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Abstract STABLE ISOTOPE RATIOS OF STELLER SEA LIONS AS EVIDENCE OF ENVIRONMENTAL CHANGE. Amy C. Hirons1, Bruce P. Finney2 and Charles W.
Potter3 Various levels of population decline in Steller sea lions (Eumetopias
jubatus) have been observed around the North Pacific Ocean
and Bering Sea in recent years. Stable carbon (13C/12C)
and nitrogen (15N/14N) isotope ratios in bone
collagen of Steller sea lions from the Bering Sea and Gulf of Alaska
during the 19th and 20th centuries indicate a combination of factors
may have contributed to the declines. The δ13C show a significant
decline of nearly 2‰ during the 172-year period while an increase,
though not significant, was observed in the δ15N. Periods of high
and low variability in both δ13C and δ15N occurred simultaneous
throughout the 172-year period. No regional differences were detected
in the sea lions’ isotopic values and all regions showed similar
levels of variability. Even longer isotope records, spanning much
of the Holocene, show alternating periods of high and then low productivity
reflected in the sea lion bones from the region. The long-term decline
in δ13C, while δ15N increases, suggests that declining primary production
has caused a subsequent increase in the length of the food web.
Declines in the seasonal primary production in the region, possibly
resulting from decreased phytoplankton growth rates, would exhibit
itself as a decline in δ13C. The periods of high and low variability
in both isotopes reveal a positive correlation with the Pacific
Decadal Oscillation and Aleutian Low Pressure Index. During warm
periods when productivity is high in Alaskan waters, isotopic variability
in the sea lions is low. When productivity is low during cool periods,
isotopic variability is high. Decreased production has likely caused
the pinnipeds to forage at multiple trophic levels in order to find
sufficient food resources.
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