Remote Setting Techniques
Remote setting refers to the process of transporting bivalve larvae from the hatchery to the grower’s culture site. Under controlled conditions in tanks or contained areas, the larvae are cultured through setting (attachment to substrate) and metamorphosis (changes in form and anatomy). These juveniles are then transferred to nursery rearing systems. This process is most commonly done for setting oyster larvae on shells, tubes or other cultch material.
Since remote setting has received extensive coverage in British Columbia during the past ten years, it will be useful to list some of the resources available. Rather than repeat what is already available, this site will focus on providing some general information and reports of new developments.
There are manuals available to guide growers through the process. They cover setting oysters in tanks on cultch such as shell or tubes. One (Jones and Jones) deals with setting single seed on shell fragments in upwellers. The third will be useful for clam growers planning their own setting and nursery rearing system.
- Advances in the Remote Setting of Oyster Larvae by Gordon G. Jones and Bruce L. Jones, 1988 (British Columbia Ministry of Agriculture and Fisheries).
- A Manual for Producing Oyster Seed by Remote Setting by William G. Roland and Thomas A. Broadly, 1990 (British Columbia Ministry of Agriculture and Fisheries).
- Manila Clams: Hatchery and Nursery Methods by Gordon G. Jones, Cathy L. Sanford and Bruce L. Jones, 1993 (Science Council of British Columbia).
Setting Oyster Larvae in Tanks
This method requires a well-constructed tank, cultch that has been prepared by leaching and conditioning, water heating system, aeration system, a microscope, algae for feed, good planning and record-keeping and, of course, the larvae. Details for tank construction are provided in the manuals. Tanks are insulated and often will have a cover to maintain warm water temperature and reduce the possibility of contamination. Any surface of the tanks that comes into contact with water and larvae must be 100% non toxic. Most tanks are made from wood, styrofoam insulation and lined with fiberglass. Ensure that fiberglass is properly cured (by exposure to sunlight or steamed) to prevent any toxicity problems. Tank size will vary, but 1.3m is a common depth.
Operating the tank for setting larvae requires filtered, heated and aerated seawater. Seawater should be filtered using a 50-100micron bag filter or sand filter system. Heating and aerating methods depend somewhat on access to utilities. Electric immersion heaters are sometimes used but in many remote sites propane torches are the usual means of heating water for setting. Temperature required for setting is between 20oC and 27oC with 25oC considered by many to be optimum. Aeration is neccessary for even heating of the water and to distribute the larvae evenly around the tank with the aim of producing an even set. A blower or air pump that produces no oil mist can be used to force air through puntured PVC lines distributed over the tank bottom. Aeration should be gentle. Tanks are covered during the setting process to retain heat, prevent contamination and ensure larvae set evenly on the cultch. In order to set, larvae will seek darkened areas away from light.
Cultch is arranged in the tank to maximize tank capacity and to ensure an even distribution of the larvae. Shell cultch is put in vexar mesh bags and bundles of bags can be put into the tank by means of a hoist. An average of 200 bags are loaded into each tank. Tube modules can be hoisted directly into tanks. One of the difficulties with tubes encountered by some growers is that seed tends to set heavier on the ends of the tubes and sometimes on the inside of the tubes. The latter can be a problem if tubes are strung for raft or longline like beads on a string but less of a problem if strung using a clove hitch on the outside of the tube.
Larvae should be put in the prepared setting tank within 24 hours of leaving the hatchery. The larvae should be inspected on arrival for health, condition and readiness to set. Observe a sample under the microscope (e.g. 40x dissecting microscope). If the larvae are in good condition and ready to set, setting should occur in two days with a further three or four days necessary for the shell to attach firmly to the cultch. Cultch should not be moved before this time.
During metamorphosis (24-48 hours after larvae are put in the tank) larvae will not feed. Feeding algae usually begins on the second day. Either live algae cultured on site or algal paste can be used to feed the newly set juveniles. Feed rations are normally around 50,000 cells/ml/day for the first couple of days after setting, but growers should consult experienced setters or the paste suppliers for accurate feeding regimes.
During the setting process a portion of the water is changed on a regular basis, either daily or every other day. Up to half the water volume can be exchanged. The drain should be covered with a 200 micron screen to prevent the loss of any swimming larvae.
Equipped with a microscope or hand magnifier (10-15X), growers can examine samples of the cultch after setting to determine how well their setting methods have worked. It takes several weeks before spat become visible to the naked eye. Setting density should reflect expected survival rates through grow-out and the result should be well-spaced, full-bodied oysters. An average of 15 spat per shell is the common standard for this type of cultch. Tubes should have an even set of between 300 and 500 spat per tube to produce a final grow-out density of 60-150 per tube. Since only 10-20% of the larvae introduced into the tank actually set and survive, the number of larvae introduced into the tank should be about 10 times the number expected to set.
Mass Remote Setting in Ambient Temperature Seawater
Recently an experimental technology for remote setting has been developed by Island Scallops. It can be done for both clams and oyster in intertidal areas and with oysters in deepwater for growers without beach access. The larvae are conditioned in the hatchery to be able to tolerate the cooler temperatures of open seawater. Setting is precisely timed to correspond to the larvae being ready to set. The larvae are brought out to the grower’s site which has been prepared in advance. Larvae are released and distributed in temporary enclosures until they have set. This process relieves the grower of the usual demands of remote setting in tanks. However, the grower, in consultation with the hatchery, must follow specified procedures for preparing the site. Specific substrate conditions must be met to ensure setting success for clams. Cultch for oyster setting must be high quality and conditioned just as it is for tank setting