Experimental test of oyster restoration within eelgrass

Abstract

Abstract Both seagrasses and oysters are foundation species valued for their wide range of ecosystem services, but their space competition sets a constraint on joint benefits. A reserve for native Olympia oysters (Ostrea lurida) was established in lower Hood Canal (Washington State, USA) more than a century ago but is now devoid of that species and dominated by native eelgrass (Zostera marina). This situation sets up a conservation conflict because management activities for one species are at odds with the protection of another. In experimental enhancement plots, Olympia oysters were outplanted at low density, which successfully maintained eelgrass density and production. One method was used in 2013 (seeded cultch, 8% cover) and two additional methods in 2015 (anchored cultch and single oysters, the latter at 4% cover). For all outplant methods, oysters experienced 99% annual mortality, associated with the attraction of non‐native and native predators. Shell cover remained steady for a year and then declined rapidly, as shell accumulation did not exceed sedimentation rates. Eelgrass per se does not preclude Olympia oysters, given that the two species were observed to co‐occur at a coastal estuarine site (Willapa Bay, Washington). However, even when socio‐political constraints on restoration activities were overcome, ecological constraints remained from predation. Competition between these two protected species was avoided, but it may be the case that top‐down control on oysters was particularly acute owing to low oyster density and/or the environmental conditions of eelgrass beds.