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Selecting shellfish growout gear (part 2)
In my previous column, we began a discussion on selecting shellfish growout gear that focused on sea scallops. This column continues with oyster, clam, and mussel gear options.
But before moving ahead with those species, I want to add a thought on scallops - bay scallops. The same equipment used with sea scallops can and is used in farming bay scallops, which have a much shorter growout time. The difference, of course, would be in the mesh sizes and the specifics of the site.
Oyster farming involves three stages: seed, nursery, and growout.
Gear can be very diverse and is determined by the husbandry method used by the farmer. Site specifics and the regulations with regard to farming the site are important considerations.
Oyster seed supply can be from a hatchery or from wild seed collection. Polyethylene Chinese Hat Collectors, (which incidentally originated in France, however, the name was adopted due to the similarity in shape to Chinese straw hats) or a Polyethylene plenum are examples of the gear used for wild collection. Both are dipped in a mixture of cement, lime and sand to simulate the surface that the oyster spat like to set on.
Once the source of the seed has been determined, the next step is to get the oysters into a nursery habitat as quickly and as early as possible. Nursery time maximizes growth by providing the animals with the highest quantity of nutrients as possible, sort of like force-feeding them.
Oyster nurseries can be dynamic or static as well as land or water-based depending on the method and access to equipment the grower has.
A floating upweller system (FLUPSY) is a containment area for shellfish, in which food-rich water movement is increased from bottom to top. FLUPSYs can be used in either the land or water applications. However, the ones deployed on water in natural habitat produce quicker growout times and better yields on average, and the decreased need for handling reduces labour costs.
A FLUPSY can be as simple as a small pump forcing water into the bottom of a half barrel or fiberglass tank, which then flows out the top. There are more elabourate FLUPSY'S costing several hundred thousand dollars, such as the ones in use on the West Coast. Those systems are fully mechanized and designed so as to be movable from one site to another. Operated by only two men, 15 - 20 million animals can be grown from 2 millimeters (mm) to 20 millimeters in four to five months.
There are also different methods of static wild nurseries. They can be round pearl nets or lantern cages containing as much as 1,500 - 2,000 seed per level deployed in the water column for six to eight weeks. Then the oysters need to be sorted and transferred to larger mesh cages for growout.
What works well in some nursery areas are ridged plastic cages made of Durethene® , which is a hybrid polyethylene, with very small, replaceable mesh and spat collection bags inside to hold the small seed. They are assembled by dual longlines and are allowed to float on the surface until reaching the desired size for growout.
One of the newest static nursery systems, in use on the West Coast, involves using 0.75mm spat collection bags inside hanging stackable tray systems, which are also used for final grow out. As many as 30,000 seed at 1.5mm are loaded in each bag for six weeks, or until the growout cycle is accomplished. The cost of equipment in this system is very low and the labour reduction vs. output is significant.
Once out of the nursery stage, there are several methods possible for final growout: tidal flats (French Method), using Durethene® plastic bags/boxes; by hanging culture from longlines or rafts using lantern cages or stacking trays; or by natural bottom seeding.
There are also areas where a combination of these final growout techniques are best to maximize yield.
Care must be taken in the selection of equipment that best suits your operation with regards to mesh sizes, which must be as large as possible to allow good water flow, especially in heavy fouling areas.
Clam growout gear is limited due to the natural method of farming (ranching) and the need for a mud or sand bottom.
Lantern cages and stacking trays are used in nursery applications especially over wintering applications where mud and sand flats can be damaged by winter ice.
Either oriented plastic or woven netting is used for predation control from crabs and ducks, and is quite effective. Each are relatively easy to install and reusable if the proper quality of mesh is obtained.
Some farms have also experimented with tray systems that have layered mud or sand at the bottom of each level. The method has limited popularity so far, though there is some merit to be considered.
Mussel farms use some of the least expensive gear in the industry from the standpoint of capital cost and ease of use.
Seed is almost always obtained from wild collection and is done using fuzzy polyethylene rope. The new high-density artificial seaweed material, which has a much greater surface area then the fuzzy rope, can also be used. It allows approximately 25 times the seed collection per meter than the rope, and is site specific.
Mussel growout is usually done in oriented plastic tubing (socking) which is available in four mesh sizes. A similar woven material comes from Europe that is also very strong in the lateral context.
One very unique socking - common in New Zealand - is a polyethylene fuzz rope inside a dissolvable cotton sock. There are significant labour savings with this kind of system, but high additional capital costs since it must be deployed mechanically. For overall productivity, this method seems to be the current leader.
All the socking allows the mussel to migrate outside the sock, or to be exposed freely to the water column within a short time span.
In areas where duck predation is extreme, lantern cages, even though they are more costly then the socking, are used quite successfully. Ducks are unable to get a free meal when the mussels are inside the lantern cages.
Remember that shellfish aquaculture is very site specific and that you need to experiment in some cases to find out what works best for you. Look at what successful growers are doing and learn from them.
Contact Don Bishop at:
Fukui North America
110-B Bonnechere St.W.
Eganville, Ontario K0J 1T0
Email: firstname.lastname@example.org or email@example.com
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