Spring is here and we’ve got a real shot of warm weather on the West Coast. Certainly, spawning steelhead appreciate the ecological effects of this boost in thermal energy.
Last week we reviewed a recent paper on repeat spawning in steelhead in the Hood River, Oregon. This week, we return to the Hood to look at another paper out of the Michael Blouin lab at Oregon State University.
The OSU study was led by Neil Thompson, a reputable scientist who also loves to fish for steelhead. As discussed in our previous post, research in the Hood River revealed that wild steelhead descended from broodstock hatchery fish still survived poorly even though they had only been in the hatchery one generation. While it has long been known that hatchery fish don’t survive as well as wild fish, the Hood River work indicated that simply introducing a fish into a hatchery produces changes in their genetics and survival. The new Thompson study takes the research a step further to try to determine why those first-generation hatchery steelhead survive so poorly (Read the paper here).
The main culprit in the Thompson paper is growth rate. Juvenile steelhead in hatcheries typically smolt as one-year old fish. In nature, one-year old smolts are fairly common in the southern part of the steelhead range, but as you work northward smolt-age increases. This means that hatchery steelhead tend to grow really fast compared to wild fish in wild rivers.
The fast growth in hatcheries is not surprising. Food is always available. There aren’t any predators. Water temperatures are moderated and there aren’t any floods or droughts.
The problem is, selection for really rapid growth may have consequences that are maladaptive for spawning or rearing in the wild. Wild fish collected for broodstock consist of multiple genetic families. If the fastest growing and largest smolts survive at a disproportionately higher rate than smaller smolts – which is common in steelhead and other salmonids – families that produce larger-sized smolts at release would produce more adult returns.
Prior research on Atlantic salmon has found that rapid growth indeed resulted in poorer survival, but this study is the first to examine the question in steelhead. We’ll discuss why fast growth might be a bad fit in nature later, but first, let’s discuss the Thompson study’s methodology and findings.
Hatchery smolts were sampled for size and genetic family origin prior to release, and adults were collected at a dam near the mouth of the river. The combined data sets allowed the authors to determine whether larger smolts survived and whether genetic family identity played a role in smolt size.
The researchers found a couple of interesting things.
First, the authors evaluated two brood years — essentially the number of smolts emigrating from a river in a given year. Larger smolts survived at higher rates in both years, but the signal was relatively weak in one year and very strong in the other.
This is not surprising. Other studies have found similar results, but it is important to confirm because it sets the table for the genetic component of the research.
Second, there were differences among genetic families in smolt length and that difference was quite significant. In fact, family identity explained 33% of the variability in smolt length, indicating there was a strong heritable component to how fast juvenile steelhead grew in the hatchery. This means that only a sub-set of the genetic families that were produced as smolts actually survived to maturity, which is one way that selection for fast growth in hatcheries can reduce the genetic variability within their broodstock.
Other studies on salmonids have also found that genetics strongly influence growth rate. As we all know by now, life history diversity is the key to restoring and sustaining wild steelhead populations. And genetics influence the ability of generations to express family history diversity. Thus, reducing the number of families that survive is important because it reduces genetic diversity.
But there is another reason that rapid growth rate might be maladaptive.
Specifically, there is evidence in other species that selection for rapid growth may reduce survival of offspring in nature.
Why? Really fast growth rates basically mean that the underlying metabolism of the fish is running fast, which means they need food more frequently than fish with slower metabolisms. Think of a V-8 engine compared to a 4-cylinder. The V-8 goes fast, but it gets horrible gas mileage. Same with faster metabolisms. Fish that grow rapidly require a lot of food. In order to acquire that much food, individuals with fast metabolisms need to be out looking for snacks. Constantly. If they don’t, their tank runs dry.
The tradeoff is that getting that much food requires taking risks — lots of risks. Consequently, juveniles with really fast growth rates can have really high mortality rates in nature. Stick the hand one too many times in the cookie jar and a cutthroat, mink, otter, or merganser will eventually find you. Thus, feeding behavior that works in a hatchery could be less effective, or even counterproductive, in nature.
While we don’t know if genetic selection for growth rate is the underlying mechanism, Thompson’s work provides us with a guidepost that suggests it is an important component of domestication in steelhead.
It will be interesting to see what the Blouin lab comes up with next. The Hood River research has provided an impressive array of insights into the hatchery and wild issue.
Until our next Science Friday post, enjoy the weekend and remember: the future of steelheading depends on us, anglers, getting involved and doing our part to conserve these magnificent fish.