Multi-Species Marine Traps
Used Aquaculture & Processing Machinery
Fukui's Monthly News Letter
Selecting shellfish growout gear
In the July/August issue of Fish Farming News I discussed shellfish seed harvesting, the materials used, and how to maximize the seed collection effort. This column focuses on gear selection for shellfish growout. The purpose is to accelerate the growth in order to reduce the overall time to market, and of course, reduce handling and labour cost.
Some farmers will move their seed collection gear to areas that have better conditions for growout to maximize the early stages of development. Depending on the species and the site, it is generally desirable to move the seed or juveniles into the next stage growout gear as soon as possible.
Shellfish gear is usually available in a variety of mesh sizes. In deciding what to buy, it makes sense to use the largest mesh size possible relative to the size of the shell of the animal in order to minimize fouling and maximize growout.
The net mesh used in shellfish gear is measured on the square. So, when making the mesh size selection, remember: diagonal measurement must be less then your smallest seed.
If you choose a 6 mm mesh, the measurement on the diagonal will be 8.48 mm. Your smallest seed must be larger then that. (To convert: millimeters x 0.4 = inches)
The rule of thumb is to allow a 20 percent increase in calculating the minimum size of shell that will be retained by a given diagonal measurement. For example, the diagonal measure of 8.48 x 1.20 = 10.17mm as the minimum seed size for that mesh. If you have a 3 mm. mesh, the diagonal would be 4.24 mm. x 1.20 = 5.08 mm. minimum seed size.
A simpler way that is used quite often is to just take 1.5 times the mesh size to get a minimum seed size that is generally accurate. Using the mesh sizes from the examples above: e.g. 6 mm. mesh x 1.5 = 9 mm. minimum seed size; 3 mm. x 1.5 = 4.5 mm. minimum seed size.
Once you have selected the mesh size then you must make a gear choice. The selection of the gear is important to the success of your crop and it is site-specific.
With sea scallops, growout options include ear-hanging, bottom seeding, or using lantern cages or pearl nets, which I'll describe here.
When sea scallops come out of the spat collection bags in late spring or early summer, they can range in size from 3mm to 10mm on average. They are transferred into lantern cages or pearl nets, which are available in mesh sizes from 1.5mm to 21mm.
Lantern cages, which are usually considered for shallow water applications, are available typically in 5, 10, or 15-level models. There has been some availability of 20-level cages, however the weight towards the end of the growout cycle is a limiting factor for ease of handling.
Round version lantern cages have been around for decades and are available either with a horizontal, flap-type opening, "Shibitsu," for servicing each level; or a vertical opening that must be stitched closed.
The new square version allows more surface area per level (pizza box theory: round pizza-square box, corners are extra area) which allows you more growout space. Since they come with one side, which opens almost completely and re-closes with a reusable snap clip, they are extremely efficient with respect to growout yield and reduced labour cost.
It's easiest to think of lantern cages as "mini apartment buildings" for grow out of scallops.
The cages can also be used for oysters, over wintering clams, and even mussels where duck predation is prevalent.
The deep water pearl nets are also very common in sea scallop growout and within the aquaculture shellfish industry generally, are likely to be one of the initial pieces of gear ever used.
They are available in a square version with an opening along one of the top seams that requires stitching, and in a round format with a drawstring to close the opening.
Pearl nets are deployed by attaching many vertically along a set line approximately 10" apart. Up to 30 nets at a time can be strung together, though 15 or 20 are more common.
Essential net fabrication features include: wall heights must be adequate, typically 2 to 3 inches; there must be sufficient suspension rope; and the growing table should be flat when deployed. If you see nets offered for sale that are lacking in one of these areas be very wary of their quality.
Pearl nets are very inexpensive, running from less than $2 to $3 each. The price of lantern cages can range from $27 to $110, with the higher costs associated with cages with more levels and/or small mesh. In deciding what to buy for your scallop growout operation, remember the gear's value is not what it cost, but what it's worth. In actual usage trials, the more efficient square lantern cages were found to have a 40 percent savings over round lantern cages and up to a 70 percent savings over pearl nets in labour and handling costs.
In the next column I will explain more about the growout gear used for oyster, clam, and mussel farming.
A Japanese scallop growout cooperative with 80 farmer members produced 800 metric tons (mt) of meats in 1994. Canada, as a nation, in 1993 only produced 14 mt of scallop meats from farm operations, with the US numbers on a per capita basis almost the same.
The methods used by those northern Japanese farmers will be discussed in a future column. However, with respect to gear, they were the first people to identify and implement the difference between shallow water cages and deep water nets.
Contact Don Bishop at:
Fukui North America
PO Box 669
110-B Bonnechere St.W.
Eganville, Ontario K0J 1T0
**NEW**Tel: 613-559-0075 or 613-628-5266
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