Science Friday: More on summer and winter steelhead genetics

In Oregon, Science Friday by Nick Chambers

By John McMillan

Two weeks ago we reviewed a study by Prince et al. that discovered a single gene differentiated steelhead which return immature (referred to as pre-mature in the study) to freshwater (i.e., summer steelhead) and those that return in a mature or relatively mature state (i.e., winter steelhead). Anglers have long known there is something inherently different about the two runs of fish. Not only do summer steelhead enter freshwater at different times and travel to and spawn in different locations than winter runs, they are also distinct in appearance.


The key finding of the Prince analysis was not just that there is a genetic difference between the two types of steelhead. Rather, it was the specificity and temporal rarity of the genetic difference. The study’s two very interesting results which clarified these characteristics have significant implications for how we manage and recover the species.


First, while scientists have known there is a heritable component to life histories, it was believed that the formation of such races of fish of a single species was largely due to a combination of several genes. The Prince et al. study found, instead, that it was only a single gene that differentiated the races.


Summer steelhead caught on its long upstream migration

Second, the gene that differentiated the races has evolved rarely over the past several million years. It does not appear to be something that pops up frequently.


These two results suggest it may be very rare for winter runs to give rise to summer runs, which could have important implications for how fish are managed for recovery as well as for their eligibility for listing or a change in status under the Endangered Species Act (ESA).


The Prince report’s findings could lead to differential listing levels for summer and winter steelhead. A case in point: both winter and summer steelhead of the Puget Sound region are listed as Threatened under the ESA. While there is very little data on summer runs, it is clear they are rarer than winters and what data is available suggest summers are probably at a much greater risk of extinction. If winter steelhead rarely if ever give rise to summer steelhead, then summer runs could require a greater level of protection to ensure their persistence. Thus, while Puget Sound winters are listed as Threatened, an Endangered listing status may be warranted for summer fish, depending on what future research finds.


Mature winter hen close to the salt

Conversely, in areas where winter runs are in significantly better shape than summers, an ESA listing may be warranted for summer steelhead but not for winter run fish, at all.


No such change in ESA listing status is currently happening. These are potential implications if future research affirms that the “summer run” gene is exceedingly rare, and if the National Marine Fisheries Service then incorporates that information into its listing framework.


So far we only have two scientific papers that have addressed this topic. Dr. Mike Miller, a co-author on the Prince et al. paper, is collecting more samples from other populations and from resident rainbow trout too, because residents could carry the “summer run” gene and thus be another source for summer steelhead. Other researchers are doing similar work. The collective results of this work will be important in determining if we need to modify our steelhead conservation thinking and practices (both in management and angling).


It certainly is an interesting time to be a steelhead scientist and angler. Each year seems to bring a new discovery about steelhead and the factors underlying their diversity. While it is important not to react too dramatically to new research until it has been corroborated across multiple studies, it may be critical for us to adjust steelhead management policies and best practices fairly rapidly based on their results. We will keep you informed as future research unfolds and agencies begin to review and interpret the data.