The success rate of aquaculture ventures in the Pacific islands has been poor (Uwate and Kunatuba 1983; Kafuku 1986). As outlined in the introduction, Trochus niloticus, more than most species, qualifies as a suitable candidate for mariculture in the Pacific islands. Nevertheless, the current knowledge of trochus culture and ranching is incomplete. What is known is the following:
Trochus spawns with an approximate lunar periodicity, usually shortly after new moon;
Females may release about one million eggs per spawning session;
The larval biology is well understood, and very high survival rates during the planktonic larval phase are achievable;
High juvenile survival rates in tanks between larval settlement and release on the reef have been achieved;
Growth rates of trochus are well documented from places throughout their range.
Information is limited in the following areas:
Artificial spawning induction;
Means of achieving sufficiently high survival rates of the juveniles between release on the reef and harvesting.
Virtually 100 percent survival through the planktonic larval phase has been achieved (Nash, unpublished data) and recently, high survival rates of juvenile trochus in tanks after settlement have been achieved in Okinawa, Japan (M. Yamaguchi, personal communication, January 1989). The Okinawan programme is supported by the Fishery Agency of the Japanese Government. Several hundred thousand juveniles are now held for field release. Unpublished research in Okinawa has provided important information on patterns of mortality and predation of juvenile trochus released when 7 mm diameter. Future research there will address the question of size-related mortality after release.
Reasonably high survival rates between releasing the juveniles on the reef and harvesting the adults must be achieved if trochus mariculture and ranching are to have any chance of success. Since conditions affecting trochus survival may differ between countries, it may be necessary to modify or adapt the findings made in one country when establishing trochus mariculture elsewhere. There is some evidence that survival rates of juvenile trochus are highest on reefs which are more elevated (Nash 1988; see Appendix II).
Juvenile trochus inhabit the intertidal or very shallow subtidal boulder zone near the reef edge. Their distribution across the reef top may range from close to the reef edge to several hundred metres back from the edge; this is very largely determined by the distribution of boulders and crevices on the reeftop. Juveniles are seldom found on exposed, flat reef pavement that lacks such hiding places, presumably because of high levels of predation on exposed animals.
The number and density of juvenile trochus on a reef top is therefore limited by the availability of boulders, holes and crevices. This in turn will determine the number of outplanted hatchery-reared juveniles that can grow to harvestable size on each reef. The importance of choosing appropriate sites for ranching is clear.
