COS 236-6 - CANCELLED - Consequences of Japanese eelgrass establishment for Manila clams in Puget Sound, Washington

Abstract: Japanese eelgrass (Zostera japonica) is a non-native marine plant which has become established in Puget Sound, Washington. Its interactions with local species, such as native eelgrass (Zostera marina) and its effects on shellfish aquaculture, which is a major industry in Puget Sound, are not well understood. In particular, Manila clams are an important aquaculture species, and previous studies are divided in whether they find that eelgrass (native or non-native) have positive or negative effects on Manila clam populations. To address these knowledge gaps, our objective was to determine the effect of habitat type on Manila clam populations. We performed a three-month-long field experiment in Puget Sound, Washington, in which we outplanted Manila clams in three habitat types (native Zostera marina, non-native Zostera japonica, and mudflats), and determined clam survival, growth, reproductive status, and condition. Our experiment occurred at three field sites (Tramp Harbor, Dumas Bay, Viewpoint Park) and during two summers (2011 and 2012). Effect of habitat type on Manila clam survival, growth, reproductive status and condition varied inconsistently among sites. No one habitat type led to consistently higher or lower survival, growth, reproductive status, or condition across sites and years. Our results indicate that site-specific factors may be more important for clam populations than habitat type. Z. japonica does not have a consistent negative effect on clams compared to Z. marina and mudflats, indicating that increases in Z. japonica populations are not likely to negatively affect local Manila clam populations or aquaculture operations.