Eelgrass species worldwide are valued for the ecosystem service they provide to estuarine and marine habitats. One species, Zostera japonica, however, has some negative impacts outside its native range and is considered invasive. In Willapa Bay WA, USA, the nonnative eelgrass has expanded to the level where the shellfish industry is concerned about its potential impacts on its livelihood. Studies were conducted using paired plots, Z. japonica controlled with the herbicide imazamox vs. untreated controls, to assess the effects of Z. japonica on Manila clams (Ruditapes philippinarum) and Pacific oysters (Crassostrea gigas). Recruitment of new Manila clams was not affected by Z. japonica. The growth of young clams, total commercial clam harvests, clam quality and clam harvest efficiency, however, were greater on plots where Z. japonica was chemically controlled than where it was not treated. The response of oysters to Z. japonica control varied by site; there was no effect at one site, while the other sites had a 15% increase in shucked meat with Z. japonica control. The potential economic impact of a Z. japonica infestation of a shellfish bed was ~$47,000 ha^-1 for Manila clams and $4000 ha^-1 for oysters for each crop harvest cycle.