Growing Oysters in the Outflow of Mariculture Ponds

Mariculture ponds growing fish, prawns or other organisms provides the ideal food source for growing oysters. It depends on the fact that with a conversion coefficient of 2:1 from feed to fish/prawn, 90% of the feed goes into the water. I can just hear you saying "this guy can't add two numbers together" but bear with me.

    Conversion coefficient is calculated by dividing how much feed is used, (usually in the form of a pellet) into how much fish is produced. Conversion coefficients in the range of 2:1 are common. However, the pellet is typically around 7% water (to keep it from growing moulds, bacteria and so forth), and the fish is typically around 80% water. In the end, when you calculate the true conversion coefficient, of dry pellet to dry fish, it comes out at about 10:1 just as you learned in biology when the teacher said that only about 10% of the mass, transfers from one tropic level to the next. 

   For instance, 100kg of krill will make 10kg of penguin and 10kg of penguin will make 1kg of sea lion. From the point of view of the oyster, 90% of the food that is fed to the primary organism (fish or prawn) is available to feed the oyster. This is not some attempt by the fish farmer to bluff someone. 

    He buys his feed pellets at so much per kg and sells his fish or prawns for so much a kg. For him, this is a perfectly logical way of looking at conversion coefficient.  For a biologist who wants to know how much food is available for the next stage, the true conversion coefficient is the one to use.

Of course, this food enters the pond in the form of feces, excretory products and respiration. Not what your oyster wants to eat. For the welfare of the oyster it is important that the ponds are in a good sunny location. With lots of sun, a heavy crop of a wide range of single celled phytoplankton grow and use this bounty. If, as in one place I worked, the water is sucked through beach rock with a good proportion of organically derived silica, then the water will contain a good quantity of Si for the formation of diatoms. Diatoms are generally speaking the best food for oysters. In another location where we farmed, the water source was otherwise and I always attributed this to the much poorer results which were obtained.

Here a problem arises. In your typical mariculture pond in a sunny climate, even with an exchange rate of once every two days, the concentration of phytoplankton is much too great for the oysters. The Japanese oyster (Saccostrea gigas) survive and grow very well in this rich soup of phytoplankton but they waste a lot of food. An oyster uses its "gills" to separate out particles from the water with preferred particles moved by paths of cilia toward the mouth while unwanted particles go the other way. Every so often, the oyster snaps its shell closed and expels the unwanted items as pseudofaeces. When the concentration of normally desired food items is too great, much of this good food is expelled as pseudofaeces. The feces along with the pseudofaeces fall to the bottom of whatever container the oysters are growing in and turns anaerobic. Various systems are used to ensure that this bottom muck does not poison the oysters.  Part of the trick to produce the maximum crop from the available food is to present the  food at the desired concentration.  More of that later.

If you have ever been associated with oyster growing in the sea, you know how much work; how many processes go into handling the oysters. I mention this as I am going to describe the method we used to grow the oysters. It may sound like a lot of work but is a fraction of the work needed to grow in the sea and is all in comfortable conditions on land rather than at sea where you are exposed to whatever weather is thrown at you. Even better, most of the work you have to do in the land based system is with the small juvenile oysters so there is little weight to move around and later the oysters finish their growing pretty well by themselves.   In a sea based system, you are continually separating and sorting the oysters which are continually attaching themselves to each other.

The oyster we grew was Crassostrea gigas (now Saccostrea gigas), also known as the Pacific or Japanese oyster. It is able to handle high concentrations of food in its environment and grows fast producing, to my taste, the finest oyster available. It has one cup shaped shell and one flat shell. When grown free on mesh racks, the oyster sits on its cup shaped shell with the flat shell uppermost. If you grow on racks this orientation is necessary since, if the growing edge of the shell touches the rack (as it would do if you put the oyster flat-side-down), the oyster  will grow into the mesh and will have to be pried off every time you thin or harvest. However, this characteristic can be turned to one's advantage.

Early on, we noted that with stacks of racks of free sitting oysters, the build up of feces and pseudofaeces would soon smother the oysters unless we cleaned them every week or so.    Even then oyster on the bottom racks were often smothered. Even worse, with the oysters in contact with anaerobic mud, we often got infestations of shell worm. When you open an oyster which is infested with shell (mud) worm, you often break into a pocket of anaerobic mud in the shell.  Not the thing to impress the customer.  The system we came up with was as follows.

When we got our oyster from the hatchery at about the size of half a pea, or sometimes smaller, we grew them on trays of mesh until they were large enough so that they could be laid flat on a piece of Netlon with a 10mm hole.  We then made up racks of netlon (plastic extruded mesh) of about a meter by a meter.  We cut the mesh in strips leaving both ends connected. We put spacers at each end like a weaving so that alternate strips of net were up and down. We then placed a baby oyster on every third hole which it was now large enough to bridge. Sometimes we just laid them on and sometimes used a variety of glues. The oysters were placed cup-shell-down. As soon as the mantle of the oyster came out and touched the mesh, they started to grow into the mesh. After a couple of weeks they were firmly attached. At this point, we cut the mesh strips apart in such a way that each oyster now had a loop at the hinge end with which to hang it.

We then manufactured split rings by winding warmed PVC welding rods around a suitable shaped stick and cutting the spiral apart once it had cooled. We then attached each oyster to each cross of a 1 metre square piece of plastic coated wire weld mesh with 55mm wire spacing. The oysters were now suspended, hinge up, entrance and exit down under the mesh. These pieced of mesh were then stacked with about 150mm spacers in the flow of water from the fish and prawn ponds. With this system the area available for organic material to settle was greatly reduced and even if it did settle on the thin hinge end, it did not plug the water entrance or exit of the oyster which was now down-facing. The stacks of racks were suspended in a concrete trough of about a meter and a half on a side. Usually we had 5 layers of mesh in a stack.

An interesting aside which relates to our present acidification of the oceans was that with a high level of algae production, the alkalinity of the water increased. Not surprising since phytoplankton growth uses up Carbon dioxide which is causing acidification.

November 2011
I haven't read this blog for quite a while.  I just realized that I didn't deal with the problem of an excessive concentration of food in the water.  We got around it this way. 

Our trough where we grew the oysters, was closed on one end and the water flowed out the other end.  Instead of introducing the water at the closed end and letting it flow through the trough, we introduced it all along the trough.  This way, as the oysters removed food, it was replaced and the concentration of food in the water which the oysters experienced was much less that the concentration of food in the water straight from the pond.  This also allowed us to solve another problem. 

Oysters use up oxygen just like any animal.  One of the most effective ways of oxygenating water in a tank or trough is to jet the new water vertically into it.  This jet entrains and blasts air down into the water.  You would think it would cause the water to circulate.  It does but not the way you would expect.  If you blast this water downwards along one side of the trough, you would think that the rotation of the water in the trough would be in the same direction that the jet pushes it.  In fact it is the opposite as the bubble which are blasted down into the water rise up and cause the water to rotate.  To be effective, the pond must be a couple of meters above the trough and pipes must be large enough so as not to loose significant pressure.  Otherwise you would need a small centrifugal pump.  We drew "pipettes" from heated plastic pipes to make the nozzles just as you do with glass tubing.  It was then easy to cut the narrow part of the pipe at whatever diameter you wanted and to make two pipettes from each piece
From time to time, we would pull the plug on the trough and wash down the bottom.  The bottom of the trough was flat but would have been better if it had been deeper at the middle to aid the washing out of the anaerobic mud which collected there.  I never measured it but I suspect a given weight of oysters makes more feces that an equal weight of cow.

Of course the next stage would be to grow a commercial sea weed on the outflow of the oyster troughs.  This occasionally happened accidentally in our system but we didn't sort out a commercial system while I was there.