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Study Looks at Ways to Reduce Hatchery
Steelhead Adaptation to Captivity, Increase Survival

by Staff
Columbia Basin Bulletin, December 2, 2016

A hatchery steelhead soon meets its end in the hatchery from which it came. A hatchery reared steelhead fry with dominant and/or aggressive traits will grow bigger than non-dominant fry, but that doesn't mean that growth will keep pace as the juvenile steelhead is released from the hatchery.

A recent study of steelhead fry at the Alsea Oregon Hatchery Research Center also found that rearing density did not affect the relationship between dominance levels and body size at release.

Researchers hypothesized that families with a higher level of dominance at first feeding grow to a larger size compared to families with a lower dominance level, but they found that hypothesis doesn't fully stand up.

"We found that dominance is weakly correlated with body size at release, and the correlation was not effected by rearing density," said corresponding author Neil Thompson, a postdoctoral researcher at the Integrative Biology Department at Oregon State University. "Although, that finding should not be applied to production level densities until a similar study at a production scale is conducted. We also found that dominance at first feeding was positively correlated with body size, as had been reported previously in Washington state."

According to the study, "Family Dominance at the Fry Stage Weakly Influences Mean Family Body Length at Smolting in Hatchery-Reared Steelhead, hatcheries may be selecting for fish with two behavioral traits – high levels of dominance and/or aggression. Thompson's co-author is Michael Blouin, professor in the Department of Integrative Biology at OSU.

Dominance and aggressiveness is neither a good nor bad thing, Thompson said. But they are traits that juvenile steelhead express during hatchery rearing.

"Our research is focused on determining what traits are correlated with large size at release from the hatchery," he said. "This is important because body size at release is correlated with survival for hatchery-reared steelhead."

Other studies of hatchery-reared steelhead have found that hatchery-origin fish are better adapted to the hatchery than are natural-origin fish and vice versa for the wild environment (natural-origin fish are better adapted than hatchery), he said. This fitness tradeoff has also been found in Atlantic salmon where individuals who perform well in the hatchery perform poorly in the wild environment.

"Our work is attempting to identify what traits make a fish better adapted to the hatchery environment," Thompson said. Once researchers identify those traits, then they might be able to modify the way steelhead are reared in the hatchery and "potentially increase reproductive success in the wild environment."

The study used a small number of families (7) in their sample, mostly due to an equipment malfunction during early rearing and limitations at the hatchery.

A relatively small steelhead hatchery with a goal of releasing less than 50,000 smolts would use 30 to 40 female broodstock, which would create 60 to 80 families, Thompson said, with each female broodstock split between multiple broodstock males. A family is defined as the offspring from one female broodstock and one male broodstock.

In finding that "rearing density did not affect the relationship between dominance levels and body size at release," Thompson offered another caveat. In this study, high density was studied with the same number of fish, but in a smaller tank.

"At production scales, higher density is produced by a larger number of fish in the same size tank, so dominance may have a different effect in production hatcheries than in our experiment," Thompson said. "In our experiment a hierarchy could have been established in high and low densities given the approximately equal number of fish in each treatment. In production high density it would be harder for a hierarchy to be established because of the thousands of other fish in the tank."

Steelhead are reared in hatcheries for one year and released as smolts, after which they migrate to the ocean. However, natural-origin steelhead rearing in the wild will smolt at two years or older.

"Thus, selection in the hatchery is likely acting on traits that increase the ability to reach the largest body size possible in a single year," the study says.

This research is focused on hatchery-rearing dynamics and no comparison should be made to natural-origin fish in the wild environment, Thompson said. "What makes a fish survive and do well in the wild environment may be very different from what makes a fish do well in the hatchery." The hatchery lacks predators, has an abundance of artificial food and the fish are treated for diseases and parasites.

"Our research goal is to identify ways to rear fish in hatcheries that reduces adaptation to captivity by 1) reducing variance in size at release and 2) increasing post-release survival for the entire hatchery cohort," Thompson said. "Both of these aims would reduce the potential for adaptation to the hatchery to occur and may reduce the difference in reproductive success of hatchery and natural-origin steelhead that spawn in the wild."

Given the outcome of this study –there is weak support that dominance is correlated with body size at release – Thompson suggests further study on a larger or production scale. That, he said, is a multi-year project, but would more fully test "hypotheses about adaptation to captivity."

Thompson's most recent study, "The effects of high rearing density on the potential for domestication selection in hatchery culture of steelhead (Oncorhynchus mykiss)," reviewed in the Columbia Basin Bulletin and published online August 5, 2015, in the Canadian Journal of Aquatic Sciences (, found that crowding in hatcheries quickens the pace of the fish to domesticate and lowers their fitness for survival and spawning in the wild and that the fitness is due to a decline in overall length to below a threshold for survival.

Blouin's most recent study found that it takes just one generation for the DNA of steelhead domesticated in hatcheries to be altered and to be significantly different than steelhead whose parents are wild. See "A single generation of domestication heritably alters the expression of hundreds of genes," published online February 17, 2016, in the journal Nature Communications,

Related Sites:
Study Looks At Whether Crowded Hatcheries Push Steelhead To Domestication, Smaller Body Size by Staff, Columbia Basin Bulletin 10/30/15

Related Pages:
Study: Changes to Genetics of Hatchery Steelhead Occur in Just One Generation by Staff, Columbia Basin Bulletin 2/26/16

Study Looks at Ways to Reduce Hatchery Steelhead Adaptation to Captivity, Increase Survival
Columbia Basin Bulletin, December 2, 2016

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