Factors limiting oyster growth in Willapa Bay (Washington, USA) evaluated with in situ feeding experiments

Natural and anthropogenic environmental changes in estuaries affect the growth and health of organisms living there, often along spatiotemporal gradients. Throughout the world's estuaries, aquaculture and wild oyster populations support food and cultural systems, so quantifying factors affecting growth may inspire interventions to prevent future losses of oyster productivity. In Willapa Bay (Washington, USA), an estuary that produces 10–20% of US oysters, oysters are primarily harvested for market from the lower estuary, putatively due to food limitation up-estuary. We present studies in which in situ experimental systems were designed to manipulate food availability in the upper, and in one case contrasting to the lower, estuary. Contrary to expectations, food addition did not improve survival, shell growth rate or tissue mass of post-metamorphosis juvenile Pacific oysters, Crassostrea gigas. Instead, the experiment did not recapitulate the impaired growth up-estuary that typifies on-bottom oyster outplants, and irrespective of food addition, growth rates in the upper estuary were equivalent to rapid summer growth (>10 mm month(−1)) in other regions of the bay. Multiple lines of evidence suggest that elevating oysters above the substrate in the experimental systems removed food limitation caused by reduced clearance rates, thus allowing oysters to grow rapidly, even when other environmental conditions such as carbonate chemistry were poor. This observation is consistent with experience of shellfish growers and a valuable observation for managers of wild and aquaculture shellfish in estuaries.