Continued from part 1 via the Instagram account of Keepemwet Fishing Science Ambassador John R McMillan. @rainforest_steel


Here we have a very simple statistical model that evaluates the relationship between variation in annual run size and the total number of life histories in each species. Of note, I changed the juvenile life histories from one to two to include yearling coho smolts, which I assume exist to some degree in the Skagit, based on a suggestion by @fisherfreak. Thanks! Someone else raised that same point to me about a year ago, in which case the total number of life histories for #cohosalmon increases from 3 to 6. I needed to standardize the variation in annual run sizes so I calculated the Coefficient of Variation (CoV) for each species annual run sizes over the period of record, which is represented by the 1-100% scale on the bottom (x-axis). The CoV tells us how much run sizes disperse from the mean run size. The higher the percentage, the more variation in run size there is from year-to-year (think pinks). The lower the percentage, the less the variation. On the vertical (y-axis) scale is the total number of life histories for each species. I used a simple regression model that allows for non-linear relationships (which just means the association does not have to be straight, or linear, and can curve). The solid black line represents the best fit of the model, and it was a very strong fit, with total number of life histories explaining 85% of the variation in run size. All this means is: Life history diversity among salmonids (in this data set at least) in the Skagit is strongly linked to how much run sizes vary from year-to-year. The theme is pretty consistent with steelhead, diversity matters, a lot. It is their evolutionary calling card, just like home runs were for Barry Bonds, steals were for Ricky Henderson, and 100 mph fastballs were for Nolan Ryan.


Wrapping up the first part of the discussion here on steelhead life histories. As mentioned previously, they display the most life histories of any salmonid. Up to 32-38, and that does not even include repeat spawners. As smolts, most steelhead spend 1-4 years in freshwater before migrating to the ocean. However, they have found fish as old as 7 migrating to the ocean! It suggests that some fish are maturing as rainbow and then going out to the ocean to become a steelhead. Shape shifters. And once they get to the ocean, some fish spend only a month or two, while others may spend up to five consecutive years. The result of this diversity is an incredible array of ages at maturity. Some steelhead will mature by age-2, others not until age-9. Consequently, we have mature steelhead ranging from little ones as small as 2lbs to huge fish that tip the scales beyond 40lbs. Given all this diversity it is not surprising that biologists and anglers were long confused about what a steelhead actually was, and the delineation of species was further muddied by the inclusion of resident rainbows (which I have not yet touched on). Steelhead have so many choices in life history they are basically like a Baskin Robbins or a Costco. Options, options, options. Or, think of them like a leatherman tool, while species like pink are a phillips head screwdriver – not many options with the last, and if the environment in a given year requires a flat head screwdriver, the pinky phillips won’t do as well. Steelhead populations on the other hand will not fare as badly, while their phillips head won’t work well, they can simply pop open the leatherman and choose the right tool. This is what helps them occupy the greatest geographic distribution of any Pacific Salmon, all the way from Russia across the Pacific Rim down the Baja California. No other species of salmon comes even close to matching this. They are not leathermen.


Decided I can’t just walk away with figures and tables. I will continue the presentation tomorrow, until then, here is a small summer run steelhead. Nothing is more rewarding than spending all day snorkeling to finally capture an underwater photo of the most difficult species to shoot. I love these fish!