The American oyster (Crassostrea virginica) was once prevalent in many rivers of Southern Maine, as evidenced by oyster shell middens in the Damariscotta and documented accounts of abundant oysters by early American colonists. In the 18th through 20th centuries, overharvesting combined with declining water quality from industrialization nearly eliminated Crassostrea virginica from Maine. Scattered populations continue to exist in several estuaries. Today, the growth of the oyster industry throughout the state may be contributing to an increase in local populations, however, they remain at a fraction of their historical abundance.
The Basin Oyster Project has been working to restore oysters in the Basin off the New Meadows River since 2020 as a collaborative effort between multiple stakeholder groups throughout coastal Maine, including academic institutions, conservation NGOs, town commissions, and industry members. In this case study, we describe the Project’s efforts to restore oysters and the results of monitoring oyster settlement, growth, and survival and the effects of oyster restoration on biodiversity and substrate to date.
Live oysters for restoration have been acquired through a purchase program with local aquaculturists and from raising oyster seed through aquaculture on Limited Purpose Aquaculture permitted sites. Oysters of various shell lengths have been bottom planted at 2 locations for the past 4 years. In 2024, oyster spat recruitment and settlement was monitored by deploying mesh bags filled with 2L of cured oyster shells and checking the shells for the presence of oyster spat or scar marks every 2 weeks from mid-June through mid-October. Starting in 2023, the naturally settling adult population has been monitored annually through shoreline surveys in the low and mid-intertidal zone. In 2023, oyster growth and survival was quantified by measuring and checking for mortality and settling oysters in a designated group of oysters contained in trap-wire plots. A likely predator of American oysters, the European green crab (Carcinus maenas), has also been monitored through consistent intertidal surveys to determine abundance and size class distribution. Annual environmental monitoring has included tracking water quality parameters and changes in substrate type and marine biodiversity over time.
Our preliminary analysis shows promising signs for the Basin oyster population. Oysters in Maine are among the few populations that could potentially benefit from climate change. Oyster populations in Maine are frequently recruitment-limited due to water temperatures rarely breaching the minimum spawning threshold. However, the water temperature in the basin has been increasing since 1995, revealing a positive sign for future oyster spawning. Species richness in both restoration sites has also significantly increased, showing that oyster restoration can indeed help ecosystem health as whole and return community benefits (p=0.0.002642)