Larsen said in an email. "It is not the only factor, but it may be a significant factor," he said.
Mini-jacks are young male Chinook salmon that mature early. They occur naturally in the wild, probably as one of their many survival techniques, Larsen said in an interview. It helps keep the genetics of a specific year's run in play if ocean conditions are poor or competition is stiff. Rather than returning to the Columbia as a 4- or 5-year-old adult, or even a 3-year-old jack, mini-jacks return at age 2—just five months after they were released from a hatchery in the spring.
Compared to the 12- to 18-inch jack, a mini-jack is only about 8-inches long. "They are often called precociously mature males, and are obviously even less desirable for harvest or sport angling than jacks," Larsen noted.
Precocious, too, in their methods for passing on genes, he said. "They employ what is known as a 'sneaker strategy,' to sneak spawning opportunities when full-size adults are spawning in order to hopefully fertilize a portion of the eggs and pass on their genes to subsequent generations."
Mini-jacks are also detrimental to a population because their prevalence is an overall indicator of a disruption in the population's age structure, Larsen said. It results in fewer 4- and 5-year-old adults returning. Mini-jacks that stray and breed in the wild have a lower reproductive success rate. They also compete with native stocks for habitat and prey. And from a management perspective, they significantly alter the accuracy of smolt-to-adult return estimates. "They're enumerated as smolts being released from the facility, but in fact they are a maturing male," he said.
Larsen said NOAA's experiments are conducted at representative hatcheries and use applied science primarily to improve smolt quality, productivity and adult returns, and to increase the salmon's size and age at return, and with an effort to minimize genetic impacts on wild fish. "We do believe results are applicable across the Columbia Basin," he said.
What they found is that certain hatchery practices contribute to the likelihood of producing more or fewer mini-jacks. "That is why in our research we have advocated for developing rearing regimes that more closely match that of wild fish where possible. We refer to this as 'the wild fish template,'" Larsen said.
That template calls for hatchery practices that mimic conditions in the wild, because those tend to be most successful, Larsen said. It tells hatchery managers that, when possible, hatchery conditions should attempt to approximate those of wild fish in terms of seasonal growth rate, size, dietary fat composition, and water temperature.
Larsen said while it's counter-intuitive, hatcheries that release larger smolts tend to get returning adults that are smaller and younger—a higher percentage of mini-jacks. That's because growth triggers the maturation process, which usually begins in salmon about a year before they spawn.
"If they are growing very well on commercial fish diets in the hatchery that are 18-21 percent fat—compared to the natural insect diet with about 6 percent fat for wild fish—they grow very fast, sending an energetic signal to the fish that they are doing well and can initiate the maturation process early rather than spend years in the ocean," he wrote.
Water temperature also affects how quickly salmon mature, Larsen noted. But many hatcheries use well water, and the temperature does not vary throughout the year. "Since salmon are cold-blooded, their growth is influenced by water temperature. Warmer water means a higher metabolic rate, greater consumption of higher fat diet, and a larger size," he said.
The NOAA scientists have worked with federal, tribal, state and PUD hatchery managers to help them improve the quality of smolts they release, increase smolt-to-adult return rates, and increase the age at which adults return, he said. They've done that by collecting physiological samples from hatchery fish to characterize their smolt development and maturation status before they are released, and then monitor them downstream, calculating survival numbers and size and age when they return.
"Each hatchery and salmon stock is a little different, so while there are some general principals we use to inform our recommendations (i.e. The wild fish template), there is no 'one size fits all' process that can be applied everywhere," he wrote. "It is very interactive, requires multiple release years to account for variation in freshwater and ocean conditions year to year and requires adaptive management as situations change and data comes in to modify our approach accordingly."
When asked about the best thing that hatcheries can do to reduce their impact on wild fish, Larsen concluded that when the goal is harvest augmentation, using segregation programs that produce fish that are very different from the wild fish—in terms of their run timing and other factors—could go a long way toward reducing the interactions and impacts that some hatchery stocks have on wild runs.
He said there are so many areas where hatchery and wild fish differ—including family composition, rearing density, dietary composition, water temperature, ration, disease prevalence and predation.
"So, one can never achieve anything close to a natural environment, but it doesn't mean we can't do some modifications to improve the situation," he wrote, adding, "Salmon hatcheries are in many cases the only barrier to extinction for some stocks, so they are and will be a presence on our landscape into the foreseeable future. They are a tool, but like any tool they need to be used wisely."