Eelgrass Mitigation: Balancing Ecological Functions

Eelgrass is an important habitat that provides functional benefit to a variety of species. Recognition of the importance of functions provided by eelgrass has led to increasing regulatory attention and scrutiny of projects with the potential to affect eelgrass systems. Shellfish aquaculture activities sometimes overlap eelgrass beds, potentially reducing turion density and altering ecological function. NMFS policy is to recommend “no net loss of eelgrass habitat function” but, given a lack of complete understanding of the ecological functions provided by eelgrass habitat relative to functions provided by shellfish culture, mitigation ratios are based only on changes in eelgrass density and not on the alteration of ecological functions. This leads to inappropriately high mitigation ratios for aquaculture projects that affect eelgrass. Under current definitions and guidance, mitigation ratios are higher than necessary because no credit is given for positive functional offsets provided by aquaculture that occurs in eelgrass. As an illustrative example, aquaculture activities have been shown to affect eelgrass beneficially in some situations but no mitigation credit is received when geoducks stabilize sediment and cause increases in eelgrass extent and/or density. Similarly, oyster culture can reduce eelgrass density while also providing ecological benefits such as food resources and structural habitat for numerous floral and faunal species but no credit is granted based on these positive functional values provided by shellfish aquaculture. NMFS’ CEMP requires “no functional loss” but, because the ecological functions provided by eelgrass are not perfectly understood, theses potential benefits are not considered when determining the mitigation ratio. This may result in situations where projects overlapping eelgrass result in an inappropriately high debit even though some projects may provide beneficial functions similar to those provided by eelgrass. What ecological functions are provided by eelgrass beds? What ecological functions are provided by shellfish aquaculture? Do credit-debit mitigation methods provide a credible method to account for changes in ecological function due to changes in eelgrass density? This talk provides a review of the known science regarding ecological functions provided by both shellfish aquaculture and eelgrass beds and describes methods to aid in the appropriate balance of mitigation ratios