Estuarine community response to longline spacing in intertidal oyster culture

Structures on which oysters are cultured can have negative effects on other habitats such as seagrass. Intertidal longlines hold clusters of oysters above the sediment surface but may influence the light, hydrodynamics, or desiccation environment for eelgrass Zostera marina , a protected species. In this large-scale experiment, density and spacing of longlines were altered to evaluate responses by eelgrass, other trophic groups, and oyster size. At 2 sites in Willapa Bay, Washington (USA), blocks of longlines were set up across a gradient of longline density (0-1.25 lines m -1 ). Separately, with overall longline density held constant, the arrangement was changed to move some closer together (packing), creating regular wider aisles (0.9-2.5 m). Over the 2 yr crop cycle, eelgrass density was negatively related to longline density. However, packing allowed the same density of oysters to be cultured while wider aisles maintained more eelgrass. Even though packing placed some oysters at high density, it did not reduce oyster shell height or condition index after 2 yr. Epiphyte load, epifaunal load, and epifaunal community structure did not differ with longline density, and only epifaunal load increased with aisle width. For mobile species of fish and decapods, abundance and community structure were not affected by longline density or packing. Strong site and season differences occurred in all trophic groups. Weaker effects of longline spacing choices on other trophic groups than on eelgrass should be interpreted cautiously due to few true replicates in this large-scale experiment. Nevertheless, the outcome shows how oyster grow-out choices may reduce environmental effects.