Clam garden bivalve dietary responses and trophic shifts in relation to environment

Clam gardens are a method of Indigenous aquaculture, involving altering of beach gradients, to increase clam habitat that have been used since pre-contact. Ecological examinations of clam gardens have been increasingly led by First Nations groups, as restoration projects begin on clam gardens aiming to re-establish pre-contact conditions. The purpose of this study was to measure condition indices of traditionally-harvested littleneck clams (Leukoma staminea), dietary parameters and beach-level oceanographic conditions were used to investigate differences between clam gardens from non-walled beaches in Kanish Bay, Quadra Island, British Columbia. I utilized non-metric multidimensional scaling, distance-based redundancy analysis, and Bayesian 3-source isotopic mixing models to examine environmental drivers on bivalve dietary composition and condition at four clam garden sites and four non-walled sites. Bivalve FAME proportions differentiated site types indicating increased saturated fatty acids (SFAs) and decreased monounsaturated fatty acids (MUFAs) in clam garden sites compared to non-walled beaches. Stable isotope dietary proportion estimates indicated that site types were not different proportions of food particulates, with a majority of all site diets originating from oceanic food sources (POM). Stable isotope trophic positioning indicated that clam garden L. staminea bivalves feed at a higher at a tropic level with a narrower dietary compared to non-walled clams. This indicates either a major difference in dietary particulate intake, increased physiological stress driving differences between site types, or a combination of both. Correlations in a distanced-based redundancy (db-RDA) analysis indicted that both changes in feeding behavior, increases in primary productivity over clam garden sites, and decreased stress could be driving observed trophic shifts and fatty acid differences. We conclude that clam garden L. staminea bivalve trophic shifts and fatty acid changes are consistent with stress-alleviated bivalve responses, and matched correlated variables in our db-RDA. I theorize that more stable oceanic parameters lead to increased bivalve growth rates in clam gardens, as decreased stressors were correlated with filtration feeding behavior indicated by FAME profile shifts, increased condition indices, and trophic shifts observed in this study.