Under legislative mandate, Washington Sea Grant undertook a large-scale, six-year multidisciplinary study of the ecological and biochemical effects of geoduck planting and harvest. Washington Sea Grant-supported researchers conducted 15 surveys of adjacent farmed and unfarmed areas, comparing sediment, eelgrass, and faunal characteristics at various stages in the growth and culture cycles.
Puget Sound’s eelgrass provides many ecological services: anchoring and enriching substrates, removing acidifying carbon dioxide, and feeding and sheltering a wide range of fauna. Washington State considers it critical habitat and permits no shellfish culture in established meadows. But eelgrass and aquaculture inevitably interact, particularly when eelgrass colonizes geoduck beds. An instance of such colonization in Samish Bay offered a large-scale natural experiment in geoduck aquaculture’s previously unstudied impacts.
Results show evidence that geoduck harvest decreases sediment elevation within the farm, with recovery on the order of one year. Harvest activity and the biofouling of predator-exclusion nets led to the elimination eelgrass from the geoduck farm. Installation of PVC tubes to protect young geoducks increased local scour, reduced sediment elevation, and increased the rate of mortality for eelgrass seedlings. In all, results suggest that geoduck aquaculture decreases organic content of sediment within the farm; sediment loss was greater around the PVC tubes, possibly suggesting increased scouring. Eelgrass began recolonizing the farm one year after net removal. Two years after net removal, farm sediment showed higher organic content than unfarmed sediment.
Findings are informing policies for protecting eelgrass habitat and regulating shellfish cultivation. The results will help the U.S. Army Corp of Engineers and Washington Department of Natural Resources in establishing buffer zones for eelgrass meadows.
For more information, see the Geoduck Aquaculture Research Program final report.