CTSA Publication No. 144 |
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Obtaining pearl oysters
Collection from reefs
In most areas, when pearl farming starts, farmers begin by taking adult or young pearl oysters directly from the reef. The one advantage of this is that only the ability to dive is required. There are many disadvantages. First, older pearl oysters do not produce as high quality of pearls as younger ones. Additionally, most farms make a considerable amount of their revenue from the second and third pearls that a pearl oyster may produce; adults collected from the wild may be too old and past the period at which they will be useful for second and third implants. In many areas, there may not be enough pearl oysters present to support large farms. Even if a local pearl farming industry starts out by using wild-collected pearl oysters, it will soon be necessary to look for other sources.Hatchery production
Pearl oysters can be artificially bred in hatcheries. Hatcheries could be a source of large numbers of pearl oysters at relatively low prices. Only a few Black-lip pearl oyster hatcheries now exist, and the supply of spat they offer may not be sufficient to meet the demand. Most hatcheries also sell pearl oysters when they are small. The small spat take a long time to reach maturity on the farm and losses to predators may be high, making it potentially cost prohibitive. If hatcheries offer spat at economically feasible prices, then this can become a viable alternative to spat collection.Spat collection
Spat collection offers the advantages of being a relatively inexpensive and simple way to obtain spat. Spat collection occurs when any material designed to attract spat settlement is placed in the water and tended. Properly designed spat collectors also protect the small spat while they grow. They are harvested when they reach the desired size. Spat collection can only be done if there are enough adult pearl oysters in the surrounding waters to reliably produce high numbers of spat. Spat collection may not work well in areas with high water exchange, such as very open lagoons or near-shore areas that are not enclosed, since larvae may not be retained long enough for them to settle on the collectors before being washed out to open water. However, there are some cases where successful spat collection has occurred in areas with high rates of water exchange. It may be advisable to experiment with a small number of collectors before launching a costly and time consuming large-scale effort. Spat collection requires periodic maintenance of the collectors and lines for up to one year before harvesting the spat.
How to conduct spat collection
Collector materials and construction
The choice of material is very important as it will influence the likelihood of pearl oyster larvae setting upon it, and the ease and cost of collecting spat. Pearl oyster larvae set on a wide variety of materials in nature, but appear to prefer dark materials and the undersides of hard objects, which may offer protection. Although many artificial and natural materials can be used to make spat collecting devices, the best types are inexpensive, dark plastic materials that can be compressed in various ways to form lightweight, compact devices that offer a large surface area and protective spaces. The most commonly used materials are either polypropylene shade cloth or thick black plastic sheeting. If shade cloth is used, it should be of the knitted or knotted type, since this prevents unraveling. A shade rating of 60-70% is best.The following describes two of the most common ways of constructing spat collectors.
Accordian-style collectors
This style of collector (Figures 2 and 3) is made from strips of shade cloth which are threaded accordian-fashion onto a length of line, then compressed into a thick mass measuring 0.6-2 m in length and 8-12 cm in width. By using long lengths of material and by compressing it into a thick mass, a large surface area is provided for many spat to set. A general rule of thumb is to use 25 m of material for every 1 m in length of the collector. Bunching the material provides protection in the center of the collector for the young pearl oysters to hide from hungry fish and other predators. Small collectors measuring about 8 cm wide and 1 m long can be used when testing new areas (Figure 3). Commercial collectors for proven areas measure up to 2 m long, and are slightly thicker, about 12 cm.
Figure 2. Accordian-style shade cloth spat collector (Maria Haws). |
Figure 3. Experimental shade cloth spat collector (Simon Ellis). |
The core line holding the material should be knotted at both ends to hold the material firmly in place. Knots should also be tied one or more times in the middle to keep the material evenly spaced along the line. These internal knots should be placed at least every 60-70 cm along the collector. About 1 m of free line should be left at the end of the collector to attach it to the mainline (Figure 4). Melt the core line at both ends to prevent fraying.
A machine to rapidly manufacture spat collectors can be made out of
readily available and inexpensive materials (Figure 5 and 6). This machine
consists of two wooden inter-meshed, toothed wheels powered by a hand crank.
Strips of shade cloth are fed through the toothed wheels which fold the
strips of shade cloth. These are then pushed onto the long needle by the
machine operator. The needle is often a used car radio antenna or similar
object. Once a sufficient quantity of folded shade cloth has been threaded
onto the needle, it is transferred onto a length of line attached to the
needle and compressed.
Figure 4. Schmatic diagram of a shade cloth spat collector. |
(Simon Ellis). An example: dimensions may vary. |
Spat bag collectors
Another way to make collectors is to cut strips or panels of any black plastic material (shade cloth, black plastic sheeting) and place these into a fine-meshed spat bag (Figure 7). The material should be loosely wadded so that water can pass through it. The accordian-type collector can also be inserted into a spat bag. The bags are then tied closed and hung on the mainline. Spat bags are used to protect the young spat from predators such as fish. However, there is some evidence that in areas where predatory snails are a serious problem, the use of spat bags can make matters worse by trapping the young snails inside the bag where they will continue to feed on the pearl oysters.
(Maria Haws). |
(Maria Haws). |
Collector placement
It is important to locate spat collection lines in areas where larvae are expected to be present, and where they are retained long enough to reach the point of metamorphosis (2-3 weeks). The best areas for spat collection are enclosed lagoons where the water exchange rate is low enough that the larvae are not washed out to sea before they can set. More open lagoons may also be good sites provided there are sufficient numbers of larvae in the lagoon at any one time. A large number of adult oysters in a lagoon is often an indicator that spat collection will be successful. Spat collection may not be feasible in areas where pearl oysters are very rare. If this is the case, hatchery-produced spat may be the only option.When spat collection is attempted in a previously untried area, it is often difficult to determine which areas will attract the most spat. Sites must be chosen that will have the highest chances of retaining the pearl oyster larvae for the 2-3 week period it takes them to settle. Areas with high numbers of adult or young pearl oysters indicate that spat have settled there. Areas of a lagoon far from passes or areas where there are eddies in the current are often good sites. One example of this is on the down current side of patch reefs, since currents tend to form eddies there.
When first attempting spat collection, spat collectors should be placed at as many sites as possible to determine which areas are best. After the first harvest is conducted, sites that yield large numbers of spat should be chosen as the locations for the next round of collection, while sites which yield poor results can be abandoned.
When to place spat collectors
Peak spawning times will coincide with the time of highest water temperatures, although some spawning will occur throughout most of the year. In the South Pacific, March and April are the peak spawning months, with a smaller, second peak in October. Although there is little data available for the U.S. Affiliated Pacific Islands and other areas in the northern hemisphere, it is expected that similar spawning peaks will occur during periods of high water temperatures. In order for them to work most effectively, collectors should be placed in the water one month before the expected spawning peak. There is some evidence that pearl oysters may prefer to set on materials that have been in the water long enough to be covered with bacteria and algae, but not so long that other large animals have covered the available space.
Since it is not known when the spawning peaks occur in the Northern Hemisphere, it is recommended to set some collectors out each month for a year when first attempting spat collection. By observing when the heaviest spat fall occurs, peak spawning times can be determined. During the second year of spat collection, most of the collectors can then be put in the water one month before these peak times.
How to set out the collectors
When the collectors are first set out, they will float upwards. As animals
and plants begin to colonize them, the increased weight will cause them
to hang downwards. This is normal and should be expected. However, during
the first week or so when the collectors float upwards, care must be taken
to make sure that they are always submerged so that they do not become
a hazard to boats. Keeping the collectors submerged will also protect them
from excessive wave action, which can easily detach the small spat.
(modified from Gervis & Sims, 1992). |
Predators of pearl oyster spat
Fish and octopus
Trigger fish, puffer fish, tusk fish, eagle rays and many other fish that regularly eat shellfish can quickly devour large numbers of pearl oyster spat. Other large predators include octopus that can pry open the valves of the oysters. Fish and octopus predation can generally be avoided by setting the longlines in 20-30 m of water and at least 30 m from any reef areas. Any predatory fish seen in the area of the longlines should be speared or trapped.Ranellid snails
Ranellid snails (gastropods) of the genus Cymatium (Figure 9) are probably the most consistently destructive predator of pearl oyster spat and the hardest to control. These snails are ubiquitous throughout the tropical Pacific and have a planktonic larval phase that can last many months until the larvae encounter a suitable settling substrate. Cymatium infestations occur when larval snails settle on a collector line. The snail attacks the oysters by inserting its large proboscis into the shell. One adult or subadult Cymatium can attack and kill up to 10 juvenile oysters per week.Cymatium are easily noticed on the collector lines and their presence is also indicated by large numbers of dead shells in one area or on one collector.
Prevention of Cymatium predation in spat collection is not possible.
The only known method for control of these animals is regular inspection
and removal of visible snails. Care should be taken not to arbitrarily
remove non-predatory snails and other animals from the cages as they may
be helpful. If inspection is carried out on a 1-2 week schedule, then mortalities
related to this predator can be kept to a minimum. It should be noted that
Cymatium outbreaks are episodic and have been correlated to heavy rainfall.
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Maintaining collectors and lines
The mainline should be deployed at least a month before the collectors will be set out. This gives the line time to stretch and be readjusted to provide a stable base for the collectors. The mainline should be checked and adjusted weekly after it is placed in the water. The line must be maintained at the proper depth, and there should be no sideways shifting of the line. If shifting occurs, new anchor lines or guy lines must be added to stabilize the line.
Once the spat collectors are tied to the line, they should also be checked weekly for the first month. They should be observed at both low and high tide to be sure that they are at the correct depth, that wave action is not too high causing excessive movement of the collectors, and that any additional weight is not causing the line to sink.
After the first month, the line and collectors should be checked every
two weeks. If the line begins to sink, more floats must be added. Remove
biofouling from the longline and floats, but do not attempt to remove this
from the collectors, since small pearl oyster spat may be lost doing this.
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Spat identification
Pearl oyster spat can be difficult to identify when very small. Pearl oysters of the genus Pinctada are easily distinguished from bivalves that are not pearl oysters, but distinguishing between Black-lip pearl oysters (Pinctada margaritifera) and related species is often difficult. Figure 11 shows pearl oysters that are between 2 and 5 months old. The most obvious identifying feature of Black-lip pearl oyster spat is the dark emerald green color near its hinge. This green fades to brown near the edge of the shell, although the brown area may be tinged with green. As the spat grows, the green area may fade almost completely until only the smallest hint remains near the umbo. Pearl oysters more than 5 cm that remain a bright green over most of the shell are probably not Black-lip, but a related species (Figure 12). Black-lip pearl oysters also have growth processes (spines) that give the edge of the shell a jagged look and the body of the shell a scaly appearance. The growth processes are usually broad and flat and the tip is often wider than the base. In related species, the spines are more pointed.
If there is doubt about the identity of a particular pearl oyster, it
is best to let it grow until it can be identified with certainty. When
pearl oysters reach a size of about 4-5 cm, their identity should become
clear.
2 and 5 months of age (Simon Ellis) |
Figure 12. "Look-alike" pearl oyster spat (Simon Ellis). |
When to harvest
In cases where pearl oysters do not set in high numbers, it may be advisable to remove the pearl oysters when they are small and grow them out in protected containers. In this case, the spat should be cut off the collectors when they are large enough to identify (2-3 cm) and placed in spat bags, lantern baskets, or covered trays (Figure 13).
Post-harvest care
It is important not to crowd pearl oysters in any container, since
this will limit the amount of food available for each one. Except for very
small spat (less than 1 cm), there must be at least 12 cm of space between
each spat. Young pearl oysters tend to clump together and stick to each
other by the byssal threads, which may interfere with their ability to
feed. Containers must be inspected at least weekly. Any spat that are clumped
together must be gently separated by cutting the byssal threads.
(Gervis & Sims, 1992). a. Pearl net, b. Circle net, c. Lantern net,
d. Box net,
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Note: Pearl oysters should never be pulled from a substrate. The byssus is attached directly to the gut of the animal and tearing them from the substrate can kill them. Use a sharp knife to gently cut the byssus at the point where it is attached to the substrate.
The containers or spat bags must be cleaned weekly so that water flows freely over the spat and supplies sufficient food. Although it tends to be hard to clean spat bags this frequently, it is necessary in order to quickly grow the spat large enough so that they can be placed in pocket panels. As spat size increases, they must be redistributed into new containers to allow them sufficient room to grow.
Although containers can protect spat from most predators, some can still enter the containers and attack the spat. The most problematic are Cymatium and crabs. All containers must be inspected weekly and predators removed. If this is not done, just one of these predators can kill all the spat in a container.
Once spat reach the size of 5-10 cm, they can be placed in special pocket panels with fine mesh. Pearl oysters measuring 10 cm or more can also be drilled and hung on chaplets. Once spat are transferred to pocket panels or chaplets, they should be cleaned monthly before grafting occurs.
Pearl oysters must be a minimum of about 13 cm long or about 18 months old before they can be implanted. Many of the publications in the “Further reading” section (page 8) provide further details on the later stages of pearl oyster culture.
Economic benefits
Once the time of spat set is determined in a particular area, a spat farmer has to set the collectors only once or twice per year. While there is some cost and time involved in initially setting the mainlines, proper treatment and maintenance of this equipment will result in years of use. Collectors have a limited life span of 1-2 years depending on the level of spat set and must constantly be replaced. Collectors are often made by the women in the family or as a family activity in the evening. Making spat collectors can often provide more income than traditional handicraft production.
Further assistance
CTSA, Hawaii Sea Grant and the College of Micronesia Land Grant program
operate an aquaculture extension network designed to provide information
and assistance in all forms of tropical aquaculture. They can be reached
at the following addresses:
| Mr. Simon Ellis
Regional Aquaculture Extension Agent COM Land Grant P.O.Box 1179 Kolonia, Pohnpei, FM 96941 Tel. 691-320-2728, Fax 691-320-2726 e-mail: sellis@mail.fm |
Cheng-Sheng Lee, Ph.D
Center for Tropical and Subtropical Aquaculture The Oceanic Institute 41-202 Kalanianaole Hwy. Waimanalo, HI 96795, USA Tel. 808-259-7951, Fax 808-259-8395 e-mail:chenglee@hawaii.edu |
University of Hawaii, Sea Grant Extension Service
2525 Correa Road, HIG 237 Honolulu, HI 96822, USA Tel. 808-956-2862, Fax 808-956-9106 |
Dr. Maria Haws, Director
Pearl Research and Training Program University of Hawaii 200 W. Kawili Street Hilo, Hawaii 96720, USA Tel. 808-933-9460, e-mail: Haws@aol.com |
Further reading
Clarke, R., D. Sarver and N.A. Sims. 1996. Some recent history and prospects for the Black-lip pearl oyster, Pinctada margartifera in Hawaii and Micronesia. Twenty-sixth Regional Technical Meeting on Fisheries, Noumea, New Caledonia. 10 pp.
George, C.D. 1967. The cultured pearl, its history and development to the present day. Lapidary J. Am. July-August-September: 1-6.
Gervis, M.H. and N.A. Sims. 1992. The biology and culture of pearl oysters (Bivalvia: Pteridae). ICLARM Studies and Reviews 21. 49 pp.
Haws, M. 1999. Pearl farming: a manual of basic methods. CTSA publication #127, in review.
Haws, M. and S.C. Ellis. 1999. Producing pearls using the Black-lip pearl oyster (Pinctada margaritifera). CTSA Aquafarmer Information Sheet, #141. 8 pp.
Periodicals
Out of the Shell. Coastal Resource Research Network Newsletter, Lester Pearson International, Dalhousie University, 1321 Edward Street, Halifax, NS Canada B3H 3H5.Pearl Oyster, Information Section Marine Resource Division, Secretariat of the Pacific Community, B.P. D5, 98848 Noumea Cedex, New Caledonia.
Glossary
Fouling, biofouling: small plants and animals that colonize the shell of the pearl oyster.
Gonad: reproductive organ producing either sperm or eggs.
Grafting: also known as seeding or nucleus implantation. This is a surgical procedure in which the nucleus and a small piece of mantle tissue are inserted into the gonad thus starting development of a cultured pearl.
Larva, (pl. larvae): an early developmental stage of the pearl oyster life cycle lasting 2-3 weeks when the pearl oyster is a microscopic and free-swimming organism.
Metamorphosis: developmental stage involving anatomical and behavioral changes that transform the free-swimming larvae to a settled adult.
Spat: juvenile pearl oyster.
Umbo: area just above the hinge of the shell.
Acknowledgments
This publication was funded in part by a grant/cooperative agreement from the National Oceanic and Atmospheric Administration (NOAA), project #A/AS-1, which is sponsored by the University of Hawaii Sea Grant College Program, SOEST, under Institutional Grant No. NA86RG0041 from NOAA Office of Sea Grant, Department of Commerce, UNIHI-SEAGRANT-TR-99-06.
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