Assemblage shift following population collapse of a non-indigenous bivalve in an urban lagoon

The quahog Mercenaria mercenaria has been introduced repeatedly to the Pacific coast of North America, but only one population is known to have become established. In the 1970s, the population of M. mercenaria at Colorado Lagoon, in Los Angeles County, California (33^o46′16″N, 118^o08′05″W), was estimated at more than 300,000 individuals. To determine the current status of this non-indigenous species (NIS), in 2009, we sampled 57 intertidal and 20 shallow subtidal plots, identifying and quantifying collected bivalves. No quahogs were found among the 2,490 living bivalves in our plots, though two were found intertidally outside of our plots. The M. mercenaria population has thus collapsed since 1980, but the native community has not recovered. Six of the fourteen living bivalve species we encountered were NIS; three are new records for the location, including the clam Venerupis philippinarum, which made up 87.6% of collected individuals. Though M. mercenaria is likely on its way to extinction on the US Pacific coast, the bivalve assemblage at this location remains heavily dominated by NIS.