Blue crabs and oysters are probably the two marine organisms for which the Chesapeake Bay is most famous. Oysters are filter feeders. They pump water over their gills and extract both oxygen and small particles of organic material, including algae, on which they feed. In 1989, R. Newell estimated that at the end of the 19th century, oysters could filter the entire volume of water in the Chesapeake Bay in four or five days. Today it takes more than a year. Between 1870 and 1890, Maryland harvested more than 10 million bushels of oysters each year. The harvest today is less than one percent of that figure. What happened?
The cause of the decline in natural ecosystems worldwide has a single cause - the explosive growth of the human population since the industrial (agricultural, medical) revolution. We humans, collectively, are responsible. In the case of oysters, it is generally accepted that there are several specific factors that have contributed to the decline in harvests. Firstly, oysters are susceptible to two diseases, "MSX" and "Dermo". Although still somewhat controversial, many scientists believe that both diseases are caused by "exotic organisms" that were introduced into the Bay, probably by the discharge of ballast water from ships that filled their ballast tanks in the European Mediterranean Sea and/or Asia. There is no question that over-harvesting oysters, and especially the method of harvesting, is also responsible for the decline in harvests. Just as is true of ecosystems worldwide, habitat destruction is a paramount problem.
Healthy oyster reefs build up from the bottom toward the water surface. In this way many oysters can grow above the mud, in relatively clear water, and juveniles can settle and begin to mature safely in the interstices between the larger shells. Healthy oyster reefs provide habitat for many other kinds of organisms of commercial and recreational value, and were once extensive enough to be serious hazards to navigation. No more! Repeated dredging and tonging have flattened the mounded reefs so that young oysters no longer have a stable substrate to which they can attach. The flattened layer of shells makes it difficult for oysters to stay above the mud and provides little protection from predators.
Selectively harvesting the largest oysters is also a problem because it removes the most efficient spawners from the population. Worse, by harvesting the largest (and oldest) individuals (and killing all the baby oysters attached to their shells) we remove individuals which may have developed a resistance to disease. By selectively harvesting the older, potentially disease-resistant individuals, we are stopping evolution in its tracks and preventing the species from naturally evolving toward resistance to the diseases.
The most recent attempt to improve the oyster harvest is to construct large reefs and to declare them as sanctuaries. The idea is to build piles of oyster shells in places where natural reefs once existed, based on historical records, and then declare them "off-limits" for harvesting. In this way, it is hoped that individuals which survive the diseases will spawn and transfer resistance to their offspring. The abundant spat from the artificial reef will also colonize substrate outside the sanctuary area and, over time, increase the commercial harvest. This scheme has the added advantage of increasing the number of oysters that can filter algae from the water. If oyster populations increase, then water clarity will improve, submerged aquatic vegetation (SAV) can become more common, and the entire Bay ecosystem will improve. In another attempt to improve the commercial oyster harvest, large individuals which may have developed disease resistance are spawned in the laboratory. Unlike many aquatic organisms, oysters are relatively easy to culture. The abundant spat can then be used as "seed" on artificial reefs, or made available to individuals who wish to culture oysters for their own consumption and, at the same time, filter the water in their creeks or rivers. Keeping track of the mortality of each strain, as is being done at VIMS, will identify which strains survive the best. More information about the selective breeding programs, the artificial reef program, and how you can help, can be found at the VIMS web site, www.vims.edu, or at www.oystergardener.org, the Tidewater Oyster Growers Association.
The next article will discuss the expanding practice of "oyster gardening", and provide information about how waterfront property owners or individuals with access to the water can participate. If you missed a NAPS Stewardship tip, they can be found, along with links to other sources of information at the NAPS web site.