What is a B-run steelhead?

In Oregon, Science Friday by Nick Chambers

Fisheries managers predict this will be a very bad year for returning B-run steelhead in the Snake River. The Snake, with its headwaters straddling the Teton Range on the Idaho-Wyoming border, is the largest tributary to the Columbia and its intact habitat and steelhead runs are vital to the overall health of Columbia River steelhead populations. Despite the high quality habitat in the upper Snake B-run wild steelhead have declined in abundance. But what are B-run steelhead?


Simply, the Snake River’s summer steelhead are generally delineated into two distinct sub-populations/stocks, known as “A-runs” and “B-runs.” Managers distinguish these two sub-populations according to size and return timing at Bonneville Dam, the lower-most dam in the Columbia River.


A recent, excellent paper written by Dr. Timothy Copeland and others from the Idaho Department of Fish & Game looked into the life history of A- and B-run steelhead in more detail, to evaluate whether the criteria of age, size and entry timing in fact support the long-held delineation of two sub-populations, and if such groupings were useful for management and conservation.


The authors found important results. First, yes, there were general differences in size and run timing of A- and B-run steelhead. For example, B-runs smolted at an older age. Over half of B-runs were three year old smolts, compared to only 17% of A-runs, and in general the B-run population was composed of older fish. B-runs also tended to be larger in size, but that is not a hard-and-fast rule as some A-runs were also older and larger.



The authors also found some differences in run timing. B-run fish tended to pass Bonneville Dam later than A-runs. Like age and size however, there was overlap and significant variation between the many populations that make up the A- and B-run stocks.


The results of this analysis highlight that there is overlap among the two stocks, both in age and size and in the time during which they pass Bonneville. This underscores the remarkable diversity in steelhead, and suggests that steelhead life histories occur along a continuum rather than in discrete groupings.


These results have implications for management and conservation. The authors suggest that genetic stock identification at dams and monitoring of stocks in specific sub-basins is necessary to fully understand the complexity within the sub-populations.



It is very important to improve our understanding of steelhead populations in order to manage them effectively, especially in years such as this one with probable critically low numbers of B-run wild steelhead. As we have mentioned before, diversity is a key element underpinning the resilience of steelhead populations. Diversity in age structure, habitat use, and other factors provides a biological buffer for steelhead that helps them persist through changes in freshwater and marine conditions.


In order to recover wild B-run steelhead, and all steelhead populations for that matter, we must adopt management strategies that conserve this diversity instead of managing for the most abundant or prevalent life history strategies. Such strategies will require even more research like that published recently by Copeland et al. In support of this need Wild Steelheaders United is committed to reviewing, critiquing and sharing the results of all peer-reviewed wild steelhead science.