Climate and ecosystem linkages explain widespread declines in North American Atlantic salmon populations

Abstract

North American Atlantic salmon (Salmo salar) populations experienced substantial declines in the early 1990s, and many populations have persisted at low abundances in recent years. Abundance and productivity declined in a coherent manner across major regions of North America, and this coherence points toward a potential shift in marine survivorship, rather than local, river-specific factors. The major declines in Atlantic salmon populations occurred against a backdrop of physical and biological shifts in Northwest Atlantic ecosystems. Analyses of changes in climate, physical, and lower trophic level biological factors provide substantial evidence that climate conditions directly and indirectly influence the abundance and productivity of North American Atlantic salmon populations. A major decline in salmon abundance after 1990 was preceded by a series of changes across multiple levels of the ecosystem, and a subsequent population change in 1997, primarily related to salmon productivity, followed an unusually low NAO event. Pairwise correlations further demonstrate that climate and physical conditions are associated with changes in plankton communities and prey availability, which are ultimately linked to Atlantic salmon populations. Results suggest that poor trophic conditions, likely due to climate-driven environmental factors, and warmer ocean temperatures throughout their marine habitat area are constraining the productivity and recovery of North American Atlantic salmon populations.

Linking Behavior, Physiology, and Survival of Atlantic Salmon Smolts During Estuary Migration

“…Although changes in physiological smolt development and the timing of initiation of migratory behavior could shift run times initially (McCormick et al 1997), these changes are also both entrained by circannual rhythms in photoperiod , so there may be limits to how early physiological and behavioral smolting can occur (Otero et al 2014). The resilience of many North American stocks to strong selection pressures imposed by these shifts is unknown (Mills et al 2013;Friedland et al 2014;Hayes and Kocik 2014).…”

“…Through improvements in estuary survival by stocking below dams and monitoring smolt gill NKA activity, we expect that the proportion of fish exiting the estuary each year could approximately double in a best-case scenario. Based on a lack of differential survival in marine habitats after leaving Penobscot Bay (Sheehan et al 2011), and the fact that patterns in marine mortality are similar among North American stocks (Friedland et al 2003;Mills et al 2013), this gain would likely translate directly to increases in returning adults.…”

“…Information about these relationships could have timely implications for the management and conservation of fisheries. Elevated marine mortality in recent years, due in large part to changes in ocean climate (Friedland et al 2003;Mills et al 2013;Friedland et al 2014), is thought to be a driver behind the recovery failure of many endangered populations of Atlantic Salmon Salmo salar (Chaput et al 2005;Chaput 2012;Miller et al 2012;Mills et al 2013;Lacroix 2014). However, high mortality during migration through freshwater and estuarine corridors (Thorstad et al 2012b;Hayes and Kocik 2014) also likely contributes to reduced population sizes (Parrish et al 1998).…”

“…Estimates of marine survival also often include estuarine mortality due to the difficulty in separating these processes (Friedland 1998). A better understanding of factors that influence estuary mortality could help to enhance the management of Atlantic Salmon stocks in the face of changing ocean climates (Mills et al 2013). Despite the importance of estuarine habitats during migration, little is known about Atlantic Salmon smolt behavior and survival in North American estuaries compared with in freshwater and nearshore marine environments (Weitkamp et al 2014).…”

Impacts of Intraguild Predation on Arctic Copepod Communities

“…Consequently, many small visual predators such as fish larvae and juveniles, would reap a larger energetic reward for a similar harvesting effort, making these Arctic copepod species the preys of choice. Hence, changes in the assemblage of copepods communities could impact marine predators' recruitment, resulting in a form of bottomup control (Mills et al, 2013;Greene and Pershing, 2014). For example, along the West coast of the Spitsbergen island in the Svalbard archipelago, the increase of warm Atlantic water masses that bring along abundant C. finmarchicus may have a negative impact on the reproductive success of little auks by reducing the relative abundance of its preferred prey, C. glacialis (Kwasniewski et al, 2010).…”