Growing Oysters in the Outflow of Mariculture Ponds

Mariculture ponds which are 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) by how much fish is produced. Conversion coefficients in the range of 2:1 are common with all animals including fish. 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. Note that 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 Carbon dioxide. 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. I suspect an addition of Sodium silicate (water glass, isenglass) would help and possibly an addition of ferric chloride or sulfate in some locations.

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 the normally desired food items is too great, much of this good food is also 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 it is and 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  all the work is in comfortable conditions on land rather than at sea where you are exposed to whatever weather is throwing 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.  And a fast growing oyster has very sharp edges.

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  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 on the bias once it had cooled. We then attached an 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.

 

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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 in at the closed end and out the other end.  Instead of introducing all the water at the closed end and letting it flow through the whole length of 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.  Such a jet entrains and blasts air down into the water.  You would think it would cause the water to circulate and so 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 the opposite occurs as the bubbles which are blasted down into the water rise up and cause the water to rotate in the opposite sense to the jet.  To be effective, the pond must be a couple of meters above the trough and feed 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 black plastic pipes to make the nozzles just as you do with glass tubing.  It was then easy to cut the drawn 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 oyster racks and the trough 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.

Algae Culture - Alternate Systems

I learned my mariculture based on the traditional western model. Growing larval fish, crustaceans and mollusks into juveniles is a core part of most mariculture ventures and most of them use cultured, single cell algae as part of the feeding regime. However, through the years, I have come across three alternate systems which were very impressive. First, though, what is the classic "western" system of algae culture. 

 

"Western" Algae Culture" One usually starts out with a sample of sea water either from the wild or from a contaminated culture which contains at least some cells of the algae species one wants to culture. This sample is serially diluted with sterile, nutrient-spiked-sea-water and sub samples of this water are put into a large number of small glass vessels (10cc test tubes for instance). The dilution is done sufficiently so that there is an expectation that there will only be one cell of the desired species in at least some test tubes. The rack of test tubes, corked with cotton wool to allow air exchange while excluding contaminating organisms, are incubated on a light table for an appropriate length of time. 

 

Microscopic examination then determines which test tubes have been successful in growing the desired species and these samples, individually or pooled, are used to inoculate larger vessels (Erlenmeyer flask for instance) Three to five stages are typically needed to bulk up the algae to a useful quantity for feeding to larval animals and in the larger cultures.

 

Carbon dioxide enriched air is often bubbled through the culture. In a modest sized algae growing facility it is typically the full time job of one or more people to manage the algae cultures. The first alternate system I saw was a Bio-stat system.

  

Biostat System The biostat system is closest of the three systems to the classic western model. Algae are isolated and bulked up in the same way. However in the last stage, a growing vessel called a bio-stat is inoculated. A bio-stat (possibly better named a bio-dynam), is a transparent or translucent vessel which is sealed except for two inlet ports and two outlet ports. One inlet port allows the constant in-flow of sterile (or very well filtered) carbon dioxide charged air and the air is typically inserted through an air stone at the bottom of the vessel. A second inlet port allows a continual stream of nutrient charged, filtered and sterilized sea water to flow into the bio-stat. 

 

One outlet port has a "brewers trap" to allow the excess air to exit and the second outlet port lets out the overflow algae culture into a holding container or directly into the larvae culture. It can be shown mathematically, and experience supports the fact, that the maximum algae production (not the maximum concentration) occurs when the exchange rate is the same as the doubling time of the organism which is being cultured. In algae cultures, this is typically a few hours to a few days, depending on a number of environmental factors, all of which the operator endeavors to control. Bio-stats don't always work and if the "clean technique" of the operator has slipped somewhere along the line, contamination can occur and the culture crashes or takes off with the wrong organism. However once a bio-stat is up and running, it can operate for a very long time (months), producing log phase algae with a minimum of effort.

 

Log phase refers to the sigma curve of growth and is the part of the growth curve where the algae are growing at more or less an unrestricted rate. Fast growing algae generally have the best food value and freedom from toxins. They are the "tender young peas" of the mariculture trade, rather than the old woody carrots. 

 

Besides being great production vessels, bio-stats are excellent for research. Very subtle changes can be made in the inlet parameters such as temperature, light, nutrient mix and so forth and the results easily monitored, even automatically, with something like a spectrophotometer set to the wave length of chlorophyll absorption. Of course much more elaborate observations can be made from microscopy to sedimentation volume through to chemical composition . The best example I have seen of this system was at Seasalter Oyster Hatchery in Kent in England. John Bayes was the manager and he had bio-stats consisting of plastic sleeves of about 3m circumference and 2m high. As I remember, there were about ten of these in his light room and he had banks of fluorescent tubes immersed in the bio-stats to increase the light availability. The sleeves were supported by cylinders of wire weld mesh with about a 2" hole size. Amazingly, he could insert a needle into the plastic for the removal of a sample or the introduction of more air without the sleeves splitting. At the bottom he would pour a cylindrical vessel of the same diameter as the sleeve, full of runny concrete, put the bio-stat in place and fill it with water. The concrete then hardened with the shape of the bottom of the bio-stat and from then on, perfectly supported it. As far as I remember, John and his assistant, Rayner, ran the whole hatchery by themselves, including the algae culture and all the other units that made up the operation, producing bivalve spat. The next method that impressed me was the "Chesapeak Bay" system. 

 

 Chesapeak Bay System This system was used in the Eastern Shore laboratory of The Virginia Institute of Marine Science. Mike Castagna ran the unit and when I visited, many many years ago, he had successfully raised the larvae of well over a hundred different mollusks (including the Indian Wampum, otherwise known as the jingle shell) Mikes system was pure elegance in the sense that a mathematician or a physicist talks about elegance. In other words, a system that solves a problem with less steps than has hitherto being discovered. Mike took sea water straight from the ocean and pumped it into open tanks. The water was first filtered to eliminate large algal species and to mainly let through small diatoms. Standard nutrients were added, making sure to include sodium silicate for the diatoms. The open tanks were located on a balcony around the top of the laboratory just under the roof and the roof was translucent. It was probably fiberglass. At that time, I don't think poly carbonate was available. I seem to remember that he bubbled air into the tanks but I don't think he even enriched with Carbon Dioxide. I suppose for the short blooming he used there was enough carbonate in the water and the introduced air. 

The water was only allowed to 'grow' for a day or two before it was used and if you looked through the tanks they showed a sort of milky cloudiness. They hadn't even turned visibly green yet. Talk about tender young peas. Mike probably raised more different species of mollusks with this system than anyone else has even attempted. The third impressive system was one I saw in a Thailand prawn farm.  

 

The Eastern Prawn Algae System The algae grown in this system is the chain diatom Sleletonema sp. which fortunately, larval prawns take to at an early age. Under the microscope one can occasionally see a larval prawn riding a strand of algae like a lion on a buffalo and eating it from one end. The critical fact that allows this type of culture is that the skeletonema can be filtered from a water stream by what can only be described as a pillow case. 'Pillow cases' of the correct material are available at all hatchery supply stores in the East. 

This sort of culture is carried out in open tanks of about 2 to 5 cubic meters, sometimes in a special room with a translucent roof on the sunny side of the hatchery. I saw one hatchery, though, with the tanks under the open sky outdoors. The usual algae culture nutrients including sodium silicate are used and in Thailand it is common to filter the incoming water through a sand filter. Tanks are typically painted blue or white inside. Enough aeration is supplied to keep the water moving. For inoculation and feeding, a "pillow case" is tied with a string around the end of the outflow pipe of a previous successful culture and the outflow pipe (on the outside of the tank) is lowered to the ground and allowed to flow for 5 or 10 minutes. The pipe is hitched back up and the pillow case removed. Inside will be a thick soup of skeletonema with some unicellular diatoms. 

 

The algae is re-suspended in a bucket of water and either fed to the prawn larval tanks or used to start a new culture. Typically, each culture must be renewed at the most after 2 days as the growth is so fast. Three day cultures are usually over the hill. The filtration of the incoming water through a sand filter is mainly to remove animal larvae which are not wanted in the prawn culture. Sand filters are typically the size of small swimming pools and produce sparkling clear water from the very opaque water of the gulf of Siam. 

The filtering through the pillow cases continually cleans the culture. There are always other diatoms and some other algae growing in these cultures but the inoculation of each new culture is massive and the other algae selectively pass through the pillow cases so every transfer is a re-cleaning of the culture. I imagine that another chain diatom such as Chaetoceros might cause problems with this sort of culture but I never observed a problem. In a prawn farm I ran, the running of 6 such tanks was the work of at most, half an hour a day and there was always an abundance of algae available. 

 You mustn't think I am knocking the western system. It is a great research tool, especially if your interest is in, for instance, finding out the nutritive quality of a particular species of algae. However for a working mariculture system, cost is always important and a system which eliminates one or more full time jobs can be critical. Even if these systems occasionally fail, the success over most of the time more than makes up for the occasional glitch. I think, sometimes, the problem is that we westerners are control-freaks. We want to have absolute control of everything that happens. We may, for instance, in our homes, eschew pure solar water heating for electrical heating since we aren't willing ever to be without hot water. It may be the same with algae culture. We may not be willing to operate a little closer to nature when we have been used to having complete control. I suspect, with the need for sustainability, a philosophical shift may be necessary in the west. I know that since we put solar water heating on our roof, we have become a little more conscious of the weather outside. A small step but perhaps the first of many. In the case of algae culture, the above systems were so successful that they gave their practitioners quite an economic advantage.