Oyster Reef Restoration in New Hampshire, USA: Lessons Learned During Two Decades of Practice

Eastern oyster (Crassostrea virginica) populations in New Hampshire were approaching historical low levels in the late 1990s when broadly supported oyster restoration projects were initiated. Since then, the state’s collaborative program has conducted approximately 20 projects. Most consisted of constructing a hard substrate (usually mollusc shells) reef base then adding live juvenile oysters (“spat-on-shell”) produced in remote setting tanks. Assessments conducted in 2013, 2015, and 2019 provided long-term (up to 13 years post-construction) data on success and identified potential improvements to the restoration protocol involving two design criteria: reef base and site location. All three long-term assessments found excessive base burial (sedimentation) at many sites, leading to attempts to balance base height, bottom area coverage, and available funds. The result for most projects was multiple, haphazardly positioned shell mounds extending < 0.5 m above the bottom. The site location process initially focused on areas where oyster reefs occurred historically, but after the 2013 assessment and subsequent field experiments, the focus changed to identifying areas with the highest potential for recruitment from wild oysters. This research, which involved three natural reefs over two years, found > 75% of total recruitment occurred < 400 m from the natural reefs. Most current restoration projects center around these two design criteria: multiple shell mound structure and proximity to a healthy natural reef. The New Hampshire experience thus far confirms the emphasis by others on the value of long-term assessments, but as is the case in most areas, much remains to be learned and accomplished.