Science Friday: Some come early, some come late: Genetics of Dean River steelhead

In Science Friday by Nick Chambers

There are a handful of rivers that every steelheader wants to fish. Among these is the Dean River in British Columbia, perhaps the most revered wild steelhead river on the planet. Not only is this river remote and beautiful, it is also home to some of the hardest fighting steelhead anywhere.


The Dean’s fame as a wild steelhead fishery has prompted some attention from the scientific community to better understand its steelhead population and to promote sustainable management. One study, conducted during the summer of 1996 in the lower river, looked at different entry times for summer steelhead to determine if there were physiological or genetic differences in early-run and later-run fish.


The fishing season on the lower Dean runs from early July through late September. During that 1996 season Michael Hendry (lead author of the study) examined wild steelhead caught by anglers, noting the sex of the fish and measuring length, taking scale samples and collecting genetic clips. A full copy of the study can be read here:



Anybody want to bet the over-under on what Hendry and his associates found?



In fact, the research team found several interestings things. First, the sex ratios of male and female steelhead were about equal. It is not uncommon for summer steelhead populations to be female-biased, presumably because a higher proportion of males residualize and mature in freshwater without ever going to the ocean. But that was not the case in this study.


Second, they found male steelhead in the Dean were on average a little larger than females, which is typical for steelhead and many salmonids.


Third, the authors found that body length increased as the season progressed. In other words, larger fish tended to enter later than smaller fish, perhaps because they spent a longer period of time in the ocean and thus had more opportunity for growth.


Fourth, they delineated fish into three groups based on when they were caught: early (July 2-30), middle (July 31-September 5) and late (September 6-30). There were highly significant differences genetically between the early and late groups, suggesting that there is a heritable component to run timing. This finding is consistent with those of many other salmonid studies. In addition, in the Dean study there was some evidence of further population structure in the later arriving group, which means there may have been more genetic differences than were actually detected in this study.


The Dean study referenced here is now over 20 years old, but was at the time — and remains — a very important contribution to steelhead science. In particular, it highlights that timing of steelhead migration is at least partly genetic. There is a wide range of possible explanations for this phenomenon. Early-timed fish may have to enter earlier to access spawning grounds that are further upstream. Or, they may spawn in different tributaries. Alternatively, larger and later-arriving fish may have higher spawning success owing to their larger size.


It is likely the causal factors vary from watershed to watershed across steelhead range, but the principal take-away is this: steelhead have evolved to take advantage of different habitat conditions in freshwater when they migrate from the ocean to spawn, and differences in run timing are often reflected in differences in genetics and body size.


Such differences — and our improved understanding of them — are not trivial in terms of influence on steelhead management and angling. Climate change is predicted to elevate summer water temperatures and reduce stream flows. Run timing of steelhead, while partly genetic, is also strongly shaped by those factors. Hence, climate change may select for earlier- or later-arriving steelhead depending on changing habitat conditions, and steelhead populations that have more genetic diversity are likely to have higher potential for survival.


This is one of the primary reasons Wild Steelheaders United is working to ensure some rivers are managed exclusively for wild steelhead, where the genetic diversity of wild steelhead will not be degraded by introgression with hatchery fish. It is why we have advocated for managing steelhead fisheries where harvest is permitted so that harvest is evenly spread out across the run, rather than intensively focused on early-timed fish. And it is also why, because such vital genetic differentiations will fade out if we continue to lose quality habitat, we continue to invest heavily in restoring and reconnecting important steelhead habitat throughout the West.