California mussels (Mytilus californianus) excavated from archaeological site CA-SRI-147 on the southern side of Santa Rosa Island, California, provided a discontinuous environmental record spanning about 7000 years. An earlier report of the midden remains at this site found a weak inverse correlation between changes in mussel shell length and sea surface temperature (SST), but not productivity, and a decline in mussel size from ~5800 to 5100 calendar years before present during a period of low productivity and increasing SST. This study uses oxygen isotopes from the site to directly compare environmental change to shifts in faunal assemblage. Four intervals of low temperatures are revealed in the nearshore environmental data, each corresponding to a significant change in faunal assemblage at the site. The first occurs c. 6600 calendar years before present (cal. BP) and coincides with the greatest amount of mussel shell mass and size in the site. The second (c. 5500 cal. BP) occurs during an interval of variable mussel contribution and increasing red abalone (Haliotis rufescens) contribution. The third (c. 4800 cal. BP) occurs during a time when platform mussels (Septifer bifurcatus) exceed California mussels in meat weight, while the final interval (c. 3700 cal. BP) occurs prior to a period of probable site hiatus. The overall pattern of decreasing shell contribution and size at the site is not found in the mollusk isotope data, suggesting that environmental change had a profound impact on short-term variability, while human predation of key resources provided the long-term primary stress on nearshore biota.
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