3. RESULTS AND DISCUSSION

3.1 Qualities of Mabe Oyster and Pearls

Mabe cultured in Vava'u appeared in general very healthy. The shell was thicker than the Japanese Mabe. Its shape was very similar to Japanese Mabe, but a width of golden-coloured along the lip on nacreous layer or mother-of-pearl layer seemed a bit wider compared to Japanese one. Mabe oysters commonly found in the coasts of Fiji were different from Tonga's ones. Fiji's Mabe had narrow shell width and its golden-coloured part on nacreous layer was wider and higher than that of Tonga. It means that Tonga's Mabe is superior to Fiji's Mabe in terms of production of whitish Mabe pearls which have higher value than those of golden colour.

All shells showed good growth condition. Its growth was faster than that of Amami Island, Japan. Mabe cultured close to the bridge of Vaipua showed the best growth conditions of all Mabe cultured in Vava'u. This indicates the availability of abundant feed plankton which flows out of Vaipua Bay. The bay seems like a natural feed production tank.

A higher rate of fine quality pearls was observed out of the pearls cultured in the mouth of Neiafu Bay. The results of experiments on the culture period undertaken by the Fisheries Department indicated that the group of pearls cultured for a period of 18 months was of better quality than those of other cultured periods. It was also found that appearance of golden-coloured pearls was comparatively higher in the shorter-cultured shells, while the number of whitish pearls was increasing in the longer-cultured shells. This suggests that it was better to implant nuclei into the larger shells rather than smaller shells, to improve colour quality i.e. whitish Mabe pearls.

Appearance ratio by different quality group was almost the same as the one on the Japanese farm. There are three groups of quality generally produced, i.e. 1) finest quality, 2) golden-coloured or less quality by still possible to sell after processing, and 3) no commercial value even after processing. Only among the pearls harvested since 1995, it was observed that the ratio of good pearls was slightly better than the Japanese Mabe pearls.

It was suggested to improve the site of nucleus implanting a little towards the mouth area, i.e. to the byssal part. The practice of Mabe's lip cutting before nuclei grafting is recommended to make grafting easier and to produce thick nacreous layer.

As an overall conclusion from the above results, the pearl production technique being applied in Tonga seemed to be appropriate in general.

3.2 Production Efficiency

Deployment of spat collectors started in November 1989, and a total of 73,300 juvenile oysters of 1–2cm in length had been collected (Table 1). Out of these juveniles, only about 800 Mabe pearls were obtained for further processing. It seemed that the yielding rate of Mabe pearl production was very low. It was reported that high disappearance and high mortality were observed inside the nursery baskets after being harvested from the collectors. This high loss was caused by careless handling in harvest of juveniles attached by the collectors and improper management during the nursery period. To minimize such loss, the mission made recommendations as mentioned in the next chapter.

Table 1 Record of juvenile Mabe oysters collected

n.a.: Not Available

At present 4,000 juvenile (six-month-old) collected in 1996 and 2,000 young (one and half-year-old) collected in 1995 were under grow-out. In addition, about 300 adult oysters were being farmed for pearl production. The juveniles collected in 1996 were already reduced to 50% after six months. Minimizing loss during this period represents the main goal to improve the total yield of Mabe farming.

3.3 Spawning Season and Spat Collection

All juveniles were usually collected only in the Neiafu Bay. In 1990 the Fisheries Department deployed spat collectors at three different sites other than in the Neiafu Bay, but no juveniles of Mabe oysters attached on those collectors. This seems to indicate that Mabe oysters might densely inhabit only in the Neiafu Bay. The bay should be reserved as a Mabe spats collection ground.

Most juveniles were obtained from the collectors deployed in the summer time, i.e. during December to March, when the water temperature reaches the average of 28–29° C. The reason of the failures of spat collection in 1993 and 1994 has not yet been clarified. However, it might be caused by no spawning in the wild as those years were quite colder than usual, or by ill-timing of collectors deployment. The present spat collection method was by deploying a certain number of collectors every two weeks during the summer time particularly as of January, regardless of the actual spawning in the wild. This method had been applied during the early experimental period when the spawning season had not yet been ascertained. Since the spawning season has now been identified, the spat collection method should be modified by introducing a scientific approach to enable to collect spats more efficiently.

It is known from studies in the Amami Island in Japan that Mabe's swimming larval stage takes place for about one month at 27°C. It should therefore be recommended to deploy spat collectors at the end of the swimming stage. This would reduce the workload of staff tremendously and save operational costs. Since there are fortunately only two species of winged pearl oysters found in Vava'u, a microscopic study on swimming larvae would not be so complicated for identification of the species. For this reason, SPADP overhauled an unused microscope stored at the Vava'u Fisheries Station and returned it for use at the station in December 1996.

3.4 Farming Environment

The Vava'u Islands group was found to be the ideal place for not only Mabe pearl farming, but also for other pearl farming such as black-lip and silver-lip pearl oysters as well as other aquaculture practices (such as fin-fish farming, tuna or amber jack) because there are many well sheltered bays and inlets with sufficient depth, good tidal currents and clean water. Some black-lip pearl oysters experimentally cultured in Vava'u, particularly at the mouth of Vaipua Bay, were showing good growth.

Thickness of the Mabe pearl layer was also indicating a good environmental condition in Vava'u. It is the consultant's experience that, if the environment is bad, side parts of the pearl cap tend to be thicker and the top part becomes thin.

Although there was no pearl farming experimented in Neiafu Bay, this could also be a good place for Mabe pearl farming. However, the causeway built on the Ahanga Passage at the end of the Neiafu Bay is severely blocking water exchange of the bay. The Neiafu Bay is internationally famous for yacht sailors so that over one hundred yachts moor in the bay at the peak season. Amounts of effluent from those yachts seem considerably high. Besides, the number of houses seem to be increasing recently around the bay. The blockage by the causeway would accelerate the deterioration of the water quality in the bay, which would cause pollution and subsequent drop productivity. The bay must be protected from pollution to maintain it as a Mabe pearl oyster bed. It is strongly recommended to improve the water channels of the causeway by excavating the existing channels deeper and wider and by adding more channels.

After inspecting all the proposed and selected pearl farming sites, a total of 11 sites were identified as good ones. These are marked on Map 1 (Attachment 1) with numbers. The results of water quality survey are shown in Table 2.

Table 2. Results of water quality surveyed in August 1996

The consultant divided the eleven sites into three zone-groups of different environmental characteristics as follows:

Zone A: Sites No. 1, 2, 3, 7 and 8
Zone B: Sites No. 4, 5 and 6
Zone C: Sites No. 9, 10 and 11

Zones A and C have influence of freshwater inflow, but their water catchment areas are different. Besides, Zone C has an oceanic influence as it is close to the reef and rather exposed to the trade-wind. Zone B has less influence of freshwater and more oceanic feature. Such zoning was also based on the results of water quality surveys, particularly pH.

Because of different environmental characteristics, quality of pearls produced could vary in each different zone-group. In other words, we would produce different qualities, by no means of less value, of pearls that would attract markets, or could produce good pearls by using each zone at different growth states, for instance, some sites would be best for nursery purpose, others for grow-out of shells and others for pearl farming. During the period of pearl farming, shifting of locations according to farming stages (beginning, middle and before-harvest) would be necessary to produce better quality pearls. Such environmental variation would be an advantage for improvement of pearl farming in Vava'u.

3.5 Production Potential

3.5.1 Farming area

By using a hydrographic chart of Vava'u, the consultant measured and calculated the potential farming area at each selected site (Attachment 1: Map 2-1 to 2–7 and Table 3).

Table 3. Proposed pearl farming areas by site

* Including areas of raft anchoring
** Refer Fig. 1: Layout of raft (Attachment 2)

Zone A: This farming zone would be the most important farming ground and could be used for the whole nursery cycle to pearl farming. Therefore it is recommended to start development of pearl farming in Vava'u with this zone. The farming plan would be as follows:

Spat collection site: No. 1
Grow-out sites: No.1 and parts of No. 2 and No.3
Pearl farming: No.2, 3, 7 and 8

Zone B: Since this zone is far from the town, there would be difficulties to secure labour, management, and maintenance. However, it is expected that this zone could produce a different quality of pearls compared to zone A or C.

Zone C: This zone has a potential to double the farming area, but wind velocity gets stronger. The potential area was not included in the calculation.

3.5.2 Production estimate

A typical durable long-line raft of pearl farming is shown in Figure 1 (Attachment 2). The w00-meter-long rafts can be set every 200 meters along the shore line, which is sufficient to maintain the environment with good condition.

The length of total shore line enabling to set rafts was calculated as 42,500 meters (Table 3), and the total length of rafts only would be about 21,500 meter. If oyster hanging-ropes are set at a distance of one meter, the total number of ropes deployed is calculated as follows: 21,500m/1m × 5 main ropes (long-lines) = 107,500 hanging-ropes

To establish a commercially sustainable production of Mabe pearls, the following assumptions for a farming system are usually offered:

1. Nursery of juvenile oysters for a year (Total number of ropes: A)
   • number of juveniles hanged per rope: 20 shells

   • estimated loss during the period: 40%

2. Grow-out of young oysters for a year (Total number of ropes: B)
   • number of young hanged per rope: 10 shells
   • estimated loss during the period: 20%
3. Pearl farming for one and half years (Total number of ropes; C)
   • number of mother of pearls hanged per rope; 5 shells

According to the above assumptions, the total number of hanging-ropes will be A+B+C. In Vava'u, then the following equations are set up:

From the equations (1), (2) and (3), the numbers of ropes for A, B and C are calculated as follows:

A = 26,092 = => 26,000 ropes
B = 31,310 = => 31,000 ropes
C = 50,097 = => 50,000 ropes
   ---------------------------
    Total 107,000 ropes

According to the above numbers of hanging ropes, the total annual production in Vava'u is roughly estimated as follows:

It means that a total of 1.1 million oysters from juvenile to Mother-of-Pearls are always farmed in Vava'u. This indicates that a total stocking density is 7.7m²/shell in average in the exclusive farming areas. The actual stocking density at a farming raft is 3.2m²/shell of mother-of-pearls. Stocking density of Japanese Akoya pearl farming cannot be adapted because Mabe's feeding requirement is probably over 100 times as big as Akoya pearl oyster, either than black-lip pearl oyster. The stocking density of 5 shells per rope would be reasonable for Mabe mother-of-pearls farming after nuclei implanting. Oysters can be hanged every 30cm on rope. The present stocking density of 90 adultoysters per 6m-length PVC pipe with 2 meter distance is about 0.11 m²/shell. This would not cause problems at the moment as the farming scale is quite small in Vava'u. It would, however, be considerably high if and when a full-scaled farming took place.

3.5.3 Economic appraisal

It is possible to implant 3 to 4 half-round nuclei in a Mabe pearl oyster. Of the total production of pearls, a ratio of the finest pearls, medium-quality pearls and poor-quality pearls would be in general 1:1:1. This ratio of quality averages apply in Japan, but the consultant estimates that almost similar ratio can be obtained in Vava'u. The poor-quality pearls must be crashed. Its powder may have some commercial value.

For instance, 250,000 mother-of-pearls could produce at least 750,000 Mabe pearls. Out of these, one third, i.e. 250,000 pearls would be the best quality pearls. If these were sold at the average price of US$30 per piece, its sale would be of US$7.5 million. An additional revenue would be obtained from the sales of medium-quality pearls and Mabe shells as well.