Spawning and tank culture of T. niloticus were first carried out by Heslinga and Hillmann (1981). Standardised induced spawning and mass production of juveniles were achieved by Lee (1997). Trochus have been cultured at the Northern Territory University, Darwin, and at the pilot hatchery at One Arm Point, Dampier Peninsula. Juveniles will be produced on a commercial scale in the Multi-Species Hatchery (MSH) currently being constructed in Broome for the Kimberley Aquaculture Aboriginal Corporation (KAAC). As trochus juveniles grow in tanks, they will eventually graze the tank's microalgae ( Navicula spp. and Nitzschia spp). too heavily, resulting in slowed growth or starvation. Juveniles can be cultured in tanks to adult size, but this is uneconomic because densities need to be lowered to avoid overcrowding. The estimated cost to culture juveniles to a size of 1 - 3 mm is as low as 0.7 cents/juvenile, but increases to 29 cents/juvenile if the animals are grown up to 6 - 10 mm in land-based systems (Lee, 1997). Therefore, it is most cost-effective to remove juveniles from tanks and release them onto reef sites at less than 3 mm in diameter, a size attained at an age of about two months. Hatchery produced trochus can often have much smoother shells than small wild juveniles of similar size and this may influence their vulnerability to predators (Purcell, in review).2 Hatchery Production In recent years, research has refined the culture techniques for trochus. Broodstock can now be readily induced to spawn throughout the year, without damage to the shell or animal. Broodstock are collected from intertidal reefs and kept together in tanks or tubs in seawater provided with strong aeration at ambient temperature (for example 28 o - 30 o C). Between 30 and 100 animals are needed for successful spawning induction as only a small percentage of them will be ready to spawn at any one time. The adults can be induced to spawn using a 'temperature shock' method (Lee, 1997). At sunset, the water temperature in the holding tank is raised by 2 - 3 o C and maintained. Periodically, the spawning tank is drained and 'new' seawater is added. Adults will usually commence spawning within one hour, although sometimes this procedure needs to be repeated. The neutrally buoyant, fertilised eggs are gently removed from the holding tank using a fine (100 µm) mesh net. Several hundred thousand eggs are transferred into each culture tank (1000 to 3000 L) containing high quality, aerated seawater. Seawater should be filtered (to 5 µm) to exclude the larvae of animals which may eat or compete with trochus juveniles. Alternatively, treated marine bore-water can be used. Trochus larvae do not feed until after they settle, so there are no requirements for algae until this time. The culture tanks should be situated under shade-cloth, which reduces UV light and allows partial sunlight to provide good growing conditions for the benthic diatoms (a group of algae). The bottom of each tank is covered with a thin layer of coral rubble, acting as substrate for benthic diatoms upon which the juveniles feed. Cultures of benthic diatoms are introduced into the tanks up to a week prior to spawning and fertilised with 20 - 50 ppm 'Aquasol' (a commercial fertiliser used in horticulture). For optimum larval survival and juvenile growth, the salinity in culture tanks should be maintained at 35 ppt and the temperature kept at 30 - 33 o C. More than 50,000 juveniles can be grown to three millimetres size in each tank (Lee, 1997). |