As the earlier CFTC programme discovered, the Vava'u Group of islands appears to offer extensive reef flat areas with the desired physical conditions for growing cottonii by the off-bottom constant depth farming technique (Fig. 1). More than 26 trial sites were investigated in this programme. Areas where growth was not continually consumed by herbivorous reef fish were identified, Utungake, Fafine Is., Ofu Is. and Faio Is. However, even plants at these fertile sites were not immune from the seasonal (January - February) surface grazing by thousands of juvenile rabbit fish. Stock loss to these fish occurs very quickly, and many tonnes of healthy plants can be lost in just two weeks. Protection of farms with nets is impractical and creates an excessive labour input to maintain clean nets in order to ensure water flow. Even the protection of a centrally run seed stock farm with fixed pens proved impractical because of the small size of the fish (Fig. 2). The small volume of surviving stock remaining after the grazing season was never sufficient to maintain economically viable farms. The fish grazing problem is further confused by the fact that the annual rabbit fish breeding does not always produce very high numbers of juveniles. However, massive spawning years are frequent enough in Vava'u to prohibit a cottonii development using the off-bottom farming technique.
The seasonal appearance of juvenile rabbit fish in the shallow waters of Vava'u is
well known by local people. The juveniles (40–80 mm total length) of the two
common Siganid species are known locally as 
. They are sometimes caught
in large numbers by two or three people simply holding a large cloth sheet in
shallow waters, while others herd shoals of the fish towards the sheet.
There are two shoaling species of rabbit fish common in Vava'u waters, Siganus vermiculatus, known locally as Pokumei and Siganus argenteus. (forktail rabbit fish) known as Ma'ava. S. vermiculatus is also recorded as S. spinus (spinefoot rabbit fish) on some Tongan checklists. The two species are very similar, and S. spinus may be the correct identification for the vermiculated rabbit fish known as Pokumei. Siganid species are reported to take about 30 days after spawning to reach a total length (TL) of 22–25 mm, and size class distribution data from the Vava'u CFTC programme data (Fig. 2 and 3) indicates that spawning must occur from late November through December. The adults, which can be up to 20 cm in length, reach breeding maturity in one year. In January and February juvenile and young rabbit fish invade shallow reef flat areas in shoals of 300 or more individuals.
The CFTC programme found that in January and February rabbit fish shoals, with fish 30–80 mm (TL), destroy off-bottom cottonii farms by grazing just the surface cells of plants. The outer pigmented (coloured) cells then quickly die, resulting in first a white mottled plant appearance, progressing within days to an overall whitening of dead plants. The sudden appearance of white branches and decaying white whole plants is often mistakenly identified as a disease. Over three consecutive January/February periods juvenile and young rabbit fish numbers were high enough to cause the mass destruction of planted hectares on Vava'u reef flats. Mature adult rabbit fish are considerably less common than the juveniles, indicating a high mortality rate from predators during the first year of the life history. It is conceivable that the presence of cottonii farms on reef flats increased the numbers of fish surviving to breeding maturity, and hence promoted an increase in hatchlings in the following years.
In view of the results from the previous CFTC development programme, it is recommended that no further off-bottom farming trials be carried out in Vava'u.
There have been no changes since the original trials in the 1980s to suggest that further work with off-bottom farming on reef flats would lead to any viable commercial farming.
The question remains, would a different farming technique resolve the problem of the seasonal destruction of stock by juvenile rabbit fish? Floating rafts and longlines are used at some production sites in Malaysia, Indonesia, and the Philippines. Where floating techniques are employed in deeper water away from coral reefs, grazing by herbivorous fish is reported to be significantly less than that experienced by off-bottom constant depth farms.
A small project could be undertaken by Fisheries Division in Vava'u to establish a floating long-line farm, monitor the impact of grazing Siganids and investigate production costs. Floating raft culture is not recommended because of the higher capital and maintenance costs of this method, and the absence of any significant quantities of suitable construction bamboo in Vava'u.
Cottonii was recently (July 99) re-introduced to Vava'u from Tongatapu. The stock has been planted on a reef flat near Mafana Island. During the mission the author recommended to Fisheries Division staff in Vava'u that they quickly relocate their trial seaweed plot to the Lotoa'u channel (Fig. 4) between Utungake and Pangaimotu Islands. Trial plots should be inspected every two to three days, and a small number of trial cuttings on the eastern reef flats may initially be consumed by just one or two adult herbivorous fish. The Lotoa'u channel is easily accessible by road and foot from Neiafu. The channel is also well protected from potential storm driven wave damage and loss of trial plantings. The strong current through the channel with the tidal flow in and out of Lotoa Bay promotes faster cottonii growth than that recorded on the eastern reef flats. Hence a trial in the Lotoa'u channel provides the best opportunity to quickly increase the volume of stock in Vava'u before the onset of seasonal rabbit fish grazing in January 2000. Stock produced in the Lotoa'u channel could be used for a pilot floating long-line farm (Fig. 5), sometimes referred to as the hanging long-line technique.
The long-line farming technique is generally less resilient to wave action damage and storm destruction than the off-bottom method. Consequently, suitable sites for long-line farming in Vava'u are located where there is sufficient current and wave exposure to promote viable growth rates, but some degree of protection from direct exposure to prevailing south-easterly and easterly trade winds. Well sheltered semi-enclosed areas such as Neiafu Harbour, Vaipuna inlet, Kolotahi Bay, and Neiafutafi Harbour, all lack sufficient water movement for viable commercial growth rates. These sheltered areas are also subject to periods of low salinity from land run-off during heavy rain. Figure 6 shows areas in the east of the Vava'u Goup that would be suitable for a long-line farming development. Deep water farming in the southern and western regions of the islands is considered unlikely to be economic due to the high capital costs of anchoring and constructing floating line structures that can withstand the frequent turbulent sea conditions.
The long-line method has resulted in production from regions where no suitable shallow water areas exist for off-bottom constant depth farming. There is only one advantage of this method over the off-bottom technique, and that is a decrease in fish grazing damage. The comparative disadvantages are as follows:
The economic viability of long-line farming in Vava'u can only be assessed by a pilot scale production unit at one of the recommended sites (Fig. 6). Previous trials on nearby shallow water areas at Faioa Is., Ofu Is. and Pangaimotu Is. certainly indicated that the region can sustain commercially viable growth rates for ten months of the year.
Even if it can be demonstrated that long-line farming is free of the destructive seasonal Siganid grazing and economically attractive, a policy decision will still have to be made as to whether a proliferation of individual production units in the eastern region is a desirable development. The recommended farming area is already used extensively by Vava'u yacht charter companies and by local fishermen, and the development of floating line farms in deep water may well meet with considerable objection from these other resource users who already contribute towards the region's economy.
