Ecosystem services of geoduck farming in South Puget Sound, USA: a modeling analysis

Provisioning and regulatory ecosystem services of Pacific geoduck clam (Panopea generosa) culture were simulated for an intertidal shellfish farm in Eld Inlet, South Puget Sound, Washington, USA. An individual geoduck clam growth model was developed, based on a well-established framework for modeling bivalve growth and environmental effects (AquaShell™). Geoduck growth performance was then validated and calibrated for the commercial farm. The individual model was incorporated into the Farm Aquaculture Resource Management (FARM) model to simulate the production cycle, economic performance, and environmental effects of intertidal geoduck culture. Both the individual and farm-scale models were implemented using object-oriented programming. The FARM model was then used to evaluate the test farm with respect to its role in reducing eutrophication symptoms, by applying the Assessment of Estuarine Trophic Status (ASSETS) model. Farm production of 17.3 t of clams per 5-year culture cycle is well reproduced by the model (14.4 t). At the current culture density of 21 ind m⁻², geoduck farming at the Eld Inlet farm (area: 2684 m²) provides an annual ecosystem service corresponding to 45 Population-Equivalents (PEQ, i.e. loading from humans or equivalent loading from agriculture or industry) in top-down control of eutrophication symptoms. This represents a potential nutrient-credit trading value of over USD 1850 per year, which would add 1.48% to the annual profit (USD 124,900) from the clam sales (i.e. the provisioning service). A scaling exercise applied to the whole of Puget Sound estimated that cultured geoducks provide a significant ecosystem service, of the order of 11,462 PEQ per year (about USD 470,600) in removing primary symptoms of eutrophication, at the level of the whole water body. The modeling tools applied in this study can be used to address both the positive and negative externalities/impacts of shellfish aquaculture practices in the ecosystem and thus the trade-offs of the activity.