There is growing interest in North America in the farming of shellfish such as clams and quahogs. A variety of husbandry methods can be used, depending on the specifics of the site. The majority, however, tend to be labour intensive.
The method now most widely used consists of planting seeds on intertidal flats, which are then covered with predation netting for protection form crabs. At harvest time, the netting is removed and cleaned for later use, if possible. The clams are harvested in the traditional way using a clam hoe/rake. Reseeding is then done and the process starts over again.
While that method works, it's hardly ideal. The labour portion of the business is quite high and, for very large sites, can be a problem. Further, the predation threat is not always solved with netting because over time there is the possibility that the crabs will find a way underneath it, or, by chance, the net may get damaged.
So, the question is: how can we reduce labour cost and increase yield for this valuable shellfish? Dr. Joth Davis, who runs his own shellfish company, Baywater Inc., as well as working for Taylor United Hatchery near Quilcene, WA, has developed a culture method for Manila clams that answers the question.
The method relies on the use of ADPI CBC-1 bags. While the site specifics have to be considered shellfish growers on the East Coast may want to look closely at this approach. Baywater Inc., which is located on Bainbridge Island, WA, has successfully practiced it for the last eight years.

ADPI Bags for Clam Grow Out

Whether you call it a cage, bag, or box, the ADPI CBC-1 is 36" x 16" x 4 ½" with a 3/8" square mesh.
The Baywater site is on Thorndyke Bay on the Hood Canal. Clam culture is able to be done on the intertidal beach because the tidelands are largely sandy-silt. There is no natural set of clams, so the operation relies on hatchery seed. Relatively productive water in the spring though fall allow the company to practice both clam and oyster culture (rack and bag) at this intertidal site.
The basics of the culture method as described by Dr. Davis, are as follows:
1.   ADPI bags are obtained, which must be assembled into the box shape, using stainless hog rings (5) to seal one end of the bag. The area of the bag is approximately 4.5 square feet. The folded bag depth is generally about 4".
2.   A rectangular stainless steel frame is fabricated, with dimensions slightly larger than the size of the folded bag. It is pressed into the sand at the spot selected for the grow-out bag, similar to pressing a cookie cutter into dough. The sides of the frame enable the sand to be removed, while preventing the sides of the excavation from caving in while the depression is being dug. The sand being dug out is placed into the open end of the bag, which is sitting upright nearby.
3.   When enough sand has been removed, the bag is pinned shut using three stainless steel hitch pins, and, full of sand, is rolled into the depression. If the depression has been properly prepared, the top of the bag will extend above the level of the substrate by about ¾".
4.   The frame is removed from around the deployed bag, and the process is repeated.
5.   The bag is seeded by sprinkling Manila Clam seed, which is 1/4" - 3/8" over the exposed top. Even with the tidal flow, the seed quickly digs into the substrate and tends to stay put unless windy weather is at hand.
It should be noted that clam seed larger that the 3/8" mesh is added to the bag just prior to closing it up and rolling it into the depression.
6.   Stocking densities are going to be site specific, but the Baywater people use approximately 400 clams per bag on their site. That will yield about 18-20 lbs. of clams per bag. They have tried stocking bags with as many as 600 clams, but the shellfish did not grow nearly as quickly.
7.   The rate of growth to market size, which is about 1.5" shell height, is about 2.0 to 2.5 years, depending upon when bags are started.
8.   A good worker can deploy between 12-15 bags per hour. Since Baywater pays by the bag ($1 each), a worker can make good money deploying bags over a tidal cycle. A typical tidal cycle will last about five hours, and at this rate 60-75 bags can be deployed daily per worker.
9.   Bag retrieval is very efficient with about the same amount of time required to remove and harvest as it was to deploy. The bags are lifted slightly from the bottom, where they are shifted back and forth several times to remove as much silt as possible. The clipped end is opened and the clams and remaining silt are dumped into a harvest basket.

Culture economics
The following numbers are based on a per acre basis, and assume that the basic infrastructure for a shellfish farm are already in place. These figures reflect Baywater's approximate labour and materials costs associated with putting the project in the ground.
Cost of Production
ADPI Bags (2000): $7000
Bag Preparation: $1000
Bag Deployment: $2000
Seed costs (1,000,000): $4500
Nursery Costs (100 bags): $1000
Harvest Costs $2000
Total Costs $17,500 / acre
This cost structure breaks down to about $9 per bag. Obviously, the second cycles does not include the costs associated with procuring and preparing bags, so the second cycle costs about $5 per bag to deploy with added seed.
At a stocking density of 400 clams per bag, Baywater realized little mortality and harvest between 18-20 pounds per bag. On a per acre basis, this represents between 36,000 - 40,000 pounds of clams per acre per harvest (2000 bags).
There can be some problems with this culture method, such as the sanding in of bags during the winter and early spring. As a general rule, if you see significant ripple marks on the sand at your site from wind and tide based circulation, you will not be able to conduct clam culture because the bags will be sanded in over a relatively short time.
You must be attentive to the husbandry of the culture method and physically lift up the bags so that the entire outline of the bag surface is free of the sand (at least with short-siphoned Manila clams).
There is also the time constraint of working the low tide time needed to deploy bags. There is never enough time to get the work done, the major caveat of shellfish culture generally.
As you can probably see by now, using this method could save a lot of labour, especially on the harvest end. And, predation from crabs is nonexistent.
Joth Davis and I discussed some other methods of deployment and harvesting that are worth developing, however neither one of us are aware of anybody who has actually worked them as of yet:
1.   It may be possible to lay pre-seeded cages out like patio slabs over a large area of bottom and then, using a pump, discharge water and silt over the top of them. The silt would drift though the mesh of the cage. Normal tidal flow will eventually flush the balance of the sandy silt substrate into the cages.
2.   It may be possible, depending on the type of bottom, to gang a number of pre-seeded cages together in a grid type format so that they may be gently dragged across the site. The movement will allow silt and sand to settle inside the cage as a substrate. Harvesting would involve the simplicity of pulling the grid though the water column to clear the sand and then to open the bags for collection at the surface.
Both the above husbandry methods could be tested very inexpensively in a variety of site conditions.

Joth Davis, Ph.D. can be contacted at Baywater, Inc., 15425 Smoland Lane, Bainbridge, Island, WA. USA 98110, or through my office.

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