Science Serving Maryland's Coasts


In Situ Setting of Crassostrea Virginica Larvae on Restored Reefs: A Complementary Method for Restoring Oyster Populations

Principal Investigator: 

Cecily Steppe

Start/End Year: 

2012 to 2014


U.S. Naval Academy

Co-Principal investigator: 

David W. Fredriksson, U.S. Naval Academy


Strategic focus area: 

Sustainable natural resources of coastal Maryland


OBJECTIVES Significant restoration of Bay oyster populations is necessary to improve the fishery and to counter anthropogenic stresses. Proven methods include increasing the quantity of substratum available for oyster settlement and planting hatchery-reared larvae already set on substrata. Given the scale of restoration goals, combined with declining quantities of natural shell available and extensive handling required to seed reefs with spat-on-shell, we suggest the need for a complementary restoration method. This study seeks to expand the restoration toolbox by testing the efficacy of setting competent larvae directly onto restored reefs using a temporary structure during a three-day setting period. We will measure site environmental conditions; engineer a larval barrier structure and assess hydrodynamic forces on it; test structure efficacy in setting spat on approximately 0.01 acre of clean shell base (Year 1) and biofouled shell (Year 2); perform a detailed economic analysis of scaling up in situ setting; and share results with key stakeholders. METHODOLOGY A larval barrier structure and anchoring system will be designed from specifications obtained from measurements of currents, tides and waves. In year 1, shell base will be added to a 42mz portion of an existing reef. The enclosure will surround the base; trays and bags of shell will be added within and outside the enclosure, and to two shore-side tanks. Larvae will be added and allowed three days to set. The enclosure will be removed and shell bags brought to the lab to grow for 10 days, then transferred to a grow-out pier for monthly measurements. Divers will recover and replace oysters in on-bar trays at the end of the experiment; after one month and again the following spring. Experiments will be repeated in Year 2 without additional shell base; differences will be quantified via Kruskal-Wallis tests. Engineering economic analysis will determine the potential of applying this method at different scales RATIONALE This study will test the efficacy of setting C. virginica larvae directly onto reefs in tidal tributaries. First, it will identify potential forces on larval barriers for in situ setting due to changes in tides, currents, and wakes. Data gathered will guide the development of a structure to contain eyed larvae. The structure facilitates our second objective: to assess the efficacy of setting larvae in situ over a portion of a restored oyster bar with added shell base. Spat set and growth will be assessed by substratum and set method. To approximate setting on a previously restored bar, the set will be replicated in Year 2 without clean shell. We will examine the financial feasibility of the technique for different restoration scenarios in Chesapeake Bay, and disseminate results to key stakeholders including NOAA, Maryland DNR, Chesapeake Bay Foundation, local watershed groups, Army Corps of Engineers, University of Maryland, the Oyster Recovery Partnership, and STEM summer camps.


RECAP: This ongoing study seeks to expand options for oyster restoration by testing the efficacy of setting larvae directly onto restored reefs using a temporary enclosure to enhance larvae deposition (spat set).

RELEVANCE: A high priority of the State of Maryland and federal policy makers is to restore the Chesapeake Bay’s decimated oyster population and associated commercial fishery. Current restoration methods include increasing the quantity of substrate available on the Bay’s bottom to serve as habitat for oysters and planting hatchery-reared larvae on this substrate. These existing methods face various challenges. Available substrate, such as oyster shell, is in short supply. The researchers in this project are investigating the efficacy of an alternative restoration technique: setting competent larvae directly onto restored reefs within a temporary enclosure.

RESPONSE: The principal investigators are Cecily Steppe and David W. Fredriksson of the U.S. Naval Academy. The researchers designed and built an enclosure system to contain the larvae. Shell base was added to form a “reef.” During a three-day setting period, the researchers measured larvae deposition under varying conditions.

RESULTS: Spat on shell deposited on reefs were recorded at average densities of 1.9 spat per shell or 251 spat per square meter. The goal of many restoration initiatives in Chesapeake Bay is to maintain reefs with more than 50 oysters per square meter. The rate of oxygen depletion recorded by the researchers illustrated the feasibility of applying this method to short-term larvae deposition efforts (spat sets) even in the absence of aeration of the enclosure. The enclosure and mooring system withstood moderate tidal, wave, and ship-wake activity typical of coastal tributaries. This study served as a basis for a K-12 curriculum unit on aquaculture and environmental engineering that was taught to over 200 middle- and high school students.

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