Science Friday: How summer steelhead are genetically different than winters

In Science Friday by Nick Chambers

Many anglers over the years have no doubt wondered why some steelhead return to freshwater during the hottest and driest part of the year. The reasons why are both simple and complex.


The most obvious difference between summer steelhead and their winter run brethren is that they enter freshwater streams in summer and fall, not winter. Duh. But that is only a symptom of a larger difference. From a genetic perspective, it turns out that summer run steelhead are actually quite unique.


Summer steelhead may spend nearly a year in freshwater as adults before spawning

The most unique attribute of summer steelhead is that they enter freshwater in an immature state, often highly immature, and utilize their lengthy staging period in freshwater to complete th e maturation process. Summer run fish basically show up to the party early, before the beer is out and the brats are cooked. In contrast, winter steelhead tend to return to freshwater from the marine environment in a mature state, and sometimes will spawn within days after doing so. Their maturation occurs completely in saltwater. They show up to the party just as the music is getting loud and the beer is flowing.


Biologically speaking, neither life history is necessarily better than the other, but each evolved because that is what their respective environments require. For example, summer runs often undertake more arduous and/or longer migrations to their spawning grounds than winter runs — presumably because it is too difficult for a fully mature fish to migrate several hundred miles. A fish needs to arrive early to cover such a long distance.

While there are numerous examples of winters and summers in the same basin, they are generally thought to utilize different parts of the watershed. The Columbia Basin is a large and perfect example where only summer steelhead are able to migrate to its upper reaches.


Despite a long-term interest among anglers and biologists in understanding these life histories, scientists have only recently developed the genetic tools to identify the potential traits underlying such different strategies. A recent study out of the Mike Miller lab at UC Davis has finally uncovered something very cool and remarkable about the genetics of such life histories. The paper was authored by Daniel Prince, a graduate student under Dr. Miller.


The authors looked at several populations of steelhead in California and Oregon, and also several populations in Chinook salmon in California, Oregon, and Washington. Why Chinook too? Because they are like steelhead in that some life histories, like springers, enter the river immature while others, like fall run Chinook, enter in a mature state. This means that there could be a common trait that links the biology of the two species.


Indeed, that is what the research team found. The tendency to return to freshwater prior to being fully mature is termed “premature migration.” Prince et al discovered that premature migration in both species was associated with the same locus across multiple populations. A locus refers to the location on a chromosome where a gene is found. Basically, they identified the area where there was variation among the life histories.


Winter steelhead can spawn within days of entering freshwater in some cases (particularly for females) their freshwater residence time as an adult can be only a week or two.

The patterns they observed indicate there were strong differences in the alleles on that locus. Alleles are variants of the same gene that occur on the same locus, such as what we see when people have brown or blonde hair. In this case, variations in the alleles determined whether a fish entered freshwater as a mature winter steelhead or a premature/immature summer steelhead. The take home point is: the research team not only identified where the fish had genetic differences, they actually pinpointed the specific allele that differentiated the premature and mature life histories. The allele in question has not been fully studied, but information to date suggests it has effects on metabolism. This makes sense given that summer runs seem to store fat efficiently while winter runs burn through the fat to generate gonads and eggs.


Regardless of the gene’s function, what is most remarkable is the fact that a single allele seems to explain the life history variation. It has long been thought that such life histories in salmon and steelhead arise due to lots of genes and alleles working in combination with one another.  Hence, it was also thought that summer runs could be produced by winter runs given a long enough period of time and the right environmental conditions. That now appears not to be the case.


Further, it appears the evolution of this allele is exceedingly rare, having been produced only once in the past 10-15 million years. Thus, if populations with the allele are lost, it would be incredibly difficult to recover those life histories.


This is a complex topic, which is why we are breaking this down into two posts over consecutive weeks. In the wake of this recent research, it’s also a vitally important topic. The results of this study will likely have a strong influence on how we approach wild steelhead conservation moving forward. Next week we will evaluate what this study means for summer steelhead management and angling. Until then, read the paper and feel free to ask us questions.