Watershed‐scale climate influences productivity of Chinook salmon populations across southcentral Alaska

Abstract

The ecosystems supporting Pacific salmon (Oncorhynchus spp.) are changing rapidly as a result of climate change and habitat alteration. Understanding how-and how consistently-salmon populations respond to changes at regional and watershed scales has major implications for fisheries management and habitat conservation. Chinook salmon (O. tshawytscha) populations across Alaska have declined over the past decade, resulting in fisheries closures and prolonged impacts to local communities. These declines are associated with large-scale climate drivers, but uncertainty remains about the role of local conditions (e.g., precipitation, streamflow, and stream temperature) that vary among the watersheds where salmon spawn and rear. We estimated the effects of these and other environmental indicators on the productivity of 15 Chinook salmon populations in the Cook Inlet basin, southcentral Alaska, using a hierarchical Bayesian stock-recruitment model. Salmon spawning during 2003-2007 produced 57% fewer recruits than the previous long-term average, leading to declines in adult returns beginning in 2008. These declines were explained in part by density dependence, with reduced population productivity following years of high spawning abundance. Across all populations, productivity declined with increased precipitation during the fall spawning and early incubation period and increased with above-average precipitation during juvenile rearing. Above-average stream temperatures during spawning and rearing had variable effects, with negative relationships in many warmer streams and positive relationships in some colder streams. Productivity was also associated with regional indices of streamflow and ocean conditions, with high variability among populations. The cumulative effects of adverse conditions in freshwater, including high spawning abundance, heavy fall rains, and hot, dry summers may have contributed to the recent population declines across the region. Identifying both coherent and differential responses to environmental change underscores the importance of targeted, watershed-specific monitoring and conservation efforts for maintaining resilient salmon runs in a warming world.

Redistribution of salmon populations in the northeast Pacific ocean in response to climate

“…An important caveat for our simulations is that we assume the abundance of the focal stocks is represented by the abundance in recent decades (1983 to present). While this assumption allows us to identify predicted shifts that stem solely from ocean climate, it is likely that future climate conditions will differentially affect the productivity of individual stocks through changes at various stages in the lifecycle (Crozier et al, 2008;Jones et al, 2020;Oke et al, 2020).…”

“…An important caveat for our simulations is that we assume the abundance of the focal stocks is represented by the abundance in recent decades (1983 to present). While this assumption allows us to identify predicted shifts that stem solely from ocean climate, it is likely that future climate conditions will differentially affect the productivity of individual stocks through changes at various stages in the lifecycle (Crozier et al., 2008; Jones et al., 2020; Oke et al., 2020). Currently, stock‐specific abundance projections are not available for all stocks, but a broad literature suggests climate change will affect the productivity and population dynamics for many salmon populations during freshwater life‐stages (Battin et al., 2007; Crozier et al., 2008; Kovach et al., 2015; Morita et al., 2014).…”

“…To date, investigations of the consequences of climate change have predominantly focused on the freshwater component of their life‐history. Important responses of anadromous species to a changing climate include physiological shifts anticipated under warming temperatures (Muñoz et al., 2015) and low pH (Ou et al., 2015) in freshwater, phenological shifts in migration timing of both outmigrating juveniles (Cline et al., 2019; Otero et al., 2014; Scheuerell et al., 2009) and returning adults preparing to spawn (Finstad & Hein, 2012; Jonsson & Jonsson, 2009), and responses of populations to changing riverine hydrological regimes (Crozier et al., 2008; Jones et al., 2020; Sturrock et al., 2020). However, oceanic environments comprise the majority of many anadromous species’ lifespans and favourable ocean conditions are an important determinant of growth and survival (Beamish & Mahnken, 2001; Duffy & Beauchamp, 2011); up to 90% of mass can be derived from ocean growth (Quinn, 2005).…”

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

“…These regime shifts alter fishery quotas and recovery timelines and are associated with the widespread decline of wild Pacific salmonids across the Pacific Northwest (Dorner et al, 2013;Peterman & Dorner, 2012;Teresa A'Mar et al, 2009). Despite marine regime shifts having been frequently observed in Pacific salmonids (Welch et al, 2020), descriptions of freshwater regimes shifts are rare (but see Atlas et al, 2015;Jones et al, 2020;Scheuerell et al, 2020), and relatively few studies have assessed regime shifts in both marine and freshwater contexts. Moreover, studies generally focus on a single species even though salmon rivers often support diverse communities.…”

Evidence of prevalent heat stress in Yukon River Chinook salmon

“…Spawning abundances are the primary data used to assess management success in escapement-based management. In fact, the productivity (log recruits per spawner) of Chinook salmon populations across several southcentral Alaska watersheds was recently shown to decline steeply when spawning water temperatures exceed 18°C, the same water temperature threshold identified for heat stress in this study (Jones et al 2020). More research is needed that links water temperature, heat stress, and mortality in northern rivers to assess the sensitivity of population dynamics to environmental conditions during the spawning migration.…”