Science Friday: The different life histories of adult steelhead
Last week we reviewed the terms used to define different stages of the juvenile steelhead life cycle. This week we break down the life stages of adults and describe their different life histories, which are remarkably diverse.
The adult steelhead life stage may be delineated into two categories: adults and kelts. Adults are fish that are mature or preparing to mature and will eventually spawn in freshwater. Most winter and summer run fish caught by anglers are adults. (There is a sub-category of adult steelhead, however, which we discuss below.) Kelts are steelhead that have completed spawning and are preparing for, or are in the process of migrating back to the sea.
Individuals may adopt one of several potential life histories to achieve maturity and spawn in freshwater. In fact, there are more possible combinations – somewhere between 30 and 40 – in steelhead than any other salmonid, at least based on the science to date.
In many populations, most individuals adopt a fully anadromous life history. A fully anadromous individual spends 1-5 consecutive years in the ocean before returning to spawn as either a summer or winter run steelhead. These are what most anglers think of when they are dreaming about steelhead. These fish go to the ocean and return at ages ranging from 3-8 years and at sizes ranging from 20” to over 40” in length.
In addition to full-sized adults, in many populations we find a number of individuals that undertake shorter migrations and mature at younger ages and smaller sizes. For example, many anglers are familiar with the term half-pounder. Half-pounders migrate to the ocean for a short period of time, then re-enter freshwater in an immature state where they overwinter. After overwintering in freshwater they return to the ocean the following spring before eventually returning back to the same river as a fully mature adult. Complex, right? Indeed, it is.
There are a number of variants on this theme in rivers draining the Kamchatka Peninsula in Russia. In those populations, there are estuarine and river-estuarine life histories. Estuarine individuals seasonally enter estuaries repeatedly over successive years before maturing. River-estuarine individuals alternate years — in one year they migrate to the estuary and in the following year they remain in freshwater. Basically, they are alternating between being a resident rainbow trout and a steelhead. Shape shifters!
A life history is a solution to reproduction in a given environment, which means that life histories essentially are a reflection of habitat diversity within a watershed. Steelhead diversity is amazing and reflects the multiplicity of habitats to which they have adapted to be successful. Adults may return to freshwater every month of the year in some populations. Maintaining and rebuilding steelhead diversity — the key to rebuilding durable, abundant runs of wild fish — thus requires conserving their habitat, all of it, to ensure that they have the maximum diversity possible.
But this conservation goal also requires managing fisheries in a way that promotes diversity, particularly for those life histories that are less common. In sum, steelhead rely heavily on diversity to sustain their abundance and population resilience. Ensuring fishable populations of wild fish into the future will require improving both habitat conditions and management policies and practices, which is why we at Wild Steelheaders United are working hard to improve the alignment between how we manage habitat and fisheries.