Archaeologists working on the northern Channel Islands of California have proposed that during the Late Holocene, foraging decisions of the islands' prehistoric occupants were progressively less affected by environmental changes as increasing human exploitation pressure shaped abundance and size of intertidal shellfish species. This proposal, although supported by archaeological data from habitation sites, does not incorporate past small-scale ecological and oceanographic variability around the islands nor the effect of this variability on Mytilus californianus (California mussel), the main intertidal shellfish species exploited through prehistory. Consequently, the lessening influence of environmental conditions on shellfish harvesting during the Late Holocene has not been definitively demonstrated.

This study evaluates this proposal through the study of archeological, ecological and stable isotopic data from two shell midden sites, SCRI-195 and SCRI-496, on the coast of Santa Cruz island, both occupied during the Late Holocene (2200-500 cal B.P.). The correlation between changes in abundance and size of M. californianus and past sea surface temperature (SST) through the chronological sequences at each archaeological site was statistically evaluated, regional and local SST records spanning the Late Holocene were compared, and modern data concerning ecological and oceanographic factors affecting M. californianus productivity around Santa Cruz island were used to interpret past natural conditions of this shellfish species. The results of this study show that fluctuation in regional and local past SST does not explain variation in abundance or length of collected M. californianus through the occupation of SCRI-195 and SCRI-496.

Instead, the factors determining this variation are the intensity of human occupation at the archaeological sites, together with past productivity of M. californianus beds near the human settlements. Differing intensities of human occupation at SCRI-195 and SCRI-496 defined harvesting pressure on M. californianus beds, and the frequency and intensity of upwelling conditions (cooler SST) influenced past productivity of M. californianus during the Late Holocene at each site location. This study reached the conclusion that the factors of site occupation dynamics and upwelling occurrence together influenced human foraging on M. californianus during the Late Holocene around Santa Cruz Island and explain variation in archaeological shellfish assemblages.