A Successful harvest is the outcome of good farm planning, management, and husbandry practices that begin well before the seed is ordered and extends throughout the rearing process. Harvesting methods for farmed shellfish in British Columbia range from labour-intensive hand harvesting to machines requiring a crew of several workers to operate. For animals that are grown out intertidaly, primarily clams and oysters in British Columbia, hand harvesting is still the norm, though there are on-going attempts to mechanize. Deep water culture has also proven to be a challenge for harvesting. However, significant developments in this area have been emerging from the efforts of growers in BC.
Beach Harvesting Clams and Oysters
High value intertidal shellfish aquaculture tenures are being increasingly utilized for higher value crops, mainly Manila clams. Clams can be nursery reared in the water column in upwellers, trays or other devices. However, they must eventually be transferred to an intertidal area where they are able to burrow into the substrate and continue growth. At harvest the clams must be dug and separated from the substrate.
In France and other areas of Europe, cultivated clam beaches are accessible to machinery and mechanical clam harvesters are common. In British Columbia the development of mechanical harvesting of clams will depend upon the slope and substrate characterisitics of the beach and the possibility of land access. Many clam leases exist on small, isolated beaches with no land access. In some cases the substrate may be composed of a variety of materials ranging from mud to rock. In these instances a mechanical harvester will need to be small enough to be transported by and off-loaded from boats. Progress towards the development of mechanical harvesters has been made in British Columbia and a few prototypes have been built.
At present almost all clam harvesting in BC is done by hand. Rakes with precisely spaced long tines and shortened handles allow harvesters to quickly move through the substrate and hand pick the clams. Despite the speed at which some harvesters are able to work, it is still expensive and physically demanding. Some growers have devised special hand tools to deal with special conditions at their clam beaches.
Rearing oysters intertidally is often done in conjunction with clam culture on the same sites. Oysters can also be spread in areas where substrate is not suitable for clams. Some growers who culture oysters in deep water transfer some of the high grade stock that can be sold as singles to intertidal areas for conditioning. This may be from a few weeks to a few months. This process will remove most of the fouling and harden the shell. Harvesting these oysters from the beach is usually done by manual labour.
Deepwater culture methods, on rafts or longlines, have been more amenable to the development of mechanical or machine-assisted harvesting. Below are some examples of oyster harvesting devices and methods developed by British Columbia growers.
Harvesting Strings of Cultch or Tubes from Rafts: Hoists
Although there is still a lot of labour involved, certain devices will enhance the efficiency of the harvest operation. Happy as a Clam Shellfish Company grows out its oyster stock on rafts using the string and cultch method. Other than making sure the rafts are still stable and floating, there is virtually no labour input necessary until harvest. The company has a floating work and harvest platform that is moved alongside the row of rafts. The boom and battery-powered winch on the platform is set up to be able to haul up three 20 foot strings of large to jumbo oysters. The strings are laid out on a long table where the clusters are broken up and oysters sorted and highgraded: oysters destined for shucking are shipped while single oysters are put out on a beach site for a short period of hardening and defouling.
Machine Harvesting Strings/Cultch and Tubes from Rafts
Recently a large scale harvesting machine has been developed by Doug Stirn operating as Northern Gulf Harvestar Ltd. The machine is built on a catamaran type of craft which can serve as both harvester and general working boat.
Mechanized Harvesting of Tubes on Longlines
Harvesting from longlines on which tubes or lines of cultch are hung can be done an a small scale using a skiff equipped with a star wheel and/or a hoist. The skiff moves along the longline lifting the line out of the water to the gunnel where lines can be attached to a hoist and moved on the deck. Cluster breaking and tube stripping is then done mechanically or by hand on shore or on a work platform. (See the section on handling.)
No “off-the-shelf” harvesters are available in BC and growers have had to devise, design and build their own. Great Little Oyster Company Ltd. has developed a mechanical harvester, a tube or pipe stripper, built on its own platform. The entire platform moves along the longline, feeding the tubes into the harvest machinery. Tubes are stripped and the oysters fall onto a conveyer. They are washed and then placed into nets for temporary wet storage.
Harvesting from trays would be done in a similar manner to all other occurrences of tray handling such as loading trays with seed or sorting and grading. The trays are hoisted from the longline or rafts, the tray stacks taken apart and oysters removed, graded and assembled for delivery to the processor. For more details see the pages trays on longlines, Dark SeaTM Trays, High FlowTM Trays, and handling.
Temporary Wet Storage
Harvested product may not be shipped immediately to processors or markets and in the interim will normally be held in temporary wet storage. Proper wet storage on the lease site will enable the grower to maintain top quality and condition of harvested shellfish stock.
Harvested and sacked clams and oysters can be stored in the mid to mid-low tide range of intertidal areas. The substrate should be composed of firm gravel or small rocks and not be subject to siltation. The storage area should be well away from creeks with a high seasonal run-off and should also be protected from excessive waves. It is also advisable to have 24-hour surveillance of the storage site to prevent theft of the product. Intertidal storage is inexpensive and easily accessible but the product is also more subject to environmental extremes, contamination, predation.
Deep water storage, on the other hand, is less stressful to shellstock, not subject to environmental extremes, and less affected by siltation, and beach-related predation and contamination. The stored product is also less visible reducing the possibility of theft. Storage near the surface, however, may still leave product vulnerable to the effects of fresh water run-off such as seasonal warm water, fresh water or faecal coliform loading and should be avoided.
Shuck oysters are commonmly held in cargo nets suspended below 45-gallon plastic drums or suspending clams and oysters in totes or Vexar� pouches from longlines or rafts. Surface sink floats have also been used where product is stored on a platform with flotation and loaded so that it floats just below the surface. Surface sink floats are not recommended, however. The effects of the surface water lens may expose the product to warm water and bacterial loading. An alternative to the surface sink float is the “sinking sink float” used by Taylor United Inc. in Washington State. A platform, supported by a floating winch on either end, is loaded with harvested shellfish and then lowered to a depth of 2-4 meters below the surface.
Growers should note that wet storage should only be maintained for a specified period depending on the season and product. Shucking oysters should not wet stored more than ten days in winter and five in summer. If the oysters have been grown in deep water, the soft fouling organisms may be decaying and may contaminate and smother the stock so storage time should be as short as possible. Single oysters held in sacks (not burlap or natural fiber), totes or pouches should not be wet stored more than 2-3 weeks in winter and in summer no more than 7-10 days. Wet storage on the beach should be as low as possible (0.5-1.5m tide level).
Because of the Manila clam’s sensitivity to stress, wet storage requires special attention. Plastic mesh sacks or Vexar� pouches (not natural fiber, burlap or loose-knit “onion” sacks) can be laid flat (not stacked) in the low intertidal zone. Exposure to bright sunlight in summer may cause sunburn to the clams and should be avoided. Exposure of the clams to temperature extremes, hot or cold, should also be avoided. Each day (or at least each second day) the bags or pouches should be gently shaken and flipped. Care should be taken not to shake sacks that are in the water as this may cause the clams to close suddenly and pinch off the siphons. Wet storage of clams is also done in deep water using the “sinking sink float” technology.
For more details on temporary wet storage and other post-harvest handling of shellfish, see: Kingzett, B.C. and Pirquet, K.T. 1995. Towards Quality Assurance: An Information Manual for BC Shellfish Growers. (B.C. Ministry of Agriculture, Fisheries and Food)