School Science/Oyster cultivation in seawater ponds

Crassostrea gigas, the Japanese oyster, is ideally suited for cultivation in sea water ponds or in the water from seawater ponds. Major advantages of this oyster are:

1. Gigas does well in water with high concentrations of phytoplankton, the normal situation in marine ponds which are growing fish or prawns and which therefore have high nutrient loadings.
2. Spat (the juveniles) of gigas are readily available from hatcheries all over the world and are also not too difficult to reproduce in a modest mariculture hatchery.
3. Gigas has a ready market and is a fast grower. In phytoplankton rich water it becomes very fat (lots of glycogen stored) and as a result is very tasty.
4. Gigas can survive a wide range of temperatures from near freezing up to 30 degrees although it grows fastest and fattest at around 20 degrees.

Experimental Cultivation of gigas

A simple way to test if the oysters will thrive is to construct some simple baskets of stiff plastic mesh (netlon for instance). Put in a few tens of small, young gigas and suspend the baskets from the monks (water outlet structures) of the pond. Use mesh of at least 10mm opening so that it doesn't plug up too quickly with marine fouling organisms. A simple way of doing this is to cut two disks of, say, 30cm diameter one for the top and one for the bottom and a strip of, say 10cm wide for the side. Sew up with plastic cord and leave a place where the baskets can be opened to add and remove oysters as necessary. Attach three cords equally spaced around the upper rim, tie them together and attach a cord long enough to suspend baskets in the pond. Gigas grown this way in Elat reached a total weight of 75 grams in 9 months and had very high quality indices.

Crassostrea produce large amounts of faeces and pseudofaeces when they are in plankton rich water so every few days they will need to be agitated to remove this material. Otherwise they will smother. Further along in this module we describe a way of growing the oysters so that they are pretty well immune from this build up of sediment.

Oyster Morphology

If you cup your left hand and cover the cup with your right hand which you keep flat, this represents the shells of an oyster. The hinge is at the part of your hand nearest to the wrist and the opening at the ends of your fingers. Imagine your body shrinking until it will fit inside your hands in the same orientation as you are sitting now. The cupped left hand is the left shell of the oyster but since this is the shell that is attached to the substrate (often intertidal rocks in the case of Saccostrea sp.) it is commonly referred to as the lower shell while the flat right shell is commonly called the upper shell. A membrane or skin attaches along the oysters back and lines the shells. This membrane is called the mantle.

Oyster Feeding

Oysters feed by means of their gills. They close the gap between the shells with their mantle, leaving an openings where your thumb and little finger are in the above paragraph. Cillia on the gills beat, creating a flow of water in one opening and out the other. The mucus on the gills captures particles and the action of cilia move the mucus with its edible particles along pathways on the gills towards the mouth. Inedible particles are sent along other pathways away from the mouth. From time to time, the oyster closes quickly, blowing out the unwanted particles which are stuck together with mucus. This expelled material is referred to as pseudofaeces. As it is now in larger particles than when it was individual cells of phytoplankton, it sinks faster and forms an organic layer along with ordinary faeces on the bottom of its growing vessel (pond, trough, tank etc.)In waters with high concentrations of plankton, even particles which are edible by the oysters are expelled as pseudofaeces. This potential food is therefore lost to the oysters. Below a method to make better use of more of the available food is described.

More Efficient Use of Phytoplankton

Oysters produce pseudofaeces when exposed to edible particles in high concentrations. This, of course, wastes food that could have been used to grow more oysters. It also produces more grunge which rapidly turns anaerobic and must be cleared more often from the growing vessel. Since marine fish and prawns are fed with large quantities of prepared feed, in sunny areas, where much of this type of cultivation occurs, very rich blooms of phytoplankton develop in the ponds. Any system which presents this food at the correct concentration to the oysters will increase the quantity of oysters that can be grown from a given area of sea water pond. A way which has been used successfully is to build a trough (1m x 1m for instance) which is a little lower than the pond. The water leaving the pond is run through the trough. Instead of letting all the water in one end and out the other, the water is introduced all along the trough. This keeps renewing the phytoplankton as the oysters remove it and ensures that all the oysters are growing in an algae soup which is more dilute in phytoplankton than the water in the pond.

Cleaning the Trough

Oysters produce very large amounts of faeces and if in rich water, large amounts of pseudofaeces. After a time this material builds up on the bottom of the trough, becomes anaerobic and begins to produce hydrogen sulphide an ammonia. It is wise to fit the oyster trough with monk boards or some similar system so that the water can be let down completely and the trough flushed out. At the same time the oysters can be washed off although this becomes much less critical with the following growing method.

Self Cleaning Oysters

While a common method of growing oysters it to put them on mesh trays with their lower shells down, in this orientation, the oysters can easily be smothered by the solid wastes of oysters above them. The trick is to suspend them hinge up, opening down. A method which worked well in a pilot study in Elat was to glue spat on to netlon mesh on trays and place them in the ponds or troughs. As an oyster grows it attaches by its lower shell to the substrate. After a few weeks, the trays were removed from the water and the mesh cut up to leave an eye of mesh distal to the hinge on each oyster. The oysters were then suspended from the corners of vinyl coated wire weld mesh with a hole size of 55mm. This was fairly labour intensive but when compared with the work that goes into cleaning oysters on trays or the work done to grow oysters in the intertidal zone in many parts of the world then it can be seen that even this experimental method was not that onerous. Once the oysters are suspended this way, they are only handled once again for harvest.

Shell Worm

In many parts of the world there are worms that settle on the lip of oysters and then grow inbetween the layers of the shell. When an oyster is opened, the cavity thus produced is often broken into and spills its contents of anaerobic mud on to the oyster. These worms are particularly prevalent in muddy conditions. One way of reducing such attacks is to clean the oysters often. However, a less laborious way is to ensure that these animals do not enter the pond. Since they enter as microscopic larvae, this necessitates the microfiltration of the water entering the ponds. With the high exchange rates typically used in marine ponds, this is impossible or at least very expensive by man made mechanical methods.

A better way is to take the water from the beach or sea bottom. A number of methods have been used, depending on the individual location. In desert areas or areas where, due to the particular geology of the site, there is no fresh water flowing underground out into the sea, a well can often be dug above the high tide mark and a pump lowered into the well. If you are lucky enough to have such a site, this is probably the least expensive system to build and to run. The entire beach adjacent to the site becomes the filter for the well. Since the area is very large, the flow rate becomes very slow and the filter will often act as a live filter rather than as a mechanical filter.

As water is sucked through the beach or seabottom, oxygen goes with it. This allows many organisms to settle in the spaces between the particles. Many of these filter particulate material from the water. A good indication that the filter is actively filtering is a level of Oxygen in the water entering the pond which is lower than the oxygen level in the nearby ocean.

Oyster Quality

There are a number of measurements of "oyster quality". One of the most accepted indices is to divide the dry weight of the oyster meat in grams by the volume of the space where the oyster lives in cubic centimetres. The dry weight of the oyster meat is obtained by drying to constant weight the meat of the shucked oyster at just over 100 degrees. The volume of the cavity is found as follows. The unshucked, closed oyster is suspended by a thread from the hook under a top loading balance. The weight is noted. A container of water is brought up under the oyster and raised until the whole oyster is immersed. The change in weight in grams is equal to the volume of the oyster in cubic centimetres. After the oyster is shucked, the same procedure is followed with the empty shell. The difference in the two measurements is the volume of the cavity of the oyster.