It is well known that oyster populations today are a few percent of levels found when Europeans first entered the Bay. Over-harvesting, habitat destruction by dredging that flattens the reefs, and human introduction of a disease (MSX), have devastated oyster populations. Artificial reefs, formed by dumping shell in piles in the water, have met with very limited success, despite claims to the contrary by ACOE. Little spat set (strike) takes place, the shells disintegrate rapidly, and cow-nosed rays (like the one that almost killed Capt. John Smith at “Stingray Point”) wipe out many of the oysters that do manage to grow, perhaps because humans are eliminating their major predator, sharks. But a rapidly expanding aquaculture industry is providing hope.
Why do so few larvae “strike” or attach to hard substrate any more? Each year, the shells from the oysters I grow and eat are piled on land. In early June, the clean shell is placed in the water near my pier in hopes of starting a “reef.” I have had no success despite there being thousands of oysters in floats attached to the pier to provide larvae. Why doesn’t the shell attract natural “strike?” In spring of 2006 I took clean shells, drilled a hole in each one, and suspended the shells concave-side down off my pier, adding two shells every three days. The photographs (
On string.jpg;
Spread out.jpg) show the result (
www.bayjournal.org article =3256.) After little more than a week, the bottom two shells were densely coated, top and bottom, with “slime” or, as it is properly called, biofilms. It is well known that oysters require clean substrate for spat to attach “strike or set.” The classic book “The Oyster” by William K. Brooks, published over a century ago, clearly states this in several places. For example, on p. 99 “The first thing found out was that the floating spawn would not attach itself to or ’set’ upon anything which had not a clean surface; smoothness did not hinder - but the surface of the object must not be slimy.” And on p. 116, in order to attract spat for commercial purposes “...it is important that the shells or other substances which are employed be perfectly clean, and that they be not put into the water until spawning has commenced.”
Dr. Jeremy Jackson, a respected scientist at the Scripps Institute of Oceanography, has advocated the “rise of slime” as one of the consequences of nutrification of the coastal ocean (
www.shiftingbaselines.org.) In his 2001 paper in the Proceedings of the National Academy of Sciences (vol. 98, pp. 5411-5418) he states “Today Chesapeake Bay is a bacterially dominated ecosystem with a totally different trophic structure from a century ago.” Nutrification of coastal environments leads not only to the prolific growth of suspended algae (phytoplankton) that cloud the water, but to biofilms composed of both plants and the microbes that consume them (slime.) By continuing to pour nutrients into the water, we have changed the Bay from an ecosystem dominated by a grazing food chain (phytoplankton, zooplankton, oyster reefs, fish and apex predators) to a microbial food web beginning with dead phytoplankton being consumed by bacteria/fungi and ending in jellyfish.
Clean substrate is the secret to getting oyster larvae to settle. A technique called “spat-on-shell” is gaining popularity, and nvolves forcing larvae from disease-resistant strains to settle on clean shell in tanks, and then spreading the shell with its attached spat onto oyster grounds. This technique provides not only the opportunity for improved harvest (by tonging not by dredging!) but hopefully it improves the gene pool for disease-resistance. Spat-on-shell is proving to be extremely successful in the rapidly expanding oyster aquaculture industry in the Bay, which is just catching up to advances in aquaculture that are well established in many other places in the world. We need more hatcheries and more spat-on-shell, not more "sanctuary" reefs that are small and exorbitantly expensive. Watermen are clever and should be allowed to develop techniques that are practical, and they should not be micromanaged by bureaucrats.
Although the practice of settling oysters onto clean shell in tanks prior to placing the shell on the bottom is useful for aquaculture, until the “rise of slime” is curtailed, natural oyster spat set (“strike”) is unlikely to improve. Not only does the slime cover hard substrates, and prevent benthic organisms like oysters from settling, it also covers the leaves of what little submerged aquatic vegetation (SAV or sea grass) remains in the Bay and further restricts the amount of light the plants can receive. The “slime” is caused by over-fertilization of the Bay because we value maximum agricultural profits, cheap food and low wastewater bills over satisfactory water quality. Until nitrate and phosphate pollution is reduced by improved agricultural fertilization practices and upgraded wastewater treatment facilities, water quality in the Bay will not improve, the ability of oyster larvae to attach naturally will not improve, and the Chesapeake Bay ecosystem will remain dominated by bacteria. The reasons filter-feeders like oysters and menhaden cannot solve the Bay’s abysmal water quality problem is summarized on the "State of the Bay" page.
First posted in 2006, revised extensively. ©2012
Webmaster is Aaron Land.
