Institute of Marine Science, University of Alaska Fairbanks, 99775, Fairbanks, AK, USA.
Oecologia. 2001 Dec;129(4):591-601. doi: 10.1007/s004420100756. Epub 2001 Aug 4.
Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon ((13)C/(12)C) and nitrogen ((15)N/(14)N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951-1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in δ(15)N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen δ(15)N was found for any of the three species during the past 47 years in either the Bering Sea or the Gulf of Alaska. However, the (15)N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in δ(13)C (almost 2 ‰ over the 47 years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in δ(13)C over time with no accompanying change in δ(15)N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in δ(13)C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.
在 20 世纪后半叶，阿拉斯加水域的海狮和海豹数量经历了不同程度的下降，而下降的原因仍不确定。为了验证在这一期间，营养级是否发生变化并导致了种群数量下降的假设，我们测量了来自白令海和阿拉斯加湾的野生北太平洋巨型虎头海狮（Eumetopias jubatus）、北方海狗（Callorhinus ursinus）和港海豹（Phoca vitulina）的骨骼胶原中的稳定碳（13C/12C）和氮（15N/14N）同位素比值。1951 年至 1997 年的数据表明，如果肉食动物的营养级发生显著变化，海豹组织中的 δ15N 值会发生显著变化。在过去的 47 年中，无论是在白令海还是在阿拉斯加湾，三种海豹的骨骼胶原中的 δ15N 值都没有发生显著变化。然而，与北方海狗和港海豹相比，北太平洋巨型虎头海狮的胶原中的 15N 明显更高。北太平洋巨型虎头海狮的 δ13C 显著下降（47 年内下降了近 2 ‰），而港海豹和北方海狗的 δ13C 则呈下降趋势，但不显著。觅食地点的变化，加上营养级的变化，可能提供了一种可能的解释。然而，随着时间的推移，δ13C 减少而 δ15N 没有变化，这表明是由于食物网基础发生了环境变化，而不是由于猎物转换导致的营养级变化。该地区季节性初级生产力的下降可能是由于浮游植物生长速度的降低，其表现为 δ13C 的下降。生产力的下降可能表明北太平洋的承载能力下降。对于这些鳍足类动物，尤其是尚未发展出足够觅食技能的幼年期和亚成年个体，最佳猎物的数量可能已经降至维持其种群密度的阈值以下。
