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Fukui's Monthly News Letter
Collecting shellfish seed
Fundamental to successful shellfish farm operation is having access to healthy seed (spat). With almost all shellfish species, seed can either be collected from the wild or produced in a hatchery. Depending on logistics and cost, the operator will decide on which source works for his or her farm.
The collection of seed (spat) from the wild is done with materials specific to the target species. The choice of substrate, which is the surface or type of material the shellfish larvae attaches itself to, will depend on the setting demands of the shellfish species and the site specifics where larval stage development is occurring.
The most desirable substrate is a material that has the highest surface area as possible and at the same time has a texture or physical property with the ability to attract and retain the larval stage of the species. There have been many improvements in substrate materials, and that development is expected to continue as the need for the precious seed increases with the rapid growth of the industry.
The specifics of the site where the seed is collected are equally important to success. Parameters to consider include such things as current direction, water depth, relationship to existing seed-producing wild stocks, the best time of year for optimum collection, etc. They can be determined by doing some experimentation with known data from existing agencies or individuals familiar with the species and the geographical area.
In general with respect to seed collection in all species, there has been a shift in thinking to use materials that will maximize collection. Some of the older, more traditional materials used may have a low cost, but they just don't produce enough seed per collector to meet industry demands.
Mussel seed collection has typically been done using an extruded polyethylene mesh material or used polyethylene/nylon rope that is worn enough that its surface has a fuzzy appearance.
The theory of the fuzzy rope is that it provides a higher surface area and a way of byssal thread attachment for the larvae.
While this method is widely used and the cost is relatively low, there are some materials new to North America but have been used in Japan for more than a decade that are able to increase the surface area tenfold. The "Artificial Spawning Seaweed," which was just introduced to some mussel growers on the Atlantic coast for testing, has the potential to collect seed at much higher counts per meter with less labour.
The structure of artificial spawning seaweed is a very fine polypropylene mesh in a flat, very thin ribbon approximately ¼" x 6" that is attached to a main line hanging rope at its center point. It is reusable and easy to strip and maintain.
Scallop seed collection is accomplished by deploying a polyethylene mesh bag, stuffed with either discarded nylon netting or a specialty material called Netron, at known collection sites.
The scallop larvae, once released, move around at the mercy of the currents for about 40 days, and then they set.
The idea behind the collectors is that the larvae will pass through the mesh bag and attach to the nylon net or Netron. They start to grow, becoming larger than the mesh material, and are collected in the bag. Spat sizes will vary and are dependent on the ocean location's nutrient richness.
Collections can vary from less than 100 to more than 500 seed, with 800 being a cut-off point for seed.
In the early days of scallop spat collection, onion sacks were used because they were easy to find. The problems with onion sacks were that they could be used only once, and they had inconsistent mesh sizes. That made them less than desirable when it came to the collection of the valuable seed.
The bags used for scallop spat collection are still called onion sacks by some. However, today's true spat collection bags are made from knotted polyethylene. They measure 40 centimetres (15.75") by 80 centimetres (31.5"). Mesh sizes available are: 0.75mm for very deep water or slow growth areas; 1.5mm for standard growth; and 3.0mm for rich oceans.
Fouling can slow growth rates, especially in rich water. So, the larger the mesh, the less the fouling and of course, the quicker the growth. It is important, however, to experiment with collection counts at your site to check which mesh size will give you the optimum results.
Until a few years ago, it was the opinion of the industry that old discarded nylon monofilament fishing nets were likely to be used in spat collection bags because they were free.
But Netron is now more likely to be used. The advantages of Netron include:
Oyster seed collection is typically done with "Chinese hat collectors." They are round plastic stackable discs covered with a cement limestone-type mixture that provides a textured surface which is favorable for oyster spat to set on. Once the seed has started to grow the plastic discs are flexed so that it breaks free. The seed can be sorted according to size and shape.
An interesting note is that the oyster industry has started to use 0.75mm mesh scallop spat bags to grow out seed in the wild. The growers say that there is far less labour and that the seed grows much more rapidly than it does in a nursery environment.
Contact Don Bishop at:
Fukui North America
110-B Bonnechere St.W.
Eganville, Ontario K0J 1T0
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