Sub-tidal aquaculture of surf clams

Project Contact

Seafood Divers Inc.
P.O. Box 1145
Provincetown, MA 02657-1145
(508) 237-0875
seafooddivers@gmail.com

Report Summary

Our primary objective was to develop shellfish aquaculture techniques for surf clams (Spisula solidissima), for use on our sub-tidal grant site in Provincetown Harbor. Our conclusions from this project were as follows:

1. Planting unprotected surf clam seed leads to 100% mortality due to predation.

2. Growth of surf clams in kitty litter trays covered with Tenex mesh is better in Provincetown Harbor than in Pilgrim Lake or Hatches Harbor, and reach market size in less than one year.

3. If shifting sand covers the Tenex mesh, clams grow more slowly, or die.

4. Clams grow well at densities of 1 per 5 sq. inch and 1 per 2.5 sq. inch, but greater density is probably possible.

We are continuing to experiment with larger and more economical ways to grow and harvest the surf clams.

Goals

Our objectives were to:

1) Develop shellfish aquaculture techniques for use on our sub-tidal (20 to 30 feet deep) grant site in Provincetown Harbor to grow an alternative species, surf clams (Spisula solidissima).

2) Determine the maximum density at which seed can be planted for full grow-out to marketable size.

Farm Profile

Seafood Diver's farm is a leased grant comprising two acres located in the Provincetown waters of Cape Cod Bay. Since this project started, two new sub-tidal grants, one acre each, have been laid out and buoyed. One grant applicant is a local fisherman and one is a local diver. Both grants have corridors between them in accordance with USDA and Massachusetts Aquaculture Association Best Management Practices. Both have been surveyed by Jerry Moles of the Mass. Division of Marine Fisheries and subsequently approved.

In addition to surf clams, we have some oysters in trays shelved in five tier racks. Non-aquacultured grant inhabitants are numerous, with cunners and tautog, lobsters, starfish, and crabs, all attracted to the various structures.

Participants

Our technical advisor, Bill Walton, was instrumental in supplying us with the original idea of placing netting under seed clams and has been available and answered our questions throughout the project.

Our collaborator, Dr. Parker Small, has been invaluable; not only was his use of kitty litter trays for growth and density sampling the innovative high point of the project, but he accessed and sampled the trays himself, using scientific diving techniques.
Without Henry Lind, the Town of Eastham Natural Resources Officer and Director of the Eastham Aquaculture Training Center, we would have had no surf clam seed.

Project Activities

The project began with dives to obtain the brood stock for spawn in April, 2005. These clams were conditioned and spawned at the Eastham Aquaculture Training Center and yielded just over three million seed, the majority of which does not survive to planting size.

On June 23rd, 2005, we had a lot of small Spisula seed in the micron size so we attempted to sow it in kitty litter trays @ 70,000/tray at 750 microns. These trays were a nest of three trays constructed so picking up the top tray would leave the substrate (kitty litter or sand), and just bring up the clams when they were larger. We experienced difficulties because the sand and/or seed washed out as we tried to lower the trays in the water. On June 30th, another experiment where we tried to add the clams to the trays prior to planting, did not work. On July 8th, we placed four kitty litter trays on the grant, filled them with sand underwater, and planted them with one mm seed: two trays with 50 seed (one covered, one uncovered), one with 100 seed covered, and one with 200 seed covered. We also placed 50 seed, unprotected, within a 2'x 2' area outlined by leadline.

A year later, June 13, 2006, Dr. Small dove on the density sampling trays. He found no sign of the two 50 seed trays, and the 2'x 2' square leadline was intact with no seed inside its perimeter. He sampled the covered tray w/100 seed and found 85 clams ranging from 35 to 50 mm.
This density study, using kitty litter trays, was a change from our original idea of using different densities in the fabloc bags and the 11 'x 11' nets. With the kitty litter trays we were able to sample densities much quicker and easier than by opening bags or lifting netting and digging. Also, Dr. Small had kitty liter trays in Pilgrim Lake and in the Truro waters of Cape Cod Bay. These sites were accessible by foot at low tide, so the sampling was more often and more extensive.

We didn't get our field plant size Spisula until the end of September when we picked up 9,000 seed @ 9-16 mm (median 12.9 mm). As of the third week in October, 2005, we had nine nylon 4'by 3' Fabloc bags w/ 3/8" mesh with 1,000 seed clams in each. These bags were stapled into the substrate by each corner and had two small floats to lift up the top netting. In early May, 2006, we dove on the grant and found the bags heavily fouled w/calcium concretion and small barnacles. They were hanging from their floats with one or two of the corner staples still anchoring them, but the mortality was greater than 95% We cleaned the bags and recovered some seed, which had grown to market size.

Our last activity on the grant in 2005 was attempting to bury the hauling netting. We did get one in place, but it was so buoyant we couldn't get it covered. Also, our pump, a venturi design powered by a two-inch centrifugal pump, didn't have the power to dig the sand, nor to throw enough sand to cover the netting. In July, 2006, we bought a new type of pump, a venturi pump that is powered by a pressure washer, and it works well.

Instead of the proposed four overlapping sections of 11 'x 11' netting, we used a 5' x 10' galvanized re-bar rack with six-inch sides and lined with small mesh plastic coated wire. We lined the bottom with scrim to hold the sand. We then cut three hauling net sections and placed them on top of the scrim, fastening them to the rack corners with different lengths of plastic ties so we'd know which net to pull and could get three different hauls from this one rack. Next we used our pressure washer pump to fill the rack with sand, covering the scrim and the three layers of hauling net on top of it. After the first net section is hauled, bringing up just the clams, the sand is reseeded. When these clams are ready for market, they're brought up by the second hauling net, then this is repeated for the third net.

In early November we used the hydraulic winch and boom of CHASE, our support vessel, to haul the top layer of netting off the rack. It came over the side with nothing but clams. Our diver, who'd rigged the pick, was off to one side; he saw and recovered the single clam that fell through the net, it had fallen back into the rack.

Results

We've shown that surf clams grow well in Provincetown Harbor, but that they need protection from predators. Growth to market size is achieved in one year at densities of one clam per 2.5 sq. inches. We've also determined Spisula densities by using the kitty litter trays. They are perfect for ease of measurement and recoverability of seed, but are not economically feasible because of their small size. The 5'x10' trays were the best economic adaptation of kitty litter trays. Growing clams in fabloc bags did not work.

Two unexpected results were that the Truro Shellfish Advisory Committee began considering the feasibility of open water grants in their town, right beside Provincetown, on Cape Cod Bay, and we have two more open water grant applications in Provincetown.

Conditions

In December, a high altitude jet stream dove into Lower Cape Cod with tremendous winds that wreaked havoc with the shallow, inter-tidal aquaculture grants in Wellfleet, two towns south of Provincetown. Grants wound up with large tangles of twisted re-bar racks and crushed grow-out trays windrowed on them. Our deep-water farm was another story; twenty-five feet underwater, it was completely unaffected and untouched by the weather.

Economics

Kitty litter trays are not economical for commercial production even if grown at one clam per square inch. They would produce approximately 200 clams and at 10 cents per clam would yield only $20. It would require at least 20 minutes of dive time to plant and harvest them.

Assessment

We've made significant progress in demonstrating that surf clams grow well in Provincetown harbor and are evolving more economically feasible methods of planting and harvesting.
The next step is a comparison study to determine the optimal economic planting/harvesting process among: 1) 5' xlO' trays, 2) lift nets, and 3) hand raking. We also need to begin marketing activities.

Adoption

We don't plan to use more of the 5'x 10' racks as they're expensive. They were intended as nursery trays for another aquaculturist. We may continue using the racks we have already, but future planting will probably have to be directly in sand, either over hauling mesh or bottom planting with harvesting by raking.
Exploration of this more economical approach is one of the critical issues addressed in the next SARE grant.

Outreach

We've had articles published in the two local newspapers, "The Cape Cod Times" (March 21, 2005), and the "Provincetown Banner" (March 17,2005). We've also had the "Times" article published world wide on the net by Harlyn Halvorson (e-mail at: hhalvors@mbl.edu).
We've spoken to our technical advisor, Bill Walton, and will send him a copy of this report for dissemination. We will also present our findings to the Truro and Provincetown Shellfish Advisory Committees.

John H. Baldwin
Nov. 29, 2006