A 19-year analysis of fish raised in an Oregon hatchery found dramatic genetic changes can occur in a single generation, with the domesticated fish responding rapidly to artificial evolutionary pressures found in a crowded, human-regulated tank.
But out in the wide-open world of riverine predators and ocean migrations, these same hatchery-bred smolts faltered when compared to their wild-raised brethen.
"The impact of hatcheries on salmon is so profound that in just one generation traits are selected that allow fish to survive and prosper in the hatchery environment, at the cost of their ability to thrive and reproduce in a wild environment," wrote a team of four scientists in a new paper published this week in the Proceedings of the National Academy of Sciences.
The paper — which tracked a run of Hood River steelhead for 19 years — confirmed that hatchery life had the power to trigger genetic shifts that were then passed onto the next generation. A release about the study called it "evolution at warp speed."
"We've known for some time that hatchery-born fish are less successful at survival and reproduction in the wild," said Michael Blouin, a professor of zoology at Oregon State University, in this story about the research. "However, until now, it wasn't clear why. What this study shows is that intense evolutionary pressures in the hatchery rapidly select for fish that excel there, at the expense of their reproductive success in the wild."
Which traits were being favored by hatchery life wasn't clear, the scientists said. But they speculated that growing up indoors inside giant tanks might hard-wire a tolerance for "extreme crowding" into the fish — and that trait might work against fish attempting to reproduce under real-world conditions.
"If research can determine exactly what aspect of hatchery operations is selecting for fish with less fitness in the wild, it could be possible to make changes that would help address the problem," the scientists explained in this story.
Evolution at 'warp speed'
In the study, scientists found that offspring that had adapted best to the tank world were about three times worse reproducing in the wild than fish that hadn't made the shift out of a hatchery.
"First-generation hatchery fish had nearly double the lifetime reproductive success (measured as the number of returning adult offspring) when spawned in captivity compared with wild fish spawned under identical conditions, which is a clear demonstration of adaptation to captivity," the authors explained in the study.
The findings suggested a dramatic "tradeoff" had taken place rapidly, the authors said.
"Those with the greatest fitness in a captive environment produced offspring that performed the worst in the wild," they wrote. "Specifically, captive-born individuals with five … or more returning siblings … averaged 0.62 returning offspring in the wild, whereas captive-born individuals with less than five siblings averaged 2.05 returning offspring in the wild.
"These results demonstrate that a single generation in captivity can result in a substantial response to selection on traits that are beneficial in captivity but severely maladaptive in the wild."
One implication from the study suggests that hatcheries alone cannot save the Pacific Northwest's damaged salmon runs, argued the Eugene Register-Guard in an editorial published Tuesday morning.
"There is still reason to believe that hatcheries can play an important role in the recovery of endangered and threatened fish species. But nearly a century and a half of experience with hatcheries has proven they cannot adequately compensate for the wholesale destruction of habitats," the paper argued here.
"No one should believe the myth that abundant, healthy fish runs can return to the Pacific Northwest as long as its rivers remain dammed, diverted and polluted, its watersheds overdeveloped and its forests harvested with scant regard for the impact on salmon."
What about Alaska hatchery salmon?
What the findings say about fish management in Alaska — where a sophisticated network of state and private hatcheries release millions of salmon and other fish every season — is less clear, and the authors cautioned against applying their results to other salmonoid populations elsewhere in the North Pacific.
"It remains to be seen whether results from this one study on steelhead generalize to other hatcheries or salmon species," Blouin said in a release.
"Nevertheless, this shows that hatcheries can produce fish that are genetically different from wild fish, and that it can happen extraordinarily fast," he added. "The challenge now is to identify the traits under selection to see if we can slow that rate of domestication."
The issue does have an urgency in the Pacific Northwest that's more intense than in Alaska. Many salmon runs in Oregon, Washington and California have dwindled over the decades as rivers were dammed for hydroelectric power and habitat was degraded due to logging, agriculture and development. As a result, many unique salmon populations are now listed under the federal Endangered Species Act, and hatcheries come to play an important part of an effort to restore the fish.
By contrast, Alaska wild salmon populations remain healthy. At the same time, the state follows a decades-long practice of using hatcheries to augment wild runs and also to jumpstart recreational fishing, especially in lakes on the road system.
In 2012, the state-run hatcheries plan to released more than 8 million fish spread among six species, ranging from tiny fingerlings to the catchable-length Chinook salmon. In an overview of Alaska's commercial fishing hatchery program through the early 2000s, a federal analysis reported that 33 Alaska production hatcheries had released more than 100 million juvenile salmon annually.
That report also characterized Alaska's hatchery regime as a "balanced program designed to enhance fisheries while maintaining healthy wild stocks."
So far, using hatchery fish to supplement harvests "has not had detectable adverse impacts on the abundance of Alaska's wild stocks," biologist Steven McGee wrote here.
"Alaska has not yet repeated the mistakes made in the Pacific Northwest, where habitat destruction, hydropower development, and use of hatcheries to replace damaged wild stock production have resulted in serious depletion of most salmon resources."