Indigenous sea gardens within the Pacific Northwest generate partial trophic niche and dietary fatty acid shifts in littleneck clams (Leukoma staminea)

Abstract

Pacific Northwest Indigenous communities historically managed terrestrial and marine environments to increase the productivity and access of traditional foods. Sea gardens, also called clam gardens, were an Indigenous aquaculture method that increased traditional food habitat for targeted food species such as bivalves. This study utilized fatty acid biomarker analysis alongside stable isotopic analysis with bivalve condition indices of traditionally harvested littleneck clams (Leukoma staminea) to investigate dietary and trophic differences between sea gardens and non-walled beaches in Kanish Bay, Quadra Island, British Columbia. We utilized non-metric multidimensional scaling, distance-based redundancy analysis (db-RDA), and Bayesian three-source isotopic mixing models to examine impacts of environmental drivers on bivalve dietary composition and condition at four sea garden sites and four non-walled sites. Bivalve tissue saturated fatty acids (sum SFAs) showed statistical significance (Mann-Whitney U-Test, W = 3447, P ≤ 0.01), indicating that sea garden clams (x̅ = 34.96; SD = 7.56) had higher mean relative percent areas of SFAs compared to non-walled sites (x̅ = 31.76; 7.71). Bivalve monounsaturated fatty acids (MUFAs) showed statistical significance (Mann-Whitney U-Test, W = 1202, P ≤ 0.01, a = 0.05), indicating sea garden clams (x̅ = 3.48; SD = 3.78) had lower mean relative percent areas of MUFAs compared to those in non-walled sites (x̅ = 6.97; SD = 5.90). Sea garden bivalves were found to feed at a higher trophic level, with a slimmer dietary range, compared to non-walled clams. Both dietary marker differences and differentiated trophic positioning suggest decreased physiological stressors impacting feeding of bivalves, differences in dietary particulate intake, or a combination of both occurring at sea garden sites compared to non-walled sites. Correlations within a db-RDA indicate that bivalves with high body condition indices at sea garden sites were benefiting from decreased stress related to favorable salinity and oxygen parameters, which positively impacted bivalve feeding and respiration. Observed trophic niche differences and fatty acid differences would also be explained by this suggested mechanism. We conclude that partial trophic shifts and fatty acid changes of sea garden L. staminea bivalves are consistent with stress-alleviated bivalve responses, and match correlated variables in our db-RDA. Trophic niche differences and fatty acid shifts between sea gardens and non-walled sites should be further explored by using a quantitative method to identify sea gardens from non-walled sites in addition to established field verification methods.