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Sustainable Development: Taking the guesswork out of shellfish aquaculture planning

By Eric Scigliano

Everyone knows that growing shellfish is big business in Washington: it produces more cultured oysters, mussels and clams than any other state. However, while the cultural and economic significance of shellfish aquaculture in Washington is well recognized, its future in the face of population and environmental change is much less certain.

To guide and plan for this uncertain future, Washington’s legislature directed Washington Sea Grant (WSG) in 2013 to develop tools for finding answers. WSG staff joined forces with University of Washington (UW) and NOAA Fisheries scientists to produce Shellfish Aquaculture in Washington Statea multifaceted exploration of the geographic, economic and ecological dimensions of shellfish farming and a potential model for multidisciplinary analyses of resource industries elsewhere.

Of the five component studies, perhaps the most surprising was led by WSG researcher Bridget Ferriss. Her team updated an existing quantitative ecosystem model with information on geoduck farming in the Central Sound. The model suggests that, “at a basin scale, the food web could support a substantial increase in geoduck aquaculture over current production levels, with only minor changes in the biomass of individual species.” This is important news for communities concerned that geoduck farming harms the surrounding ecosystem.

While the impacts of the farmed geoduck themselves are minimal, the findings indicate that aquaculture can have significant habitat effects. Previous models used to gauge ecological impacts largely neglected non-trophic effects. The team corrected this deficiency and through the research they found that the sunken plastic tubes and netting used to protect young geoduck from predators may affect other species. Effects are positive for some species by providing new foraging opportunities and protection from predators and negative for others like seabirds.

The same researchers also explored use of qualitative network models — which require much less data than quantitative measures — to inform ecosystem approaches to shellfish aquaculture. They demonstrated the versatility of their models in two venues. In South Puget Sound, they modeled the effects of growing larger Pacific oyster, Manila clam and geoduck crops; of removing predatory moon snails and sea stars; of heavier nutrient loads; and of various combinations of these influences. In Willapa Bay, they evaluated the potential effects of various ocean acidification scenarios on Pacific oysters and Manila clams.

Kevin Decker, WSG’s economist and outreach specialist, analyzed the economics of shellfish farming in Washington and found that it can be quite profitable: one pound of shellfish earns on average $4.75 in revenue and $1.08 in profit, which translates to $510 profit per acre of tideland cultivated. (This compares rather favorably to, say, growing corn in Illinois, which by one estimate, yields $300 profit per acre exclusive of land costs.)

These returns vary greatly by location, market conditions and species. From 2009 through 2013 average prices varied from just 59 cents per pound for cockles and 32 cents for softshell clams (which are common but underappreciated in Washington) to $1.75 for mussels, $13.37 for geoduck, and $25.35 for the prized but pricey Kumamoto oyster. On South Puget Sound, the increasing reliance on geoduck, whose prices are volatile, exposes growers to more risk.

Such calculations don’t consider the ecosystem services that shellfish provide to everyone, from cleaner water to richly structured habitat. Oyster reefs can provide as much as 50 times more surface area as plain substrate.

In the report, researchers also developed a framework for using “geographic information systems” (GIS) to site shellfish farms. GIS had previously been applied largely to spatial planning for finfish farming, not shellfish. The researchers identified the data layers — physical, ecological, commercial and social (including scenic) — to be weighed in assessing a site’s suitability for shellfish.

To better understand the relationships between shellfish farming, phytoplankton levels and water quality, UW researchers developed a high-resolution circulation model of South Puget Sound. The results show wide differences in residence time — how long it takes water to move through a system — between deep, fast-flushing central channels and shallow, isolated inlets. Long residence times and high densities of filter-feeding bivalves in those inlets suggest that aquaculture there may control phytoplankton and improve water quality.

Together, these analyses provide a new and powerful set of tools for shellfish growers, resource managers and residents seeking to evaluate aquaculture practices and proposals to expand Washington’s shellfish industry.

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