1. INTRODUCTION

The marine gastropod Trochus niloticus has been harvested commercially since early this century, primarily for the manufacture of buttons from the shell. Fisheries for trochus throughout its range in the Pacific and north-eastern Indian Oceans declined in the 1950s when plastics replaced natural shell in button manufacture. Since the late 1970s, trochus fisheries have enjoyed a revival because the fashion houses of Europe and east Asian countries, particularly South Korea, have decided to use natural shell buttons on their high-quality shirts (Carleton 1984a,b). Prices paid for the shell by importers in Europe and Asia have been increasing in the last few years, and may now fetch around $(AUS)4,000 (FOB) per tonne of cleaned high-quality shell (Nash 1988). Trochus shell exports from the major trochus-producing countries total 3,000 to 4,000 tonnes annually (Table 1).

Table 1. Export statistics of trochus shell (in tonnes) from the major trochus-producing countries, 1980–87. (Data provided by Peter Philipson, 1989)

The recent price increases have resulted in increasing pressure on trochus stocks, which have been overfished in many parts of their range (Heslinga and Hillmann 1981; Bour and Hoffschir 1985; Nash 1985). One of the most recent examples of overfishing comes from Indonesia, where it is reported that trochus stocks are greatly diminished. Indonesians who wish to continue fishing have had to range further afield, to fish illegally on the reefs of north-western Australia (The Weekend Australian, October 29–30, 1988).

Trochus fishing is well suited to Pacific island countries: it is regarded as a traditional fishery, trochus may be harvested with a minimum of equipment (a mask and sometimes a small boat), and the final product (the shell) may be stored without refrigeration until ready for export or processing.

The suitability of the trochus industry to Vanuatu is demonstrated by the fact that, contrary to expectations, there was almost no turnover of staff employed by the “Trochus Vanuatu” button blank factory in Port Vila. Anticipating that a number of workers would drop out after the first few months, a lot more workers were trained than were needed. Contrary to expectations, none of them left (Anon., 1984), which probably indicates that the work is well suited to the islanders' life style.

Of the several management options that may be used to conserve trochus stocks (which include size limits, limited entry, and individual or total quotas), one that has received attention is the replenishment of fished stocks with hatchery-reared juveniles. Like the fishery itself, trochus mariculture is more suited to the Pacific islands than that of most other species because it is relatively simple: the eggs are lecithotrophic (contain large amounts of yolk) so that the planktonic larvae do not need to be fed; the planktonic larval phase is short (2 to 8 days at 28–30°C: Heslinga 1981; Nash 1985); and once the larvae metamorphose and settle to the bottom of the tank, they feed on the benthic diatoms, and later the algae, that grow spontaneously in the presence of sunlight. Algal requirements will depend on the survival rates, and therefore density, of trochus between spawning and outplanting on the reef (see also Section 5).

In addition, trochus spawns all year round in the warmer parts of its range (Heslinga and Hillmann 1981; Nash 1985), although in higher latitudes, such as New Caledonia, spawning is restricted to the summer months (Hoffschir 1988) with an approximate lunar periodicity (Heslinga and Hillmann 1981; Nash 1985). It should therefore be possible to rear several batches of trochus larvae annually.

Some trochus culture studies have already been carried out in Vanuatu. Martin (1984) examined the feasibility of trochus culture in Vanuatu, and suggested a five-year plan for setting up a trochus research project. Some progress has been made in that direction, with the construction of rearing tanks and a seawater system at the Fisheries Department site in Port Vila, and limited success with spawning and larval rearing of trochus (della Patrona 1986).

This report details the results of a two-week visit to Vanuatu in January 1989 to examine the Vanuatu Fisheries Department site in Port Vila, and to assess its suitability for rearing trochus larvae and juveniles. The objective is to be able to replenish fished stocks of trochus around Vanuatu with local, hatchery-reared juveniles (seed). This visit was supported by the FAO/South Pacific Regional Aquaculture Development Project.