Within Pacific Northwest estuaries, native eelgrass (Zostera marina ) is regulated as critical habitat for many salmonids and other fish species under the Endangered Species Act and is recognized as an important species for ecosystem health and sustainability . Shellfish aquaculture farms operating within these same estuaries are often prohibited from impacting eelgrass beds and must demonstrate avoidance for permitting of new activities. Thus, monitoring eelgrass coverage and temporal change is of both regulatory and ecological significance. Imagery collected by unmanned aerial vehicles (UAV) or low-flying airplanes is a powerful tool for evaluating eelgrass beds at a high resolution. Compared to traditional ground-based surveys, aerial methods allow for data collection over a large area (i.e., hundreds of acres) in a short time . This can facilitate cost-effective seasonal and annual monitoring and can also present a supplementary approach for regulatory surveys.
Eelgrass data collected annually using multiple methods highlights the strengths and utilities of each. Annual monitoring of eelgrass in relation to shellfish aquaculture has occurred in Humboldt Bay, California since 2018 using a combination of high-resolution RGB imagery and ground surveys. In Willapa Bay, Washington, high-resolution RGB imagery was collected with a UAV at discrete locations in the summer of 2020 , targeting eelgrass beds with and without shellfish aquaculture activity . Coinciding , RGBir aerial imagery was collected with a low-flying airplane for the entirety of Willapa Bay. These data are compared and discussed with regards to questions of scale, resolution, processing , and results.
Additionally , UAV imagery was collected seasonally at one location in Samish Bay, Washington in the spring , summer , and fall of 2021. Eelgrass surveys are typically completed during the summer growing season, but seasonal change is an important indicator of longevity and potential bed expansion. The spring and fall imagery help to illustrate a natural seasonal fluctuation in the density and extent of eelgrass beds. In the context of regulatory limitations on shellfish aquaculture activity within eelgrass, documentation of this natural variability adds to the body of science supporting proper management of these estuarine resources.
Although there are challenges to the collection and analysis of aerial imagery, monitoring of eelgrass beds using these methods has exciting potential for the ecological and regulatory understanding of interactions with shellfish aquaculture operations.