Juvenile salmon non-natal rearing via habitat shifting through the marine environment

Salmon exemplify numerous life history strategies as they transition between freshwater and marine environment during their complex life cycles. Importantly, this life history diversity varies among species and populations and is well understood to provide resilience. Yet such complexity has also left some salmon particularly vulnerable to habitat degradation and loss, precipitating substantial population declines. Among the myriad management actions underway, protecting and restoring juvenile rearing habitat have been identified as critical to salmon recovery. Much of that effort has focused on juvenile rearing habitat within natal watersheds (i.e., the watersheds where salmon spawn and emerge). This is appropriately due to the assumption that juvenile salmon primarily rear in their natal watersheds before dispersing to mature in the ocean. However, observations over the past two decades have revealed that juvenile salmon will also successfully rear in non-natal habitats. Much research has focused on non-natal habitat rearing in non-natal tributaries or non-natal areas downstream of spawning grounds. These types of non-natal habitats are often still within the natal watershed. However, there are also examples where juvenile salmon disperse out of their natal watershed, enter the marine environment, and re-enter freshwater or brackish habitats, including in non-natal estuaries or streams to rear. This behavior, which we term ‘habitat shifting through the marine environment’ (HSME), is largely absent from dominant conservation discussions for salmon populations. Here, we review the evidence for HSME across salmon species and geographies. We found that HSME has been widely observed throughout the Puget Sound region, particularly in the Whidbey Basin but also in Central Puget Sound (King and Kitsap Counties) and Hood Canal (Mason and Jefferson Counties). Beyond Puget Sound, we found evidence for HSME throughout the Pacific from Alaska to California and Japan. There was also evidence for HSME in Baltic Sea salmonids. The evidence for HSME was dominated by Chinook (Oncorhyncus tshawytscha) and coho (O. kisutch), likely due to disproportionate research in Puget Sound and the eastern Pacific more broadly. But we also found evidence for HSME in chum salmon (O. keta), pink salmon (Oncorhynchus gorbuscha), masu salmon (O. masou), Atlantic salmon (Salmo salar), and brown trout (S. trutta). Most HSME was observed in smaller coastal streams but there is also important evidence for HSME into barrier embayments (e.g., pocket estuaries) and into non-natal large river deltas. The evidence for HSME is more expansive across species and geographies than is perhaps generally recognized. The relatively widespread nature of HSME underscores the importance of not discounting small systems without salmon spawning as fishless habitats. Recognizing the commonness of HSME may afford new opportunities for protective and restorative measures in habitats like smaller coastal streams that have seldom been prioritized for salmon recovery. Such actions may be particularly important given the century-plus legacy of impacts that have potentially limited access for juvenile salmon to these habitats. Importantly, as individual fish travel farther from natal watersheds or over longer periods of time, the need for non-natal habitat likely diminishes as fish grow larger and become further acclimated to marine environments. Finally, the conservation community may have experienced a “shifted baseline” for what constitutes normal salmon habitat use given some of the most intensive impacts to these atypical rearing habitats occurred decades before salmon declines were considered critical. Including non-natal habitats in restoration and recovery plans is likely valuable and could benefit from a framework that considers the spatial arrangement (relative to natal sources) and condition of habitats as well as desired current and future population abundances. Such a framework would rely on increased coordination among recovery jurisdictions as benefits of non-natal habitat use will often transcend population and jurisdictional boundaries.

Funding: This project was funded by the Puget Sound Partnership through the Monitoring to Accelerate Recovery program with support from the Puget Sound Ecosystem Monitoring Program (PSEMP). This project has been funded in part by the United States Environmental Protection Agency under assistance agreement number 2022-55 to the Washington Department of Fish and Wildlife. The contents of this document do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does the EPA endorse trade names or recommend the use of commercial products mentioned in this document.