Fouling & Predation
Controlling Fouling and Predation
One of the limits to the natural productivity of the marine environment is surface area that can be readily colonized. Many species produce planktonic or free swimming larvae that are carried about by tides, currents and waves. At some point in their development, they will settle on some suitable surface and grow through the next stages of development. Most of the “fouling” organisms fall into this class. Any object put into the water will soon be colonized. For the shellfish grower this can pose a major problem if the abundance of these organisms becomes so great that they outcompete the crop for available food, smother it or otherwise interfere with the culture effort. Among the fouling organisms that can have a detrimental effect to the shellfish grower are mussels, barnacles, tunicates, tube worms (polychaete annelids), bryozoa (either branching or flat and encrusting), hydroids (a small branching organism related to jellyfish and sea anemonies) and encrusting sponge.
There are some organisms in the marine environment that specifically target shellfish for food (predators) or as hosts (parasites). Parasites that can affect shellfish culture range from microcells (such as Mikrocytos mackinii the causative agent of Denman disease) and protozoa to flatworms and parasitic copepods. Predators have developed special adaptations to be able to penetrate the shells of their victims to feast on the meat inside or otherwise cause mortality. Crabs, starfish, moonsnails and ducks are some common examples of shellfish predators.
Methods of Control
Spraying chemicals to control fouling and predation is not an option for shellfish growers. To deal with these problems, growers use a combination of avoidance, prevention and pro-active methods.
Avoiding fouling and predation is sometimes possible. This might involve monitoring the site for mussel, starfish or barnacle sets and timing events in the production cycle to avoid the worst of the set. Juvenile oyster, clam, scallop or mussel seed put out in the spring or early summer for nursery rearing may be particulalrly vulnerable to such fouling organism sets. A heavy barnacle set on juvenile oysters that have just been set on tubes will create problems in a few months time when oysters start to grow over the barnacles, lose attachment to the tube and drop to the bottom. In the southern Georgia Strait barnacle larvae generally set between mid-March and mid-April at depths of 10 meters or less. Nursery seed put out after this time will probably avoid barnacle sets. A fouling line, with oyster shell attached at spaced intervals, hung in the water from the surface down to about 10 meters can be used to detect setting timing and quantities of fouling and predatory organisms. Monitoring (with a microscope) should be done weekly. Setting and nursery can then be timed to circumvent these natural events.
Starting with good quality seed, and maintaining a high growth-rate production system from nursery to grow-out is essential in keeping the upper hand on fouling and predatory organisms. Establishing good seed growth during nursery rearing will go a long way to prevent fouling organisms from overwhelming the shellfish.
Intertidal nursery rearing of oyster and clam already controls much of the predation and fouling problems by daily exposure to air. Starfish and crabs retreat with the tide and organisms such as hydroids and tunicates cannot tolerate drying.
In controlled nursery systems such as upwellers, normal maintenance of the screens, containers and seed takes care of most of the fouling and predation problems.
In deep water systems such as tray oyster culture the stock is regularly pulled up and graded. Removal from the water, handling, and restocking will help prevent the buildup of soft fouling organisms and allow the grower to remove larger organisms such as mussels and starfish. In systems where tubes or shell cultch are hung from rafts or longlines, fouling control will normally take place only at the time of stringing the stock and during the growout cycle the oysters are large and vigourous enough to handle most of the fouling and predation that may threaten them. When the oysters are pulled up at harvest, they will usually be heavily fouled and may even have clinging predators such as starfish. However the oysters are large enough to fall prey to only the largest of starfish. Smaller starfish and sea urchins that settle on the culture line often actually assist the oysters by consuming many of the smaller soft fouling organisms that have built up on the culture lines.
In deep water culture systems such as shell cultch or tube oysters, the accumulation of soft fouling organisms may be significant. Often the majority of the fouling can be removed at harvest either by hand or by low or high pressure hose washing. More important is managing fouling early in the culture cycle when the shellfish are most vulnerable. The most common measure of fouling control consists of dipping stock in a solution which will kill the fouling organisms without having a negative effect on the shellfish stock
The solution may be fresh water, a saturated saline solution or lime solution. Tube modules or tray stacks can be dipped for brief periods in tanks containing these solutions A hydrated lime bath is one means of effectively controlling fouling in the tray culture systems of one company. A lime solution at a concentration of 40g/L is prepared by adding 3.6kg (8 lb.) of hydrated lime to 91L (20 gal.) of water in a suitable container. The mixture is stirred frequently. The oyster trays are dipped in the solution for 5-20 seconds. Because the lime solution will affect the oysters if they open up while in the bath, it is important to ensure they are well-closed up before dipping. They should be exposed to air for at least 1/2 hour and then shaken in the trays to make sure they stay closed.