Oncorhynchus mykiss is perhaps the most popular freshwater sport fish in the United States. The geographic distribution of native Oncorhynchus mykiss within the lower 48 states is limited to a handful of states along and adjacent to the west coast — California, Oregon, Washington, and Idaho, although it extends south to Baja California and north to British Columbia and Alaska.  It is artificially propagated by federal and state fish and game agencies and in private aquaculture for game fishing purposes in hundreds of hatcheries and is now established in all of the 48 lower states, at least 45 countries, and every continent except Antarctica. Fly-fishing enthusiasts who live east of the Rockies might be surprised to know that the pan-size rainbow trout stocked in streams and rivers in diverse circumstances east of the Rocky Mountains are the very same species as the 20-pound steelhead that returning to spawn in rivers and streams along the west coast after several years out in the Pacific Ocean. An artificial line has been drawn by federal regulators between rainbow trout on the one hand and steelhead on the other that is inconsistent with our contemporary understanding of the species – a species that exhibits a dizzying array of life histories and astounding ecological and behavioral plasticity. For that reason, the time is ripe for the National Marine Fisheries Service to re-assess its approach to conserving populations of Oncorhynchus mykiss.


The diverse and complex life history of Oncorhynchus mykiss is thought to reflect evolutionary adaptation to highly variable environmental conditions, including extended drought, wildfires, floods, and dynamic habitat conditions in both freshwater and the ocean. The life histories exhibited by rainbow trout and steelheadchallenge conservation planners and resource managers. They include (i) individuals that spend their entire lives in freshwater rivers, streams, and lakes — those are typically referred to as resident rainbow trout; (ii) individuals that spawn in streams with juveniles migrating to lakes where they grow to maturity; and (iii) individuals that migrate to the ocean as juveniles that either mature in the ocean before returning to freshwater to spawn or mature only after they return to freshwater where they then spawn – those are typically referred to as steelhead. Notably progeny of a single mother fish can exhibit both a resident rainbow trout life history and an anadromous steelhead life history, nonetheless, those offspring can be morphologically identical through the juvenile rearing stage.


The construction of dams on most river systems up and down the west coast along with other human activities, including harvest of the species, hatchery effects, land and water resource development, and land uses that degrade aquatic habitat — including land reclamation and levee construction — all have contributed to reductions in coastal populations of Oncorhynchus mykiss. Freshwater hydrology, ocean-rearing conditions, and predation by native and introduced fish and wildlife also effect habitat suitability and the numbers of Oncorhynchus mykiss. The abundance of ocean-going steelhead has declined substantially over the past century. The diminished steelhead fisheries led the National Marine Fisheries Service in 1997 to issue a final rule listing five evolutionarily significant units (or ESUs) — genetically and ecologically distinct demographic units — of west coast steelhead as threatened or endangered under the Endangered Species Act (ESA). At the time of the listings the agency stated that “[f]ew detailed studies have been conducted regarding the relationship between resident and anadromous Oncorhynchus mykiss and as a result, the relationship between these two life forms is poorly understood” (Federal Register 62:43,937-43,954). The Service limited the listing to just steelhead populations or runs, the anadromous life form of the species.


In the intervening decades, research has made clear that the distinction between anadromous and freshwater forms of Oncorhynchus mykiss is more of human conceptualization of complex and vexing life-history responses to environmental conditions than a true reflection of the fish’s adaptations to diverse and dynamic landscapes. The mechanistic basis for the wide array of rainbow trout and steelhead morphs and behaviors can only be guessed at. Recent studies address the subject — Anadromy and residency in steelhead and rainbow trout (Oncorhynchus mykiss): a review of the processes and patterns, which appeared in the Canadian Journal of Fisheries and Aquatic Sciences in 2015 and Life history diversity in Klamath River steelhead published in Transactions of the American Fisheries Society in 2016. Those articles reinforce earlier analyses indicating that rainbow trout and steelhead can and do occur in sympatry — that is, they can occur in the same place at the same time. They also document the fact that rainbow trout and steelhead can and do interbreed. The investigations confoundingly found that rainbow trout can produce progeny that are both freshwater residents and anadromous ones, and that steelhead can do the same. In fact, Oncorhynchus mykiss can exhibit dozens of distinct life histories, reflecting an adaptive capacity to respond to a wide breadth of landscape conditions and water bodies and stream courses and the dizzying array of environmental conditions associated with them. The fish’s genetic and behavioral plasticity expressed in dramatically varying life history patterns in response to environmental influences, explains the success of the species. Research shows, remarkably, that the genes associated with anadromy often persist in populations with little access or no access at all to the ocean, indicating that behavioral plasticity may be hard-wired into the species’ DNA.


At this point, the existing Oncorhynchus mykiss populations listed under the ESA include only those individuals that exhibit anadromy and that occur below the lowest impassable barriers to their migration (see Federal Register 71:833-862).  But, as the Service explains in its 2006 final rule retaining 10 populations of west coast steelhead on the list of threatened or endangered species, the agency presumes all Oncorhynchus mykiss that occur in streams where listed steelhead occur are protected from take under the ESA, irrespective of whether they are resident or anadromous fish, and whether they are categorized as trout or steelhead. At the same time, the Service presumes that resident Oncorhynchus mykiss in steams in which anadromous Oncorhynchus mykiss occur do not contribute to the viability of the anadromous population. Such determinations by the Service have implications that extend well beyond listing decisions to critical habitat designations, consultations, and habitat conservation planning efforts that pertain to the protected populations. In those contexts, agency determinations based on an equivocal understanding of the uniqueness, size, and viability of the constituent demographic unit – thus the risk and implications of an extirpation or extinction event — can have substantial societal implications; for example, complicating flood control, land use planning, water supply projects, and other activities in and adjacent to coastal streams and rivers. Whereas the plasticity exhibited by Oncorhynchus mykiss indicates the species can be expected to exist and persist in varied environmental conditions, the listing of numerous ESUs suggests that Oncorhynchus mykiss cannot persist without prohibitions and protections, which can contribute to a burdensome regulatory regime, which in some cases has included actions that are not necessary to conserve the species.

The time is ripe for the National Marine Fisheries Service to re-assess its approach to conserving Oncorhynchus mykiss in light of the best available scientific information, some of it advancing a new and provocative understanding of the species. Analyses conducted since the 2006 rule contribute to clarifying distinctions among populations of the species, including distinctions based on barriers to passage and between resident and anadromous fishes. The studies highlight the varied life history strategies that the species has evolved to survive under multifarious environmental conditions. A pro-active, rigorous approach to conservation of Oncorhynchus mykiss and other listed species, informed by the best available scientific information, is essential to ensure that the Service is expending its limited resources on the species, subspecies, and distinct population segments that are most in need of protection.

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