Interestingly, site deposits dating earlier than about 6,300 years ago or later than 5,300 years ago lack red abalone shells, or they are present very rarely.  Instead, black abalone (Haliotis cracherodii) shells are present, along with lots of California mussel shells, in the deposits at these sites.  Black abalone shells are smaller in size, and historically black abalone was the only abalone species seen within the intertidal zone around the island.  This temporal pattern led me to propose in 1993 that red abalone middens were deposited at a time when ocean water temperature around the island was cooler than before or after.  The idea that prevalence of red abalone shells in midden deposits was indicative of cooler water temperatures originally was proposed by ichthyologist Carl Hubbs for a period around 8,000-7,500 years ago, the date of a red abalone midden on neighboring Santa Rosa Island.  Red abalones living in the intertidal zone prefer waters cooler than today’s around the Channel Islands, and historically red abalone was increasingly more prevalent in the intertidal zone northward along the California coast, correlating with decreasing ocean water temperature.  My proposal gained support from a study that my colleagues and I undertook of oxygen isotopes of mussel shells obtained from the same midden strata in which red abalone shells are abundant.  The results indicated that water temperatures at 5,700-5,300 years ago were approximately 2.5°C cooler than modern water temperature in the Santa Barbara Channel.  (An ongoing re-analysis of the data indicates the difference probably is less than this.)

Red abalone middens also exist on Santa Rosa and San Miguel Islands, located west of Santa Cruz Island.  They are especially prevalent on San Miguel Island.  In general, they date within a broader bracket of time on these islands, and the density of red abalone shells typically is higher.  This pattern correlates with a gradient of decreasing ocean water temperature from east to west along the chain of the northern Channel Islands.

Although cool water temperature and increased availability in the intertidal zone is a major reason why red abalones were being collected between 6,300 and 5,300 years ago, other determinants undoubtedly were operable too.  First, the size of the population living on Santa Cruz Island at this time probably was quite low, perhaps no more than 100 or 200 people.  Consequently, they were not placing much predation pressure on red abalone, and it could be a significant component of their diet.  Second, there is some evidence that island occupants were diving to obtain at least some of their red abalones, and diving in shallow water immediately offshore would have expanded the amount of red abalone available to them.  Third, at these early times fishing technology was not as developed as it became a few thousand years later, so shellfish were an important contributor of protein to the islanders’ diet.  (Santa Cruz Island has few terrestrial mammals, and none is large or abundant enough to have served as an important protein source).


A red abalone midden within a deflating sand dune


mussel bed

A mussel bed exposed at low tide. Red and black abalone presumably also would have been presentabout 5,500 years ago.