Sub-tidal aquaculture of surf clams
Seafood Divers, Inc.
POB 1145, Provincetown
Northeast Sustainable Agriculture Research and Education Program Farmer/Grower Grant Interim Report for:
FNE05-542: Sub-Tidal Aquaculture of Surf Clams
(2) Restate the goals of the project
Our first goal is to adapt inter-tidal shellfish aquaculture techniques to the sub-tidal environment. Sub-tidal aquaculture is a feasible alternative that avoids the problems of traditional inter-tidal aquaculture such as competing with recreation, winter ice damage, and run-off pollution. Inter-tidal (the area exposed between high and low tide) aquaculture on Cape Cod is accomplished using the same methods as 100 years ago; one of the few changes is that instead of horses and plows on the tidal flats, there are now pick-up trucks. It is currently used almost exclusively to raise hard shell clams called littlenecks (Mercenaria mercenaria).
The second goal of this proposal is to use an underutilized species of shellfish, the sea clam or surf clam (Spisula solidissima), that can be planted and harvested either inter-tidally, or sub-tidally, and which actually grows better when always underwater. They’re currently harvested full grown (5”- 9”) offshore by large vessels using hydraulic dredges in a public fishery, and the environmental effects of the method are the subject of debate. The adult clams are marketed as clam strips and chowder. Although dredged sea clams must be >5 inches for sale, the Massachusetts Division of Marine Fisheries has issued a permit to Seafood Divers (#1557) to raise, harvest and sell sea clams at 1.5 to 2 inches. Sea clams reach this size in about one year, and are called “butter clams”. Methods of growing these yearling surf clams on inter-tidal shore grants exist, but their thinner shells compared to hard shell littlenecks complicate harvesting. These butter clams are the product that Seafood Divers has been growing experimentally and selling on a pilot scale since 1997 on its sub-tidal site in Cape Cod Bay. The method has utilized Fablok bags and has required splitting the crop several times as the clams grow. The final density of clams has been well below the possible maximum and hence required much additional work. For sub-tidal aquaculture of sea clams to be profitable, underwater planting and harvesting methods must be developed that enable us to produce clams in commercial numbers using a minimum number of dives to plant, tend and harvest the crop.
(3) Update information on farm since project started
The project began on April 1st and 2nd, 2005, with scuba dives in Herring Cove, just inside Race Point at the very tip of Cape Cod where we obtained 15 surf clams to bring to the Eastham Aquaculture Training Center, the hatchery. These were our brood stock and just one of them spawned over three million seed.
Over the course of the summer and fall we seeded our two-acre grant with surf clam and American oyster (Crassostrea virginica) seed. We had a lot of small Spisula seed in the micron size so we sowed it in small trays @ 70,000 per tray at 750 microns. We guessed that it was futile but our collaborator Dr. Small is a true scientist and we gave it a try. The oyster spat we bought from a hatchery, Muscungus Bay Aquaculture, in Maine, 50,000 @ 1.5mm. We put the seed in .75mm spat bags stretched inside grow-out trays, 5,000 to a tray.
We didn’t get our field plant size Spisula from the hatchery until the end of September when we picked up 9,000, 9-16mm seed (median 12.9mm) and 10,000 6-10mm seed (median 7.5mm). Our oyster spat didn’t do well and we reduced the grow-out trays to four, with spat varying from 2 to 10mm and attached to the dead spat shell and thus difficult to size or count.
With the help and advice of the shellfish constable, Tony Jackett we relayed a bunch of large oyster seed from a contaminated area to the grant site with some smaller seed, too. As of the third week in October we had 28 grow-out trays of large oyster @ 100-125 bag, and one tray of small @ 290.
For Spisula we had 5 grow-out trays @ 2,000 each (median size 8.5mm) and 9 nylon 4’x 3’ Flablok bags (3/8” mesh”) @1,000 each (median size 13mm). These bags were stapled to the substrate by their corners with small floats centered inside each to lift top netting of bag off bottom.
The grant is a two-acre square with two leadlines crisscrossed in a big X, north to south and east to west. This makes it easy to navigate and chart placement of the various bag and tray groups, etc.
(4) Describe cooperators and their roles in project
Our cooperators were aquaculture divers Nancy Jacobsen and Rick Macara, and our diving collaborator, Dr. Parker Small. The project leader, John Baldwin, and Parker did the majority of the hatchery-to-grant work, its planning and recording. Our divers aided us greatly in all other phases of the project, from predator control to grow-out tray and bag handling and disbursement. We also had Mike Welch, our vessel operator and able topside tender.
(5)(6) Describe work accomplished and work to be completed
We stated two planting methods in our proposal: 4’x 3’ Fablok bags and four (4) 11’x 11’ mesh squares buried in a runway. We’ve placed nine (9) Fablok bags on our site, with approx. 1000 (13mm) seed per bag. We’ve also placed a six-tier grow-out rack with five (5) grow-out trays of 9-12mm seed @ 2000/tray.
We’ve constructed and placed the runway and have one 11’x 11’ black mesh pick-up square stapled to it. The mesh is made up of multiple strands of polypropylene and is very strong; that’s why we chose it, but it has neutral buoyancy and floats on even a slight current. Because of this we plan to cut and hem three Fablok bags to 12’x 12’ and place these in the runway. They’re 3/8” nylon, strong, and are negatively buoyant.
After the pick-up mesh squares are in the runway, we’ll cover them with substrate and sow them with the seed from the grow-out trays. These seed should reach market and harvest size by late summer. We can then lift both the runway pick-up meshes and the Fablok bags and quantify our results.
Finally we’ll have a big clambake to publicize our project and invite you all!
(7) Describe specific site conditions that may have affected results
In December, a high altitude jet stream dove into lower Cape Cod with tremendous winds that wreaked havoc with the shallow, inter-tidal 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.
(8) Describe economic findings
Economic necessity compels us to grow oysters, the “gold standard” of shellfish. The sustainable aquaculture of “butter clams” is completely experimental; growing them is only the first step, marketing them follows.
(9) Did results generate new ideas; next step?
One idea to emerge is that we need a better method of covering the runways with sand substrate. We modified our centrifugal pump and fabricated a venturi tube for the intake but it doesn’t have enough power; a larger pump should be the answer.
Once this project is complete, a comparison study should be set up to determine the time/quantity relationship between picking up nets and hand raking. Hand raking is very fast underwater because the divers rake kneeling with short rakes, but runways still have to be constructed, placed, then covered with net after sowing, and divers are expensive.
Project leader: John H. Baldwin 01/19/06