Diseases of the Oyster Garden

Disease is defined as any impairment of normal structure or function. From a practical standpoint, diseases result in reduced growth, condition, reproductive potential, and mortality.

Juvenile Oyster Disease (JOD) is a relatively new threat that affects oysters smaller than 25 mm. It arises in hatcheries and nurseries where oysters are placed into high-density situations. Little has been published on this disease, yet the serious nature of the disease warrants consideration of all information currently available. Early reports indicated that the oysters were emaciated and might be suffering from a nutritional disease. However, there is little evidence that the disease is linked directly with food supply. The most characteristic symptom of JOD is the presence of an organic deposit on the inside of one or both valves.

The onset of JOD is sudden and typically affects oysters that have been growing rapidly. There is only minor evidence of distress before mortalities begin. Survivors resume growth after the mortality episode.

Mortalities may occur from late June through September. First evidence of disease in the form of growth inhibition and mantle lesions can be detected only about one-week before the onset of mortality. The mortality episode itself lasts four to six weeks. Prevalence of oysters with disease symptoms ranges from 25 to 100 percent with cumulative mortalities of 20 to 90 percent. Mortalities are highly size specific. In a study at Oyster Bay, New York, groups averaging 5 to 20mm shell height when JOD symptoms were first noted suffered total mortalities of 60 to 90 percent. Those averaging 25 to 42 mm had losses of 0 to 30 percent. Mortalities caused by JOD typically occur at water temperatures between about 21 and 26 degrees C. Salinity ranges from about 25 to 32 ppt. One study noted that the disease process is inhibited at lower salinity.

Several studies have shown that the incidence of JOD and consequent mortality can be reduced by measures that increase the water circulation in containers of juvenile oysters. These include reducing density, increasing mesh size., and increasing flow rate in upwellers. One of the most effective strategies for avoiding JOD, however is to spawn and deploy oysters as early in the season as possible, so that they have passed the critical size threshold of about 25mm before JOD appears.

One of the most effective methods of fouling removal is air drying. Most marine fouling organisms cannot live out of water. Devising methods for shellfish culture that allow the shellfish to be in the water but holding the fouled parts of the gear above water has led to interesting innovations in culture methodology and is constantly evolving. The amount of time for the gear to air dry is site specific and depends on the type of fouling organisms present and the speed of desiccation. Once the fouling material is dry, it can be brushed off the gear or left for crabs and fish to consume.

Brine or Fresh Water Dips

A second effective fouling control method is a dip of brine or fresh water. Acetic acid or vinegar might also be used. High concentrations of salt, low
concentrations of acetic acid and fresh water are all environmentally appropriate. The amount of time for soaking in either brine or fresh water is dependent on the types of fouling organisms. If the shellfish themselves are being treated using either method, the farmer must recognize the limitations of soaking shellfish, taking care not to leave them in so long that the treatment may weaken or even kill the clams or oysters being treated.

The duration of soaking animals to remove fouling is size, density, temperature and time dependent. Oysters will tolerate brines considerably longer than clams or mussels. Brine or freshwater dips are not appropriate for species such as scallops or soft-shell clams which cannot close tightly.

Power Washing

A third fouling control method is to use a power-washer which combines the use of water and high pressure. The combination of the pressure and fresh water effectively kills and/or removes the fouling organisms from culture gear.

Manual Control

The simplest method of controlling fouling is manual control using brushes or other tools to remove accumulating organisms. Of course this is also the most labor intensive method, and has led to the development of the alternate methods for fouling control discussed above.

A potential conflict arises when the amount of material removed from any of the methods described above gets carried downstream and affects either public waters or a neighboring grower’s farm. If the fouling organisms are not killed outright, they may reattach to other gear. They can also cause foul odors and create an unsightly mess if the removed material builds up on the shore and decays. Large amounts of fouling material may also accumulate on the sea bottom and cause harm to the benthic animals and plants living there. With all of the above methods, the degree of impact to other users is proportional to the size of the farm – larger farms have the potential for greater overall conflict than smaller ones.

Minimizing the amount of fouling that has to be removed is an obvious solution. The smaller the fouling organism, the easier it is to get rid of it, and the less impact the cleaning operation has on other user groups and the environment. However, cleaning gear is labor intensive and there needs to be a balance of the time and energy devoted to cleaning gear against the reduction in growth of clams or oysters because of the fouling. But there also must be a balance of the potential impact from cleaning larger fouling communities with the better husbandry practice of more frequent cleaning resulting in fewer negative impacts. Reducing the impact to other users by frequent gear cleaning when the fouling is slight rather than waiting until the infestation is difficult to control goes a long way toward keeping adverse public opinion toward shellfish aquaculture to a minimum. An alternative is to clean gear and shellfish on shore and dispose of the fouling material in an approved manner, such as in a landfill or other public disposal facility. If a land site is used, gear and shellfish should be cleaned in an expeditious manner, in areas where odors or an unsightly operation can cause problems with the public.