Intertidal culture refers to systems in which the culture animals are exposed to air during the low tide of each tidal cycle. These systems include both bottom (beach) and near-bottom (epibenthic) techniques. A variety of intertidal grow-out sytems have been developed for oyster culture besides beach distribution of seeded shell cultch. These include oysters grown on stakes, racks and intertidal longlines. Near-bottom methods have frequently been adopted on sites where the bottom conditions (e.g. soft mud and/or silt) are not suitable for oyster growing. For information on these systems, consult the references listed on the oysters page. Nursery rearing of oysters on shell cultch or tubes is often done intertidally. See the page on nursery culture. What follows refers primarily to beach culture of Manila clams.
Assessing a Site
Shellfish culture sites, both intertidal and deep water, are assessed in terms of suitability and capability. Suitability entails an assessment of viability in terms of socioeconomic, resource use, infrastructural and marketing factors. Capability involves an assessment of the site in terms of environmental (biophysical) parameters. The biophysical variables can be classified according to the way in which they impact upon the culture operation. Temperature and food availability directly affect shellfish growth. Wave height, grow-out position relative to low tide, suspended sediments, water movement, disease prevalance, fouling potential, predation potential, substrate, and beach slope directly affect shellfish survival. Salinity, dissolved oxygen and pH indirectly affect shellfish growth and survival. Growers should also look at the site at different times of the year to assess the seasonality of the biophysical parameters. The effects of a winter storm on the proposed site, for example, will affect the overall assessment of site capability. For more information on site evaluation see Cross and Kingzett 1992.
Site Husbandry and Management
Managing and maintaining productive intertidal growing areas is no different than it would be in agriculture. The substrate will likely need improvement and this will take time and resources. It will need to be cleared and prepared for planting and those areas may be fenced or defined. A strip of rigid plastic mesh such as Vexar�, 8-12″ above and 4″ buried below the substrate held in place by re-bar pins will prevent stock from being washed out. It may also be possible to construct a breakwater berm 50-100m out from the plots to protect them from storm damage. A landscape rake can be used to clear the plots of debris and rocks and, if machinery accessible, plows used to turn the substrate to a depth of 6-8″. The plots should be observed and tended regularly. Growers should maintain an inventory control system, knowing what was planted when and how it is performing.
Specific product management principles will apply to oysters and clams. Growers should be able to do an inventory of stock, plan for harvesting, plan for seeding, track the effects of specific husbandry practices and determine at any given moment the value of the stock on site. Accurate sampling techniques using some basic statistical principles entered into a basic spreadsheet program will give the grower all the information needed for inventory control. From a random sampling (e.g. 30 samples) of density and size measurements a reasonable estimate of stock can be calculated.
Management for Clams
Many if not most intertidal leases capable of clam culture now support clam rather than oyster farming due to the current high market value of Manila clams. Both oyster and clam farming may require substrate improvement to reach acceptable levels of productivity. Clams will perform best in a substrate composed of a mixture of mud, sand, pea gravel and some shell fragments. Improvement for silted beaches may mean gravelling while for other sites it involves debris and rock removal.
Seeding of clam seed in prepared plots is usually done when seed has been reared in nursery sytems to an appropriately large size. Growers have generally found that the larger the seed planted the higher the survival rate. But clams that grow in nursery systems without substrate begin to develop deformed shells and die when they reach the 25mm range so growers target a 20mm size for transfer from nursery to seeding.
After seeding the plots are protected from predators by overlaying them with a plastic mesh cover commercially called “car cover” (so-called because it was made to put over open-top rail cars loaded with sawdust or wood chips). It is strong and lightweight with a mesh size of approximately 5/8″x3/4″.
It provides protection for the clams from their main predators: scoter ducks, starfish and crabs. Unprotected areas can lose as much as 90% of the clam stock to ducks. They are unable to scoop into the substrate to remove clams when the net is covering the surface. The netting is usually put out in a double layer to make it more durable. The periphery is threaded (woven) with a lead line (0.25-1.0 lb/fathom) which is secured to the mesh with cable ties (zap straps). This firms up the edges of the mesh so it can be handled easily.
Rocks and re-bar pins can be used to secure the netting in place. The netting laid out in uniform plots also provides a means of controlling and managing production.
Seeding clams into the plots is usually done in the spring. Seed is generally planted at the 5-8mm size at a density of 200 to 600 per square meter. Seed that starts in the spring in a nursery system at 6-9mm should be 20-25mm by fall and can be planted out. Seeding density is done in proportion to yields. Some mortalities are inevitable and should be factored in. Losses from 6-9mm seed through to harvest are usually calculted to be 40-50%. Besides losses to predation, thick summer growth of micro or macro algae on the plots may contribute to some mortality.
Some sites will benefit from natural recruitment of Manila clams that may partially offset losses.
For more details of intertidal site management and clam culture consult these documents. Aquaculture Factsheets and Pamphlets from the British Columbia Ministry of Agriculture, Fisheries and Food (Commercial Fisheries and Aquaculture Branch) include:
- Proceedings of the 1990 Manila Clam Culture Workshop, 1990 (Aquaculture Industry Development Report 90-09, AFF0407).
- Manila Clam Culture – Pilot Scale Site and Husbandry Assessment, 1990 (Bulletin 34 – AFF0385).
- Manila Clam Nursery and Growout Operations in Washington State, 1990 (Bulletin 32 – AFF0338).
- Manila Clam Aquaculture in British Columbia: Information for Getting Started, 1992 (Aquaculture Industry Development Report 92-01, AFF2899).
- Estimated Costs and Returns for a Clam Grow-out Enterprise, 1993 (Aquaculture Industry Development Report 93-05, AFF3107).
- Estimated Costs and Returns for a Seeded Clam Grow-out Enterprise, 1993 (Bulletin 40 – AFF1749).
- Cross, S.F., Kingzett, B.C. Biophysical Criteria for Shellfish Culture in British Columbia: A Site Capability Evaluation System, 1992 (AFF2965).
Toba, D.R., Chew, K.K., Thompson, D.S., Andserson, G.J., Miller, M.B. 1992. Guide to Manila Clam Culture in Washington. Washington Sea Grant (Publication # WASHU-H-92-001).