To meet global demands for seafood, aquaculture has rapidly increased in the last four decades. Yet, aquaculture is an ancient form of food production, and developing systems and practices to enhance yields has been a central challenge for coastal communities. For millennia, Northwest Coast First Nations have sustained shellfish production through the construction and management of soft sediment shoreline modifications known as clam gardens – intertidal rock-walled terraces. Experiments and surveys have revealed that clam gardens are 2 times more productive than unmodified clam beaches. While heightened productivity within clam gardens has been attributed to the reduction in beach slope and expansion of the area of optimal clam habitat, the specific underlying ecological mechanisms have not been identified. To understand the relative effect of multiple abiotic conditions modified by these traditionally managed systems, we conducted clam surveys along beach profiles at clam gardens (n=3) and unmodified beaches (n=3) and collected data on a suite of potentially important environmental variables. To test the effect of the ancient mariculture practice of returning crushed shell to beaches on the growth of clams, we transplanted juvenile littleneck clams (Leukoma staminea) in plots with and without shell hash additions in both clam gardens and unmodified beaches.
Results/Conclusions
As predicted, clam biomass was on average 1.5 times greater in clam gardens than unmodified beaches. Further, seawater temperature had a negative effect on clam biomass (RVI = 0.98). Unmodified beaches reach higher mean temperatures than clam gardens at the top of the beach. Moreover, sediment carbonate had a positive effect on clam biomass (RVI = 0.97) and this effect had the greatest magnitude of effect compared to any other factor. The higher carbonate content of clam garden sediments is associated with a dense accumulation of crushed shell. The results of the transplant experiment indicate that increasing the amount of crushed shell in sediment has a positive but imprecise effect on clam growth rates. This research illuminates the importance of this traditional tending practice on clam production. Ultimately, the results of this research will inform the degree to which this ancient form of mariculture can continue to be used today to enhance food security and confer resilience to impending oceanic changes.