The Decline of the Steller Sea Lion in the Northeast Pacific: Demography, Harvest or Environment?

Pascual, Miguel; Adkison, Milo D.

doi:10.2307/1941942

Cited by 69 publications

(52 citation statements)

References 17 publications

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“…Juvenile sea lions were captured throughout SE Alaska and tagged to monitor their movement. Juveniles were tagged because chronically low juvenile survival likely was a primary driver in the decline of the western stock (York 1994, Pascual & Adkison 1994 and no method for tagging adults is considered safe enough for permit approval. Although these tagging results apply only to juveniles, studies of the ontogeny of dive performance show that by 1.5 yr of age, juveniles are capable of sustained periods of long dives.…”

Section: Methodsmentioning

confidence: 99%

“…A diet dominated by less nutritious prey may have contributed to the decline of the western stock (junk food hypothesis) (Alverson 1992, Trites & Donnelly 2003 and has also been implicated in the population dynamics of other top marine predators (Österblom et al 2008). Chronically low juvenile survival was likely a primary driver in the Steller sea lion decline (York 1994, Pascual & Adkison 1994. Adults can meet their daily energy needs eating exclusively low energy prey, but young sea lions cannot and must obtain milk for an extra 1 or 2 yr at the expense of mothers' reproductive rates (Rosen & Trites 2004, Kumagai et al 2006.…”

Section: Introductionmentioning

confidence: 99%

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Steller sea lion foraging response to seasonal changes in prey availability

Sigler¹,

Tollit²,

Vollenweider³

et al. 2009

Mar. Ecol. Prog. Ser.

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We hypothesized that: (1) Steller sea lion Eumetopias jubatus diet choice is a function of prey availability, (2) sea lions move to take advantage of times and locations of seasonal prey concentrations and (3) the number present depends on the amount of prey available (numerical response). Over 3 yr, typically on a quarterly basis, in Frederick Sound, SE Alaska, multiple measurements were taken of Steller sea lion abundance (aerial surveys), diet (scats), dive behavior (satellite telemetry) and prey availability and caloric density (nearshore, pelagic and demersal fish surveys). We found that Steller sea lions shifted diet composition in response to changes in prey availability of pollock Theragra chalcogramma, hake Merluccius productus, herring Clupea pallasi and salmon Oncorhynchus spp. They selected intermediate-sized fish and avoided small (<10 cm) and large (> 60 cm) fish, and moved between areas as prey became available seasonally. The number of sea lions present depended on the amount of prey available; a standing biomass of 500 to 1700 t of prey in a nonbreeding area such as Frederick Sound, depending on species composition, can attract and sustain about 500 sea lions. Pollock was more frequent in sea lion diet in inside waters of SE Alaska -including Frederick Sound, Stephens Passage and Lynn Canal -than anywhere else in Alaska and contributed ~1⁄ 3 of the dietary energy in Frederick Sound. This finding implies that a diet with substantial year-round contributions from less nutritious, but abundant prey such as pollock can form part of a healthy diet as long as more nutritious prey such as herring, salmon or eulachon Thaleichthys pacificus also are consumed. Our study supports the conclusion that the Steller sea lion is an opportunistic marine predator with a flexible foraging strategy that selects abundant, accessible prey and shifts among seasonally available species.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Steller sea lion foraging response to seasonal changes in prey availability

Sigler¹,

Tollit²,

Vollenweider³

et al. 2009

Mar. Ecol. Prog. Ser.

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“…Since the late 1970s, rookeries within the western DPS of Steller sea lions have had different rates of population decline or increase, indicating that conditions may be variable among regions (Pascual and Adkinson 1994, York et al 1996, Sinclair and Zeppelin 2002, Winship and Trites 2003, Call and Loughlin 2005, Fay and Punt 2006. Nutritional stress, resulting from changes in composition, distribution, abundance, or quality of prey due to changes in environmental conditions, is one of many hypotheses for explaining the decline of the western DPS (Braham et al 1980, Merrick et al 1987, Loughlin 1998, Loughlin and York 2000.…”

Section: Introductionmentioning

confidence: 99%

Regional differences in the spatial and temporal heterogeneity of oceanographic habitat used by Steller sea lions

Lander

Loughlin

Logsdon

et al. 2009

Ecological Applications

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Abstract. Over the past three decades, the decline and altered spatial distribution of the western stock of Steller sea lions (Eumetopias jubatus) in Alaska have been attributed to changes in the distribution or abundance of their prey due to the cumulative effects of fisheries and environmental perturbations. During this period, dietary prey occurrence and diet diversity were related to population decline within metapopulation regions of the western stock of Steller sea lions, suggesting that environmental conditions may be variable among regions. The objective of this study, therefore, was to examine regional differences in the spatial and temporal heterogeneity of oceanographic habitat used by Steller sea lions within the context of recent measures of diet diversity and population trajectories. Habitat use was assessed by deploying satellite-depth recorders and satellite relay data loggers on juvenile Steller sea lions (n ¼ 45) over a five-year period (2000)(2001)(2002)(2003)(2004) within four regions of the western stock, including the western, central, and eastern Aleutian Islands, and central Gulf of Alaska. Areas used by sea lions during summer months (June, July, and August) were demarcated using satellite telemetry data and characterized by environmental variables (sea surface temperature [SST] and chlorophyll a [chl a]), which possibly serve as proxies for environmental processes or prey. Spatial patterns of SST diversity and Steller sea lion population trends among regions were fairly consistent with trends reported for diet studies, possibly indicating a link between environmental diversity, prey diversity, and distribution or abundance of Steller sea lions. Overall, maximum spatial heterogeneity coupled with minimal temporal variability of SST appeared to be beneficial for Steller sea lions. In contrast, these patterns were not consistent for chl a, and there appeared to be an ecological threshold. Understanding how Steller sea lions respond to measures of environmental heterogeneity will ultimately be useful for implementing ecosystem management approaches and developing additional conservation strategies.

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“…For instance, conservation ecologists tend to concentrate on demographic structure (e.g. Mertz, 1971;Crouse et al, 1987;Pascual and Adkison, 1994) since population growth or decline is of primary concern, and is ultimately determined by demographic parameters. Managers of harvested populations, such as fish or deer, also tend to focus on demography and devise management strategies that are based on parameters such as such as sex, size, or age (e.g.…”

Section: Introductionmentioning

confidence: 99%

Metapopulation dynamics of the softshell clam, Mya arenaria

Strasser¹

2008

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The Decline of the Steller Sea Lion in the Northeast Pacific: Demography, Harvest or Environment?

Cited by 69 publications

References 17 publications

Steller sea lion foraging response to seasonal changes in prey availability

Steller sea lion foraging response to seasonal changes in prey availability

Regional differences in the spatial and temporal heterogeneity of oceanographic habitat used by Steller sea lions

Metapopulation dynamics of the softshell clam, Mya arenaria

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