Zostera japonica management on commercial clam beds in Willapa Bay general permit

We issue coverage under this general permit when shellfish growers choose to use an aquatic herbicide (imazamox) to control the non-native eelgrass Zostera japonica on their commercial clam beds. This permit does not cover herbicide use on geoduck or oyster beds, or regulate physical or mechanical control methods. Zostera japonica is listed as a class C noxious weed because it can cause significant impacts on clam aquaculture and harvest for the shellfish industry in Willapa Bay. Read our blog to find out more about the Zostera japonica permit.

The current permit went into effect on May 2, 2025 and expires on May 1, 2030. 

Receive email updates

Join our email list to receive updates about aquatic pesticide permits.

Click the map to enlarge.

Current Permit Documents

Map of treatment areas

This map shows where treatments to control Z. japonica have been done between 2019 - 2023. An average of 82 acres of clam beds were treated each year during this time. The total area of Willapa Bay is around 80,000 acres, with approximately 8,000 acres covered by the native eelgrass Z. marina, and about 6,000 acres covered by the invasive eelgrass Z. japonica

Permit application, forms, and resources