Why does DNR utilize plastic mesh bags to hold the oyster shell they use in rebuilding the oyster reefs?
We would be happy to provide some additional information about the plastic mesh bags using for the bagged shell reefs typically constructed by the SCORE Program. This is a question that staff in the SCORE Program have been asked frequently in the past. I have asked Stephen Czwartacki, formerly of the SCORE Program, but who recently joined us in the Crustacean Research and Monitoring Section of the Marine Resources Research Institute, to provide you with some comments and information here
This question has come up a lot through the years the SCORE program has been building bagged shell reefs. I’m happy to give the same answer we’ve always given, but have CC’d Michael Hodges (head of the SCORE program) just in case he has anything more to add.
In the past, SCORE has experimented with natural fibers (coconut fiber similar to the “biologs” used in bank erosion and sediment retention projects), to use in making the shell bags placed along the shoreline. In short, these natural fibers could not stand up to the elements during the long, out-of-water quarantine period the recycled shell requires before being placed along the shoreline, or the very high energy current/high salinity areas that the bagged shell method is used for. I will address both of these points in detail, as well as the unintended (at the beginning of the project) benefits of using the plastic mesh. Apologies in advance for the long explanation. This is an issue that I, and my colleagues in the SCORE program, have put a lot of thought and energy into, and have been very open about as far as addressing without provocation on many occasions. Also, I personally have worked in watershed and wetland restoration in the Chesapeake region and have employed these natural fiber logs in some of our restoration techniques, although the purpose of these logs was to use as a substrate to plant flora in and around, and to have the log break down shortly after allowing plants to anchor; not to use as a permanent shoreline armor.
SCORE makes the bags they place on the shoreline in the fall and winter, where they sit in quarantine until the spatfall season occurs in mid spring and throughout the summer. This happens for several reasons. Oysters do not reproduce until water temperatures hit 70 about mid-spring every year, therefore we do not put shell along the shoreline where it has the potential to be buried by sediments, or wash out of the intertidal zone before having larval, live oysters settle on the dead shell. More importantly, SCORE uses recycled shell from all over the coast, including the Gulf of Mexico and the Chesapeake Bay. These areas have many oyster diseases and pathogens that may not be actively occurring in South Carolina, and we want to keep it that way. The way we combat this is with an out-of-water, at least 6 month, quarantine. The shell sits out of the water, which kills these diseases and pathogens. The natural fibers cannot handle elements for long enough to last in quarantine out of the water before being placed along the shoreline. The bags would break down too fast!
Why not quarantine the shell first, make the bags, then immediately put the bags onto the shoreline, you ask? Well there are several superficial reasons for this which are beneficial to the logistics of running a very large volunteer program (4,000 volunteers with 8,000 hours of volunteer time in 2013-2014!), but the main reason is still the natural fibers break down too soon in the high salinity, high energy inter-tidal zone. Breaking down too soon will not keep the bags in place long enough to do their job. We place the bags along the shoreline for one main reason: to attract larval oysters. BUT, that is not the only benefit of this method. The bagged shell is often placed in very high energy (fast current, big waves, high boat traffic) areas to combat erosion. The effects of inter-tidal oyster reefs on erosive shorelines, and sediment deposition behind the reef are well documented, and I will not get into them; but I want to point out that if the shell moves it will not affect the erosive shoreline, nor will it attract oysters to grow off of the shell for many different reasons. You have to look no further than the “washed shell” banks in and around the lowcounrty for this evidence. That shell has been moved and beaten to the point of not being a suitable substrate for oyster growth.
The plastic bags of dead recycled shell (SCORE bags) are not what we are looking for as a program, and reef showing these bags should be viewed as a work in progress. We are looking for the live oysters to grow vertically off of the dead, recycled shell, and through the plastic mesh. The plastic, by the way stretches to allow this growth. We are looking for the oysters growing off the dead shell to grow, attract more live oysters the next year to land, recruit sediment deposition and bury the bags, and keep recruiting more and more generations of growth on top. This continuous, vertical growth of the oyster reef will eventually bury the bagged shell under sediment and change the slope of the shoreline making re-growth of marsh behind the reef and recovery of the eroded shoreline possible. A very good example of this in action can be seen at the Live Oak Boat Landing at Edisto Beach State Park. Face the water at the landing and look to your right. See the natural reef directly adjacent to the landing with no erosion behind it, located in between the plastic mesh reef and the boat landing? Well that’s a SCORE reef constructed with plastic mesh in 2007! That reef was built in front of about a 3’-4’ erosion site. It takes a few years before the plastic becomes a permanent part of the shoreline, and successive generations of oysters are growing vertically, but it does happen. In fact, we built beside it again in successive years, and if you continue your look down the shoreline you can see the varying stages of maturation of a SCORE reef and its effects on the shoreline. Also, burial of plastics is essentially the same technique as burial of heavy metals in highly polluted waters. You cannot take mercury or chromium out of the water column, but you can let it settle out and bury it in sediments to contain it.
I, and the SCORE team, are always happy to address questions/concerns of the public, and welcome open conversations of our pristine natural resources here in South Carolina. We work every day to continue to have these natural resources.