Science Friday: Tillamook Bay Steelhead

In Oregon, Science Friday by Nick Chambers

Do you ever wonder what we might be missing nowadays in terms of steelhead life histories? Although we can’t go back in time to answer this question, we can look at historic data — in this case from Oregon’s Tillamook Bay.


By the 1940’s Oregon’s Tillamook Bay was a patchwork of homesteads and farms, appearing more like the present day than its historical condition of tidal swamps, marshes and productive estuary. During the 1941-42 winter steelhead season the Oregon State Game Commission sampled the steelhead catch of Tillamook sport anglers and commercial gill net fishermen.The data represent one of only a few snapshots we have into the population structure of steelhead prior to 1950.


The OSGC study revealed several important results. First, they analyzed scales and found 4% of steelhead showed an intermediate period of growth, indicating the fish had spent several months in the estuary. This is interesting because most research in the past 10-30 years indicates that salmon rely heavily on estuaries for rearing prior to their full ocean migration, but steelhead rarely do — except for areas in southern and central California where steelhead commonly use lagoon areas for rearing. It is clear the Tillamook Bay fish were taking advantage of the productive estuary areas, but it is unclear whether the fish were half pounders or just using the estuary prior to their full-scale ocean migration.


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Second, the study also found a broad diversity in age at maturity with fish spending 1-4 years in freshwater and up to 6 years in saltwater. Despite this variation in life history, average size only ranged from 17 inches to 34 inches, with the vast majority of fish measuring 25 to 30 inches. This indicates that size is not necessarily a good predictor of age and that it has more to do with individual growth rates.



Lastly, roughly a quarter of the fish captured had spawned more than once (up to four times for two fish) yet these fish were close in size to fish on their first spawning run, indicating that little growth occurs past their age at first maturity. All in all, while we do not have a lot of historical data on steelhead life histories, research such as this suggests that we still have a great deal to learn about steelhead — and that some once-important life histories may no longer exist.

Click the link below for the full paper

Sumner 1948 age and growth of steelhead in tillamook county