Sometimes when we dig in the clam garden terrace, we'll find layers of dead clams below the more recent sediment that fills the clam garden. This "death assemblage" of clams provides paleoecological information on clam growth prior to terrace building; these data can in turn be compared to growth rates of clams from middens that were harvested from active clam gardens and compared to clams today.
The collective work on Quadra Island provides the most complete data set for ecological and social changes in clam harvesting and cultivation through time. There, excavations into very early Holocene beach deposits (11,500 – 11,000 years ago) revealed butter clams that were small and slow-growing, likely because of the colder ocean temperatures and the silt-dominated sediments of the newly forming beaches during the early postglacial time period. As ocean temperatures and beaches stabilized through to mid-Holocene (~4000 years ago), clams grew faster and were larger in size at death than previously.
Louie Wilson, Christine Roberts, and Nicole Smith excavating a terrace of a clam garden to find “death assemblages” of shells.
Measurements of clams in middens suggest that despite growing human populations and presumably increased human harvest rates on clams form the mid to late Holocene, clam size and abundance were relatively stable. Furthermore, clam measurements indicate these cultivated clams were generally growing at least as fast as clams in the best, unwalled beaches of the mid-Holocene prior to the development of clam garden technology. This pattern supports the hypothesis that clam gardens and the maricultural systems within which they were embedded were sustainable over the long-term, even despite fluctuations in sea surface and air temperatures characteristic of millennial-scale climate dynamics.
In fact, it was not until the post-colonial era – with the cessation of traditional management practices, siltation of beaches from upslope logging, warming of ocean temperatures and increasingly corrosive sea water– that clam size and growth mimic those from the unstable environmental conditions of the early Holocene. Notably, the same pattern of temporally stable clam sizes in the pre-colonial era holds in other locations with clam gardens just north in the Broughton Island archipelago, even where sea otter predation of clams likely competed with humans harvesting these same bivalves.
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A saw is used to cut ancient clams (L) to reveal its growth rings (R). Counting and measuring these rings provides information on how fast a clam grew in the past and how old it was when it died. The more favorable the growing conditions, the faster a clam will grow and the bigger it will be when it dies.