The Marine Resources Division of the Federated States of Micronesia held a workshop on aquaculture planning for the nation, attended by 24 experts from across the Pacific, at Kosrae in April 1986. Several species of marine organisms were identified as targets for aquaculture development in the FSM, including trochus and green snail ranching (Gawel, 1986). In addition, introduction of green snail was proposed in a recent report to assess aquaculture potential for Micronesia (Croft, 1987).
The Marine Resources Division of the FSM envisaged transplantation of green snail to enhance their inshore resources, and the Chief of the Division Mr. Mike Gawel requested the South Pacific Aquaculture Developement Project (SPADP), Fiji, for implementation of a feasibility study.
During the South Pacific Commission Workshop on Inshore Fisheries Resources Management, held at Noumea in March 1988, Mr. Gawel, Mr. Hideyuki Tanaka of SPADP and one of us (MY) met and discussed this study. It was agreed that a field study would be financed by SPADP, and conducted by the senior author in association with Mr. Ken-Ichi Kikutani of Tokunoshima Fisheries Cooperative, who was an experienced green snail diver fisherman, and that the Marine Resources Division would provide logistics and field assistance.
The Terms of Reference of the present study were:
After the above agreement was made and survey schedules were planned, Mr. C. Friberg of Yap Fishing Authority contacted the senior author and requested to consider implementation of green snail transplantation to the State of Yap of the FSM. It was found, however, that the time frame available for the field survey was too short to include an additional trip to Yap. Thus, field surveys in the states other than Pohnpei and Kosrae were not conducted in this time.
The authors flew from Okinawa via Guam to Kosrae to begin the survey July 3 and departed Pohnpei after completing field investigations July 16 1988.
Green snail, Turbo marmoratus, is a large herbivorous marine gastropod belonging to Family Turbinidae. It is the largest species of the family and grows to 20 cm or more in shell width, its total weight exceeding 3 kg. Surprisingly little had been studied on its biology and ecology until the present authors initiated a study on this species in the Ryukyus. Information about the green snail is described by Yamaguchi (1988a).
Throughout its natural range of distribution, that is from western Indian Ocean localities such as Kenya and the Seychelles to the western Pacific and South-East Asian countries such as Vanuatu, Solomon Islands, Papua New Guinea, Indonesia, Malaysia and the Philippines and upward to the Ryukyus, green snail has been considered to be an important resource extracted from coral reefs.
When the islands of the Ryukyus became formally an integrated part of Japan, in the early 1880s, green snail resources of major islands began to be exploited commercially. A diving fishery for this snail was important economically, and harvesting was carried out most extensively. The outcome of this was a collapse of the green snail fishery in the early 1900s owing to extensive overfishing (Yamaguchi, 1987a).
Trade statistics of green snail and also those of trochus, another important reef gastropod as an inshore resource, indicated ups and downs in the harvest according to fluctuations of values in the international markets. Here, rises in their market value appeared to encourage extensive over-exploitation of the resources, often leading to sharp falls in quantities of exports despite rising values (Yamaguchi, 1987b).
The above accounts may indicate vulnerability of such inshore resources as green snail and trochus under commercial exploitation, even without sophisticated diving equipment. Modern technologies provided fishermen of remote and isolated islands with efficient gear and powerful boats. Thus, over-exploitation of inshore resources was intensified, leading to local extinctions or the island-wide commercial extinction in the Ryukyus. Similar trends may become prevalent among the developing nations (e.g., Usher, 1984).
The senior author proposed a scheme of reef-ranching to help alleviate problems of resource management, using hatchery-produced juveniles to enhance depleted stocks. He also recognized that such juveniles might be useful in transplantation of these gastropods into new areas, because the associated problem of quarantine procedures may be implemented easily with juveniles produced in the hatcheries (Yamaguchi, in press).
Transplantation of trochus had been carried out in Micronesian islands before WW II by Japanese fisheries authorities, with great success. Thus, many islands which did not possess the trochus resource before, such as Pohnpei and Truk, now earn considerable cash revenue from harvested shells of trochus, which is a nonperishable commodity for export (see Fig. 1). For example, more than half a million dollars were earned by the export of trochus shell from Pohnpei in 1988 (Curren, pers. comm.)
Activities of transplanting trochus have continued in Micronesia under US administration and also in the South Pacific, including numerous attempts of secondary transplantations, by many island nations and territories (Gillett, 1986; Yamaguchi, 1987a). A trochus population has been established by such secondary transplantation at Kosrae in 1959 (Gawel, 1982), although an earlier attempt did not appear to be successful (McGowan, 1957).
A few attempts to transplant green snail have been made in the southern hemisphere in the past. So far, only one population transplanted from the New Hebrides (now Vanuatu) in the 1950s has successfully established itself at Tahiti (Yen, pers. comm.). Status of other transplants (to New Caledonia and elsewhere) were either unknown or failure (Gillett, pers. comm.).
Green snail populations are found along the coral reefs in the Ryukyus, up to 29 degrees north latitude, owing to the warm ocean current, the Kuroshio. We conducted a baseline study to examine habitats of green snail at Tokunoshima in the northern Ryukyus, in May 1988, prior to the field survey in the FSM. The green snail population of this island had been subjected to heavy fishing pressure in the past several years and only small numbers of snails were found. However, useful information with regard to the present survey was accumulated by the field study on juvenile and adult habitats.
Fig. 1 Trochus shell harvest and export from Truk and Pohnpei
The information thus collected from the island located at such a high latitude (ca. 28 degree north) might have limited value in application to localities at lower latitudes. However, the reef organisms found on the reefs of Tokunoshima represented those of typical Indo-West Pacific fauna and flora. In fact, there were some other larger archaeogastropods which usually are widely-distributed and sympatric, i.e. trochiids such as Trochus niloticus and Tectus pyramis, and turbinids such as Turbo argyrostomus. These gastropods together with green snail appeared to occupy overlapped habitats. It is important to understand the extent of niche diversification and competition among these larger and potentially abundant gastropods.
Juveniles of green snail, smaller than several centimeters in shell diameter, were found wellhidden and only inside small crevices of limestone substrates on the reef crest zone. Such crevices were formed and occupied usually by an echinoid Echinometra matheai. Larger juveniles and small adults were located in the upper reef slope zone and they were also very cryptic in life habits in the daylight hours. Larger adults appeared to be less cryptic but they usually occurred under ledges or inside caves in the reef slope zone down to 10 meters or more in depth.
Judging from our observations and also information from literature published in the 1880s (Yamaguchi, 1988b), it may be assumed that green snail and trochus would occupy or require very similar habitat: mainly oceanic reef slope for adults and reef crest or outer reef-flat for juveniles. Therefore, the habitat and environmental criteria for trochus transplantation, which were identified by Asano (1963), may be adopted here for considering that of green snail, with slight modifications as follows.
