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NOAA Technical Memorandum NMFS-AFSC-248

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Decadal variation in the diet of Western Stock Steller sea lions (Eumetopias jubatus)


Steller sea lions (Eumetopias jubatus) are listed as an endangered species in western Alaska due to a precipitous decline that occurred in the 1980s and 1990s. In 2000, cascading declines slowed or ceased and clusters of rookeries between the eastern Aleutian Islands and eastern Gulf of Alaska began to show signs of population growth. Reasons for the decline and for a range-wide failure to recover are unresolved, but reduction in the availability of prey due to commercial fishing or environmental perturbation has been hypothesized.

Discerning the diet and patterns of prey use by Steller sea lions (SSL) is fundamental to isolating the mechanisms driving population health. Here we evaluate the frequency of occurrence (FO) of prey species in 3,412 scats of adult female and juvenile SSL collected during 1999-2009, across the range of the U. S. Western Stock. Thirteen primary prey are identified based on their occurrence in ≥ 5% of total scats. We reduce the dimension of the diet profile of the 13 primary prey to two categorical groups through principal component analysis (PC). A hierarchical cluster analysis of PC scores on collection site locations describes four geographic regions of SSL diet (with Amak Island as an outlier) nearly identical to those identified in a previously published 1990-1998 (n = 3,762) dataset. Geographic regions of diet continue to correspond with regional population trends of SSL.

The species of primary prey consumed by SSL are analogous between 1990-1998 and 1999-2009. However, the rangewide FO of 7 of the 13 primary prey increased significantly (p ≤ 0.05) during the latter decade. Only cephalopods (Gonatidae) and walleye pollock (Theragra chalcogramma) decreased significantly in FO in any season or region between decades. Generalized linear mixed models (GLMM) of seasonal prey FO determined that trends in the FO of primary prey between decades were locally driven by significant changes within one or more of the four diet regions and fishery conservation management areas (RCA). The most significant increases (p = 0.001) in FO during 1999-2009 were for commercial fishes: arrowtooth flounder (Atheresthes sp.), Atka mackerel (Pleurogrammus monopterygius), rock sole (Lepidopsetta sp.), and Pacific cod (Gadus macrocephalus); and one non-commercial fish species (Pacific sand lance, Ammodytes hexapterus) in diet Regions 2 and 3 (RCA zones 6 and 7) between the eastern Aleutian Islands and western Gulf of Alaska. Diversity scores (H') for primary prey were also highest in these regions compared to the previous decade, and are coincident with SSL population increases that began in the same area in 2000.

Atka mackerel continues to dominate SSL diet west of Samalga Pass, and walleye pollock continues to dominate SSL diet east of Samalga Pass (despite its decreased presence in diets in Regions 1-3 during 1999-2009). The results of generalized additive mixed models (GAMM) of annual trends (1990-2009) in summer prey FO on each rookery across the study area are consistent with the results of GLMM decadal comparisons.

The additional 11 years of data presented here support earlier conclusions that adult female and young juvenile SSL of the Western Stock collectively eat a wide variety of prey species, but demonstrate fidelity to prey types that are predictably available in seasonal concentrations over the continental shelf or other bathymetric structures, within close range of natal rookery sites. Foraging within close proximity to birth and breeding sites keeps adult females close to pups onshore and to young juveniles learning to forage. However, it also increases their vulnerability to potential nearshore environmental and anthropogenic interference which could ultimately influence their reproductive success.

It was not the objective of this study to define the relationship between SSL diet and population decline, or to identify the interactive mechanisms that drive diet change. However, coincident patterns emerged that suggest relationships between SSL diet, regional population patterns, climate and fisheries. Some of the patterns are worthy of discussion and future research: 1) the areas of greatest increases in the FO and diversity of prey (Regions 2 and 3) beginning in 1999, overlap with those of the strongest population growth since 1999; 2) the increase in primary prey FO and diversity since 1999 is coincident with increased restrictions on groundfish trawling within SSL Critical Habitat, enacted in 2000; and 3) the area of lowest prey diversity (Region 4) overlaps with those areas of continuing population declines, the most restricted foraging habitat (narrow continental shelf) and the lowest seasonal and temporal variability in sea surface temperature in all years of study.

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