Abalones belonging to the genus Haliotis are the most valuable marine gastropods and probably the most sought-after molluscan seafood in many areas. They are marketed in fresh, frozen, canned and dried forms and are eaten raw or cooked. The valued meat is the very large foot or right shell muscle, one of which can provide several sliced steaks. The shells are also of economic importance as they are used in traditional medicines and also for decorative purposes and jewellery. The major producing countries are Mexico, Japan and Australia. The USA, New Zealand, South Africa, North and South Korea and Canada also land fair quantities.
Because of the increasing demand and diminishing natural stocks, attention has been directed to enhancing the stocks through transplantation and stocking of open waters with hatchery-reared seed. Significant success
in stocking abalone beds with hatchery-produced seed has been achieved in Japan and North Korea and several million seed are annually planted. Recoveries of 15–20 per cent of the stocked abalone have proved the economic viability of this practice under Japanese conditions. Even though the main thrust of abalone culture continues to be natural stock enhancement, concerted efforts are underway in the USA, Japan and China to develop commercially viable methods of growing abalone to market size under controlled conditions. The production so far by aquaculture is rather small, but some of the research findings are of special interest.
There are about 80 species of abalone, distributed in temperate and semi-tropical coasts, which show major differences in thermal requirements and food preferences. These affect not only their growth and survival rates but also their colour and meat quality. The largest abalone world-wide and the most important North American species extending down to Mexico is the red abalone Haliotisrufescens (fig. 28.6). The green abalone, H. fulgens, and the white abalone, H. sorenseni, areknown for their high-quality meat, but contribute relatively less to commercial landings. Of about 10 species of abalone found in Japan, the most important is H. discus hannai. The others of importance are H. diversicolor, H.gigantea and H. sieboldi. Haliotis discus hannai is the important species in North and South Korea. The Australian and New Zealand species of abalone are of smaller size and therefore of less commercial value.
Abalones are nocturnal and live in rocky belts. The sexes are separate, but occasionally hermaphroditic individuals have been found. In warm climates they may spawn throughout the year, but in colder areas spawning may be only during the warm months of summer. Spawning of gravid abalones is triggered by sudden changes in water temperature, exposure to air or release of gametes by other spawning abalones. A sudden contraction of the foot muscle caused by such factors forces out the eggs and milt. They are highly fecund and a large red abalone may spawn as many as 10 million eggs at a time. Fertilization is external and fertilized eggs sink to the bottom, where embryonic development takes place. In most species, the planktonic trochophore larvae hatch out within about a day under favourable temperature conditions. The veliger stage lasts for about a week, by which time the complete shell and operculum have developed. The larvae then seek out suitable substrates and settle. In the absence of a suitable substrate the larvae can prolong the planktonic stage up to about three weeks.
They settle easily on coralline red algae (such as Lithothamnium and Lithophyllum spp.) and this is reported to be due to a biochemical inducer present on the surface of these algae. Once a suitable substrate is found, the larvae attach themselves by their feet and soon after start feeding on the attached algae, metamor-phosing into juvenile abalones. The most interesting aspect of the larval development and metamorphosis of abalone is that the larvae are completely dependent on the egg yolk for their nutrition until they reach the veliger stage. So the difficult problems of feeding early larvae do not occur.
As the juveniles grow in size they feed on epiphytic diatoms and other microscopic algae. Abalones are known for their slow growth rates, but the growth can be enhanced to a certain extent by increasing the water temperature and abundance of the preferred algal species. Each species of abalone has its own preferences for algae. For example, the preferred algae for the North American species include brown algae: giant kelp (Macrocystis), bull kelp (Nereocystis), feather boa kelp (Egregia); red algae:Gigartina, Gelidium and Plocamium; and green alga: sea lettuce (Ulva).The young (4–5mm) of the Japanese species feed on Undaria, Eiosenia, Codium, Aalymenia andUlva. The adults show greater selectivity and choose, in order of preference, brown algae, followed by green and then red. The colour and pattern of the abalones’ shells are very much dependent on the algae they feed on.
Adult abalones lead a sedentary life in crevices or rock ledges, but the juveniles are more mobile. They forage mainly at night on drifting macroalgae. The typical growth rate is 20–30mm per year. As light has a depressing effect on feeding and growth, enhanced growth can be obtained by reducing the lighting. Doubling of the growth rate has been achieved on an experimental scale.
Wild or hatchery-reared brood stock can be used for controlled production of seed. If wild brood stock are used, it is considered essential to condition them in holding tanks for two or three weeks. Investigations on the Japanese abalone, H. discus hannai, show the importance of conditioning at an optimum temperature and feeding on the preferred food of fresh seaweed (e.g. Undaria and Laminaria) for successful maturation. According to Kan-No (1975), if the rearing is carried out at 20°C, the species will attain maturity in about 80 days, even in the winter season. Maturity can be maintained for at least three months.
Since the stimuli of elevated water temperature and/or air drying do not provide a reliable means of inducing spawning, the use of ultra-violet irradiated sea water is sometimes adopted to ensure consistent spawning. The gravid animals are exposed to flowing heated sea water, irradiated with UV light. An irradiation of 800 milliwatt hours per litre is reported to be adequate, and spawning occurs in about three hours.
Another method of spawning abalones is the one developed in the USA of exposing brood stock to hydrogen peroxide.This is based on the finding that hormone-like prostaglandins regulate spawning in abalones: hydrogen peroxideactivates the natural enzymic synthesis of prostaglandins in gravid animals and so spawning can be induced. Gravid stock are kept in suitable containers of sea water (temperature 12–18°C) made alkaline to a pH of about 9.1 by the addition of sodium hydroxide. A 6 per cent solution (freshly diluted from a 30 per cent stock solution) of hydrogen peroxide is introduced into the container at the rate of 50ml for each 12l of water. After an exposure of about 2.5 hours, the water is drained and immediately replaced with isothermal fresh sea water.
Spawning can be expected to occur within 2.5–3.5 hours.
If the spawning tank contains both females and males, fertilization of the eggs takes place in the tank itself. A sex ratio of one male to four females is maintained in such tanks. If, however, the males and females are spawned in separate tanks, the gametes can be collected and fertilized separately. The fertilized eggs are washed free from excess sperm and incubated in clean sea water at a temperature of about 14–16°C. The larval trochophores hatch out 18–24 hours after fertilization. In about seven days they reach the veliger stage and if care is taken to maintain water quality and prevent microbial growth in the culture, very high larval survival rates can be expected. Species like H. diversicolor develop faster and may assume a benthiclife within 43–46 hours after hatching.
For larval settlement, special tanks made of fibreglass provided with filtered running sea water are commonly used. The tanks are ‘sea-soned’ with a growth of benthic diatoms, bacteria and microalgae, in particular red algae. Fluorescent lighting is provided to promote the growth of diatoms in the tanks. The stocking density of larvae appears to vary very considerably, depending on the water quality and methods of feeding.
Investigations have shown that the abalone requires a specific biochemical inducer for normal settlement, metamorphosis and rapid subsequent development of juveniles, which has been identified as the amino acid gamma-aminobutyric acid (GABA), contained in the red algae. The addition of a low concentration of this inducer causes rapid synchronous and completely normal settlement, metamorphosis and juvenile growth. Crustose coralline red algae, or specific proteins derived from thesealgae, cause a similar induction of settlement and metamorphosis, but in culture systems predation by microscopic faunae associated with the algae may cause mortalities. The use of GABA is therefore considered more convenient and inexpensive. Another means of inducing larvae to settle is by using the mucus of juveniles and adult animals together with diatoms (Chew, 1986).
Juvenile abalones of 5mm size are generally transferred to grow-out tanks, and are gradually introduced to macroalgae. For the juveniles of the Japanese species of abalone, particularly H.discus hannai, the most suitable food for growthare Undaria and Eisenia, followed by Codium,Ulva, Grateloupia and Rhodymenia. Benthicdiatoms are very suitable food until the juveniles reach a shell length of about 20mm.
Egregia and Macrocystis are commonly fed toabalones in the USA. When they reach a size of 15–20mm, a diet of macroalga is required. The stocking density in grow-out tanks with gravel substrates in Japan is about 2000–2500 per m2. The mortality of young abalones of 5mm size in commercial culture is very high (as high as 99 per cent), but it is much less (about 10 per cent) from the 5 to 30mm stage.
On the Californian coast in the USA, abalone growers concentrate on growing to the speciality market size of 5–10cm, rather than the normal commercial size of 18cm harvested from natural stocks. It takes two to five years to grow them to the gourmet size. Considerable research is presently underway to enhance growth rates by artificial feeds and by using intensive culture in tanks, raceways or ponds or containment systems in the open ocean or protected bays. In raceways supplied with heated water from power plants, abalones are reported to have grown four or five times faster than in the natural environment. Artificial diets for young abalones, containing sodium alginate extracted from the giant kelp Macrocystis, which also serves as a feeding stimulant and binder, have been in use for some time now. A crude protein content above 20 per cent has been found to be adequate for normal growth.
In China, hatcheryraised seed abalones are grown in onshore areas, inside plastic cylinders covered with close-meshed sieves (fig. 28.7). They are suspended from long lines in a poly-culture system with scallops and seaweeds. The sea water is fertilized regularly by the spraying of inorganic fertilizers and the algal production induced by this adds to the availability of natural food for the abalone. They are reported to grow to a marketable size of about 6cm in one to two years.
As mentioned earlier, the most successful system so far is the planting of hatcheryreared seed in protected areas of the sea. Several ways of increasing survival of the planted seed have been tried. One is the provision of portable habitats made of concrete blocks or shelves, to serve as substrates and refuges on the sea bed. Another is the use of transplantation cages to acclimatize the seed and to reduce initial mortality. Planting larger juveniles (about 22mm size) has been found to give an average recovery of 23–31 per cent.
Diseases and parasitic infections have not yet become a problem in abalone culture. Post-larval mortality is generally caused by predation by small worms and crustaceans hiding on algal substrates. Several species of fish, crustaceans, starfish and other molluscs prey on juvenile abalones. Octopus, starfish, rock fish, rays and the sea otter,Enhydra lutris, have been observed to be particulaly important predators of adult abalones.