EELGRASS DELINEATION METHODS AND SHELLFISH AQUACULTURE: WHY SCALE AND EFFECT ARE IMPORTANT CONSIDERATIONS

Native eelgrass (Zostera marina) is a common perennial aquatic plant that creates three-dimensional habitat structure and forms extensive intertidal and subtidal beds throughout the Pacific Northwest. Eelgrass and shellfish aquaculture can often occur at the same tidal elevations. While there are a suite of interactions between the two (both positive and negative), management predominantly focuses on avoidance. Avoidance depends on the ability to understand where eelgrass is located and how shellfish aquaculture interacts with eelgrass at unit and landscape scales. Current delineation methods for eelgrass have limited applicability in relation to shellfish aquaculture due to three main issues: (1) lack of a useful definition of an eelgrass bed that can be applied in the field, (2) the various scales and habitats applicable to shellfish aquaculture, and (3) the level of effort in relation to potential effects to eelgrass.

A review of criteria for defining eelgrass beds identified numerous differences in how monitoring and regulations have attempted to define beds and bed edges among various state and federal agencies. Locations with low density and ephemeral eelgrass are typically regarded as "beyond" eelgrass bed boundaries. However, recent guidance in California have defined a density as low as 3 turions per square meter as representing an "eelgrass bed." Other classifications indicate that an aquatic plant does not have a category until it exceeds a threshold of 30% cover. The lack of an approved definition for an eelgrass bed can cause challenges both in terms of applying that definition in the field and the scale that is being surveyed.

One of the main issues with scale is the type of habitat being surveyed. Eelgrass flats differ from fringe sites in that the structure of the bed is not necessarily aligned relative to the shoreline. Fringe sites typically have a unidirectional transition along a depth gradient. Conversely, eelgrass flats may be overlaid onto a more complex environment (e.g., a river delta) that may not have unidirectional transition patterns. Further, the scale of the site makes a difference in the methods used to delineate the site, especially in terms of more complex geomorphology.

Finally, the potential effect to the eelgrass bed should be a consideration in terms of the level of effort for the delineation itself. If avoidance is the main mitigation measure used, then understanding the bed edge should be the focus of delineation methods. Further, there is ample literature that indicates effects from shellfish aquaculture are short-term and insignificant compared to natural variability. However, recent regulations require extensive monitoring of eelgrass in the presence of shellfish aquaculture in order to identify potential changes. We will discuss several eelgrass delineation methods used for shellfish aquaculture projects, the spatial scale at which these methods are best suited, and challenges for the future.