Marine and freshwater regime changes impact a community of migratory Pacific salmonids in decline

Abstract

Marine and freshwater ecosystems are increasingly at risk of large and cascading changes from multiple human activities (termed "regime shifts"), which can impact population productivity, resilience, and ecosystem structure. Pacific salmon exhibit persistent and large fluctuations in their population dynamics driven by combinations of intrinsic (e.g., density dependence) and extrinsic factors (e.g., ecosystem changes, species interactions). In recent years, many Pacific salmon have declined due to regime shifts but clear understanding of the processes driving these changes remains elusive. Here, we unpacked the role of density dependence, ecosystem trends, and stochasticity on productivity regimes for a community of five anadromous Pacific salmonids (Steelhead, Coho Salmon, Pink Salmon, Dolly Varden, and Coastal Cutthroat Trout) across a rich 40-year time-series. We used a Bayesian multivariate state-space model to examine whether productivity shifts had similarly occurred across the community and explored marine or freshwater changes associated with those shifts. Overall, we identified three productivity regimes: an early regime (1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990), a compensatory regime (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009), and a declining regime (since 2010) where large declines were observed for Steelhead, Dolly Varden, and Cutthroat Trout, intermediate declines in Coho and no change in Pink Salmon. These regime changes were associated with multiple cumulative effects across the salmon life cycle. For example, increased seal densities and ocean competition were associated with lower adult marine survival in Steelhead. Watershed logging also intensified over the past 40 years and was associated with (all else equal) ≥97% declines in freshwater productivity for Steelhead, Cutthroat, and Coho. For Steelhead, marine and freshwater dynamics played approximately equal roles in explaining trends in total productivity. Collectively, these changing environments limited juvenile production and lowered future adult returns. These results reveal how changes in freshwater and marine environments can jointly shape population dynamics among ecological communities, like Pacific salmon, with cascading consequences to their resilience.

Insights into drivers of population declines offer a ray of hope for migratory fishes in the temperate rainforest

“…| 5 COMMENTARY income, food security, and cultural value to communities in the region (Johnson et al, 2019;Marushka et al, 2021). If historic logging has strongly reduced salmonid productivity throughout the temperate rainforest, as Wilson et al (2021) found in their study area, then new approaches to forest management could potentially yield major gains toward recovering fish populations and fisheries. Recent initiatives aiming to end large-scale logging of old-growth forest (https://www.fs.usda.gov/detai l/r10/landm anage ment/ resou rce ma nagem ent/?cid=FSEPR D950023) and incorporate indigenous values into natural resource management decisions (https://www2.…”

“…Thus, omitting carryover effects in the present study was justifiable, although it may have resulted in conservative estimates of the effects of freshwater conditions. Despite this simplifying assumption, Wilson et al (2021) find roughly equal overall effects of freshwater and marine conditions on steelhead population productivity. The strongest freshwater correlate of smolt production was cumulative logging activity in the watershed, and the strongest correlates of marine survival included coastal seal predation, competition with other salmon in the North Pacific, and an index of ocean climate.…”

“…In, perhaps, their most striking result, Wilson et al (2021) determine that increases in the cumulative area of logging activity in their study watershed were associated with 97% or greater reductions in the freshwater productivity of steelhead, coho salmon, and coastal cutthroat trout. These effects were relatively consistent across a range of model assumptions examined in a sensitivity analysis.…”

“…Some scientists have argued that widespread and sustained reductions in marine survival of key salmonid species are an overriding concern, and that investments in conserving and restoring freshwater habitat are unlikely to benefit declining populations (Welch et al, 2021). While Wilson et al (2021) document the importance of marine factors including predation, competition, and climate, they also provide compelling evidence that processes affecting freshwater habitat, especially logging, can also strongly influence population dynamics. The authors contend that identifying drivers in both environments is valuable because it gives managers and policy makers more levers to conserve species and sustain fisheries.…”

“…In this issue, Wilson et al (2021) meet this challenge by linking changes in marine and freshwater regimes to declines of five Pacific salmonid species in a British Columbia river. The authors analyzed a 40-year time series of adult and juvenile fish passage at a counting fence, using a state-space approach that incorporated density dependence, community interactions, and environmental drivers.…”