Ecologists bring to the clam garden discussion the knowledge of how well-designed ecological surveys, manipulative experiments, and ecosystem-thinking can illuminate by how much and by what means clam gardens magnify clam production and support clam population persistence.
Our ecological surveys and experiments on clam gardens in British Columbia show that the shallow sloping beach terraces of clam gardens can double to quadruple clam biomass by extending clam habitat at the intertidal height where clams grow and survive best. In fact, transplant experiments showed that juvenile little neck clams grow 1.7 times faster and are more likely to survive in clam gardens than non-walled beaches. Furthermore, clam gardens tend to have greater densities of small clams (1 – 4 cm), including newly settled clam recruits (< 2 mm). Clam gardens on Quadra Island, for example, contain four times as many butter clams and over twice as many littleneck clams compared to non-walled beaches. The Hul’q'umi’num Treaty Group found that the greater abundance of newly settled recruits is a detectable ecological effect that reflects Indigenous knowledge of larval clams being more likely to settle in clam gardens.
Our experiments also demonstrate that the increase in clam biomass in clam gardens can be attributed to elevated sediment carbonate associated with dense accumulation of shell hash and the moderating effect of shallow sloping clam garden terraces on temperature. Natasha Salter’s research showed that clam gardens maintain lower mean surface temperatures in the summer and higher minimum surface temperatures in the winter than unmodified beaches and tended to experience greater water flow.
While much of the Network’s focus so far has been on clams in the clam garden terraces (mainly butter clam, littleneck, horse clam, and cockles), our observations and that of our First Nations collaborators suggest that the boulder walls themselves create productive rocky reef habitat for octopus, sea cucumber, whelks, chiton, red turban snails and other critters. Many of these are valued foods for coastal First Nations. How these walls compare to natural rocky reef habitat is one of many questions remaining about the use and design of clam garden features. Perhaps as we learn more, we’ll find that there’s much more to a “clam garden” than just clams, and that a more appropriate term for these places may simply be “sea garden.”
To learn more about clam garden ecology, visit:
Groesbeck et al. (2014). Ancient clam gardens increased shellfish production
Jackley et al. (2016). Ancient clam gardens, traditional management portfolios
Lepofsky et al. (2020). Ancient anthropogenic clam gardens of the Northwest Coast
Juvenile littleneck clams are measured, weighed, and tagged with numbered vinyl tags before transplanting.
The clams are then transplanted in experimental plots with and without the addition of shell hash here by Laura Gray. Clams are left to grow for the spring and summer growing season, under different experimental treatments and at different intertidal heights in clam gardens and nonwalled beaches and are then remeasured.
Desiree Lawson sampling a clam garden on BC's Central Coast.