For more information on species & ecosystem science:

Wildlife Science

Fish Science

Habitat Science


Lead Scientist: Kirk Krueger

Ecoregions: Puget Trough

Ecological Systems: North Pacific Maritime Eelgrass Bed


Docks on Bainbridge Island


Docks on Bainbridge Island


Docks on Bainbridge Island


Hydraulic Program Compliance and Effectiveness

Sufficient Sample Size for Eelgrass Monitoring

Project Description

Eelgrass (Zostra marina) and is an important ecosystems component that provides habitat for many species, including imperiled salmon. Eelgrass is designated a habitat of special concern by the Washington Department of Fish and Wildlife and critical habitat by the Washington Department of Ecology. Eelgrass density and persistence is affected by human activities and in Puget Sound its geographic distribution is declining and increasingly fragmented.

In Washington, monitoring is required to ensure no net loss of eelgrass on projects permitted under the Hydraulic Code. However, spatial and temporal variation in eelgrass density can make detecting loss of eelgrass difficult.

Our goal was to provide information to help select more efficient study designs; increasing the probability of obtaining reliable study results. Our objectives were to:

  1. identify opportunities for improved monitoring,
  2. estimate sufficient sample sizes to reliably detect large changes in eelgrass density, and abundance.

Key Findings

  • Most eelgrass monitoring surveys likely sample insufficiently and fail to detect an effect when one occurs. 
  • Using controls can improve detection of loss of eelgrass and can identify regional declines that would have been incorrectly attributed to the work. 
  • We suggesttrying to detect changes in eelgrass density of 20 % or more.