Spatio-temporal recruitment variability of naturalized Manila clams (Ruditapes philippinarum) in Willapa Bay, Washington, USA

Manila clam (Ruditapes philippinarum) harvests from Willapa Bay, Washington, USA have increased by an order of magnitude over two decades. This increased yield of a non-native species has been supported by natural recruitment as well as some planting of small clams. Manila clam settlement was recorded over three years in clam spat collectors (mesh bags containing gravel) and occurred in June to October at water temperatures >13 °C. At 16 sites in the bay, clam abundance was measured at multiple life stages. Distributions of early-stage larvae during a spawning peak were well-correlated with wild populations of adult clams, with both stages highest in the southwest region having long residence time and little direct freshwater input. In contrast, settlement of these larvae 1-2 weeks later was correlated neither with early-stage larval distributions nor with one year-old recruits a year later; instead, settlement was spread more evenly between southeast and southwest regions, possibly due to mixing during larval development. Therefore, higher natural clam production in west than east may stem from differential post-settlement survival over the first winter, whereas low densities of all life stages in northern regions may be set by larval supply due to water residence time constraints. Despite an extended settlement period of several months, clams were no more abundant after one year than were measured for one week in clam spat collectors, suggesting generally high post-settlement losses in addition to factors that are spatially variable and break down correlations among life stages.