Tube Culture
In an effort to develop a more efficient system for rearing oysters, an increasing number of British Columbia growers have adapted a re-usable cultch material known as “French tubes”. Although this method was pioneered in Europe, high quality tubes (also commonly called “pipes”) are now manufactured locally by Polychlor Plastics Ltd. Growers have devised nursery, growout and harvest systems based on the use of tubes. These systems have reduced labour costs with increasing use of mechanized bulk handling.
French tubes are plastic tubes about 25mm (1 inch) in diameter and are usually in 2 meter lengths. The outer surface of the tube consists of closely spaced 2mm deep longitudinal grooves. The material and surface of the tube has proven to be an effective substrate for oyster spat settlement.
Conditioning and Constructing Tube Modules
As with any cultch material, tubes must be properly conditioned before they will be effective. New pipes are normally placed intertidally for up to a year to effectively leach the material and then conditioned to allow for a thin layer of microorganisms to colonize the surface. This layer probably acts as an attractant to the oyster looking for a suitable place to settle. Re-used pipes must be clean and free of decaying organic material.
For ease of handling to ensure an even distribution in the setting tank, tubes are usually fastened into modules. In this way 250 to 500 pipes can be handled at one time.
Setting and Nursery Rearing with Tubes
Unless the grower has easy access to a reliable natural oyster set (of which there are very few), setting is typically done in specially constructed tanks into which the tube modules can be loaded. To prepare for setting, tube modules are usually conditioned in seawater for a few days which may be followed by further conditioning in the setting tanks with filtered seawater for two or three days before arrival of the oyster larvae. This will encourage the growth of a bacterial film on the tube surfaces which is thought to attract the larvae to set. The number of larvae required for setting tubes will depend on the grower’s intentions. It may be to strip the tubes for seed after one year or growing out the oysters on the tubes to harvest. Generally the number of larvae required is approximately 4-5 million per 1000 tubes. Once setting in the tank is complete and the oysters have attached firmly to the tubes (approximately 6 days), tube modules are hoisted out of the tanks and transferred to a nursery rearing system.
For more details see the setting page.
Grow-out and Harvest of Oysters on Tubes
Two or more months after setting the seed will be large enough to remove from the nursery and hang for grow-out.Once the tubes have been separated out of the module, tubes can be individually inspected to determine how evenly oyster have set. If some areas of the pipe have an extremely heavy set, these areas can be thinned by hand. They are then ready to be suspended in deep water for grow-out. Approximately 100-200 oysters per tube is considered optimum for hanging. The use of a longline system is one common method. The individual tubes are fastened to longline supported by flotation where the oysters are left to grow out until harvest. For details on utilizing tubes in a longline system see the longline page.
Tubes can also be strung on rafts in lengths of one, two or three deep. Two tubes can be tied together by a length of string (clove hitch around each of the two pipes, 3-4 inches from the end) and the tubes draped over the cross pieces of the raft so one tube hangs over either side. The string can be wedged into a notch in the beam to keep it more firmly in place. Another method is to string two or even three (conditions permitting) tubes on one line like beads on a string and hang these from the raft cross beams.
Harvesting tubes involves carefully pulling them up vertically from the rafts or longlines. They may break from the weight of the oysters if they are held unsupported in a horizontal position. a tube with 150 oysters of market size may weigh 70kg when removed from the water. They can be hand stripped quite easily. Harvesting is becoming increasingly mechanized with devices now in operation that mecahnically haul up tubes from rafts or longlines and devices which mechanically strip the tubes.
See mechanized harvesting of tubes for details of new technologies recently developed by BC growers.