Trites, A. W., A. J. Miller, H. D. G. Maschner,
M. A. Alexander, S. J. Bograd, J. A. Calder,
A. Capotondi, K. O. Coyle, E. Di Lorenzo,
B. P. Finney, E. J. Gregr, C. E. Grosch,
S. R. Hare, G. L. Hunt,
J. Jahncke, N. B. Kachel,
H.-J. Kim, C. Ladd, N. J. Mantua, C. Marzban,
W. Maslowski, R. Mendelssohn, D. J. Neilson, S. R. Okkonen, J. E. Overland,
K. L. Reedy-Maschner,
T. C. Royer, F. B. Schwing, J. X. L. Wang and A. J. Winship,
2007:
Bottom-up forcing and the decline of Steller sea lions (Eumetopias jubatus) in Alaska:
Assessing the ocean climate hypothesis
Fisheries Oceanography, 16, 46-67.
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
1970's) 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 years 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.
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