DETECTING ANCIENT SUBTIDAL CLAM HARVEST BY MICRO-INCREMENT MEASUREMENT

Bivalve mollusks are among the most important foods preserved in the archaeological record, yet knowledge of ancient collection methods is limited by lack of direct artifactual data. Ethnographic accounts document that harvest from intertidal zones was common worldwide, but subtidal harvest was less common. Subtidal collection presents numerous challenges depending on taxa, habitat depth, substrate, water temperature, and available technology. Despite these barriers, some locations (e.g. coastal Peru) have a history of subtidal harvest using specialized implements and/or diving, even in cold water 10’s of meters deep. Reconstruction of ancient subsistence strategies could be improved with a method to discriminate intertidal versus subtidal clam harvest based on archaeological shell. Such data would facilitate addressing questions concerning the origin of diving, invention of collection technology, response to overharvest, and management in natural and mariculture settings. Micro-increment width patterns in certain species of bivalve permit such assessment. We report validation of this method in Saxidomas gigantea (butter clam) from Dutch Harbor (Iliuliuk Bay), Alaska. This species is commonly excavated from middens along the west coast of North America, from the Bering Sea southward to around San Francisco Bay. Previously published research on specimens collected from intertidal habitats confirmed the periodicity of lunar-daily increments and detected a clear fortnightly tidal pattern whereby increment width was linked to tidal amplitude variation over lunar cycles. We compared clams that grew contemporaneously at intertidal and subtidal (~3 m) depths along the same shore. Intertidal shells display increment widths that follow a lunar tidal amplitude pattern, while subtidal clam increments do not. This method was also applied to clams excavated from nearby prehistoric sites, all of which appear to have been collected from intertidal zones. This method may be applicable to other common species, thus may be valuable to coastal archaeologists worldwide. The technique is comparatively quick and requires little more than a saw, polishing grit, a reflecting microscope with camera, and free software, enabling archaeologists to employ the method without major cost or time investment.