Bottom‐up forcing and the decline of Steller sea lions (Eumetopias jubatus) in Alaska: assessing the ocean climate hypothesis

Abstract

Declines of Steller sea lion (Eumetopias jubatus) populations in the Aleutian Islands and Gulf of Alaska could be a consequence of physical oceanographic changes associated with the 1976-77 climate regime shift. Changes in ocean climate are hypothesized to have affected the quantity, quality, and accessibility of prey, which in turn may have affected the rates of birth and death of sea lions. Recent studies of the spatial and temporal variations in the ocean climate system of the North Pacific support this hypothesis. Ocean climate changes appear to have created adaptive opportunities for various species that are preyed upon by Steller sea lions at mid-trophic levels. The east-west asymmetry of the oceanic response to climate forcing after 1976-77 is consistent with both the temporal aspect (populations decreased after the late 1970s) and the spatial aspect of the decline (western, but not eastern, sea lion populations decreased). These broad-scale climate variations appear to be modulated by regionally sensitive biogeographic structures along the Aleutian Islands and Gulf of Alaska, which include a transition point from coastal to open-ocean conditions at Samalga Pass westward along the Aleutian Islands. These transition points delineate distinct clusterings of different combinations of prey species, which are in turn correlated with differential population sizes and trajectories of Steller sea lions. Archaeological records spanning 4000 yr further indicate that sea lion populations have experienced major shifts in abundance in the past. Shifts in ocean climate are the most parsimonious underlying explanation for the broad suite of ecosystem changes that have been observed in the North Pacific Ocean in recent decades.

Cohort effects and spatial variation in age-specific survival of Steller sea lions from southeastern Alaska

“…The role of prey dynamics in population change is a central issue to large mammal population dynamics, and a focus of Steller sea lion research (National Research Council 2003). The population decline coincided with the 1976/77 oceanic regime shift in the North Pacific and with population declines in several other top predator species, suggesting a link between population dynamics and prey conditions (reviewed by Trites et al 2007). Notes: All models fit are shown.…”

Quantifying long-term risks to sea otters from the 1989 ‘Exxon Valdez’ oil spill: Comment on Bodkin et al. (2012)

“…In particular, climate regime shifts over the past few decades have caused high variability in abundance of forage fish and many marine mammals and sea birds that depend on them (e.g. Trites et al 2007, Overland et al 2008, Estes et al 2009). The relative importance of natural variability only increases over time, as natural variability remains unabated while the signal of the effects from the oil spill continuously diminishes.…”

Australian sea lions Neophoca cinerea at colonies in South Australia: distribution and abundance, 2004 to 2008

“…Over recent decades there has been growing concern over the status of sea lion species. In the North Pacific Ocean, the Steller sea lion Eumetopias jubatus has been declared Endangered in parts of its range and is considered threatened with extinction in other parts (Trites et al 2007). Although the total population of California sea lions Zalophus californianus in California (USA) and Mexico is increasing (Carretta et al 2004), the Mexican stock is declining (Szteren et al 2006).…”