Abalones

Abalone

Biology of species

Abalones are marine gastropods (Fallu, 1991). They consist of a shell and a large muscular foot which is used for attachment onto hard surfaces. The foot is the edible part of the animal and it can account for more than one third of the abalones live weight (Daume, 2002).

Reproduction

Abalones are mainly dioecious meaning that they have separate male and female sexes (Fallu, 1991). On average abalone reach sexual maturity at 50mm, this occurs after approximately 3 years (Lus Mercedes Lopez Acuna,1999). Their method of reproduction is through broadcast spawning, this occurs when the mature male and female abalone in a certain area release their gametes into the water column (Fallu, 1991). This release of gametes is triggered by hormones and is also very temperature dependent. In cultured abalone the spawning is induced by changes in the water temperature (Clavier, 1992) exposure to UV-light irradiated seawater and to the introduction of Tris-buffered hydrogen peroxide (Morse et al, 1977) and exposure to air, This is all obviously species dependent.

Abalone may experience a high mortality rate, approximately around "90% in the transition phase from larva to juvenile (Fallu, 1991). This mortality rate though is compensated by a high fecundity of the adults which is 1 to 5 million eggs and 100s of million sperm (Fallu, 1991).

Spawning can occur when ever temperatures are favourable but in cultured conditions to avoid uncontrolled spawning the water temperatures are maintained to be very stable and warm (LusMercedes Lopez Acuna, 1999).

Feeding

Abalones experience a dietary shift meaning that the juveniles and adults have different diets. This can be attributed to the increase in mouth size and to the morphological changes of the radula as the abalone grows (Kawamura et al, 2001, Fleming et al, 1996). Juvenile abalones main diet consists of benthic diatoms whereas the adult abalone is almost entirely herbivorous with a preference to feed on pieces of kelp or large brown algae (Leighton, 1961).

For cultured abalone the sections of available diets include harvested and cultured seaweed as well as formulated feeds (Lus Mercedes Lopez Acuna, 1999). Formulated feeds though are preferred due to the increase in growth rate being attributed to the higher protein content and protein quality of the formulated feed provided (Lus Mercedes Lopez Acuna, 1999). The production costs of cultured seaweed are also more expensive than the formulated feed. There are also certain difficulties in harvesting seaweed and there is a high transport cost of wet algae from the harvest site to the abalone culture site (Lus Mercedes Lopez Acuna/ 1999). Successful harvesting of seaweed is also dependent on the sea conditions. Therefore it is preferred to keep cultured abalone on a formulated feed. Diet is important in abalone and has been cited as a contributing factor in the control of growth between wild and farmed abalone (Fleming et al, 1996)

Age and growth

Abalones are very slow growers but in turn have a long life-span (longevity) (Fallu, 1991), Abalones growth rate is very species dependent but it usually takes a minimum of four years to reach a shell length of 4-6cm (Newman, 1968, Forster/1967). The growth rate may be increased when fed a formulated feed that has a higher protein content and protein quality.

Potential

The market for cultured abalone has grown rapidly and has kept pace with world production. There has been a decline in many countries of wild abalone and therefore this shortage must be supplied from cultured species (Daume, 2002). The abalone is valued due to the ability to use the meat as a source of food and the shell for implements, trade material and decoration (Daume, 2002). The declines in wild abalone coupled with the value of abalone meat on the world market have promoted the concept of abalone aquaculture internationally. Since they are highly valued but slow-growing mollusks it has attracted more attention as candidates for aquaculture (Fallu, 1991),

They also have a high potential as an aquaculture candidate due to their ability to adapt to the culture environments, both the larval and the adult stage adapt easily and only experience a few stresses (Lus Mercedes Lopez Acuna, 1999).

Environmental requirements

Temperature is the most important environmental factor as it influences metabolic rate and energy expenditure (Fry, 1971). The growth rates of H. tuberculata have been shown to be faster when cultured at conditions of 18°C as at this temperature abalones allocate their energy to growth whereas abalones reared at 22°C allocated their energy to reproductive growth (LusMercedes Lopez Acuna, 1999). This is a problem as it is a commercial disadvantage if abalone spawn at a small size as somatic growth energy is being diverted to reproduction instead of growth (Fry, 1971). Oxygen consumption is also elevated by high temperatures that are up to 22°C (Lus Mercedes Lopez Acuna, 1999). Temperature should be high enough to increase the rate of larval development but not too high so as to increase the risk of bacterial infection in the culture system (Lus Mercedes Lopez Acuna, 1999).

Water salinity plays a significant role in growth and survival of abalone especially juvenile abalone as they are typically Stenohaline (Daume, 2002). The optimum salinity for larval development and grow-out ranges from 32 to 35 ppt (Daume, 2002), There will be high mortalities of young if the salinity falls below 24 ppt. Therefore in cultured conditions all hatcheries and farming facilities should be located away from estuarine areas. The sites for abalone aquaculture require easy access to seawater of high quality (Britz, 1996). Abalones have a low tolerance to the presence of ammonia or nitrite, high or low pH and low or supersaturation of dissolved oxygen (Burke et al, 1999). Therefore the water quality needs to be maintained constantly and should be free from harmful contaminants in the early life stages of abatone; only filtered and UV-treated water should be used.

The water flow rate needs to be sufficient to encourage feeding behaviour, maintain dissolved oxygen levels and to cause wastes to be transferred to the outlet but not so fast as to wash away the feed (Lus Mercedes Lopez Acuna, 1999). In terms of growth under crowded condition juveniles should be reared at densities of 100 to 200 individuals per 100cm2 due to that at higher densities benthic diatoms cannot meet the food demand of growing larvae (Lus Mercedes Lopez Acuna, 1999).

Culture problems

One of the major limiting factors for culture of abalone is the capability to secure a viable and cost effective source of food as well as the growth and feed conversion efficiency of the species (Hahn, 1989), Abalone farming is both expensive and time consuming since Hatiotis species are relatively slow growing (Britz, 1996).

Mortalities in abalone systems can also be caused by bacterial infections due to the individual being weakened by stress from high or too low water temperatures as well as poor water quality especially since abalone have a low tolerance to environmental stresses (Burke et al, 1999). Thus continuous maintenance of the cultured abalones environment is highly needed but time consuming.

References

Britz, P.J (1996) Effect of dietary protein level on growth performance of South African abalone Haliotis midoe, fed fishmeal-based semi-purified diets. Aquaculture. 140:55-61

Burke, C.M., Harris, J.O., Hindrum, S.M., Edwards, S.J. and Maguire, G.B. (1999) Environmental Requirements of Abatone. Final Report, FRDC Project No. 97/323, Tasmanian Aquaculture and Fisheries Institute. 154 pp.

Clavier, J. (1992) Fecundity and optimal sperm density for fertilization in the ormer {Haliotis tuberculata). In; R,A. Shepherd, M.R.Tegnerand R.A. Guzman del Proo (Editors), Abolone of the world: biology, fisheries and culture. Oxford. Fishing New Books. 86-92 pp.

Fallu, R. (1991) Abalone farming. Fishing news books, Farnham, Surrey. 195 pp.

Fleming, A.E., RJ. Van Barneveld & P.W. Hone (1996) The development of artificial diets for abalone: a review and future directions. Aquaculture 140: 5-53

Forster/ M.R (1967) The growth of Haliotis tubercutata: results of tagging experiments in Guernsey 1963-5. Journal of the Marine Biological Association of the United Kingdom. 47:287-300

Fry, F.E.J. (1971) The effect of environmental factors on the physiology of fish. In; W.S. Hoar and D.J, Randall (Editors), Fish physiology. Academic Press, New York. 9:1-98

Hahn, K.O,, (1989) Handbook of Culture of Abalone and other Marine Gastropods. CRC Press/ Boca Raton, Florida, 348 pp.

Kawamura, T.S. Takahiro, H. Takami/Y. (1995) Dietary value of benthic diatoms for the growth of early juvenile abalone Haliotis discus hannai. Journal of experimental Marine Biology and Ecology. 194:189-199

Leighton, D.L. (1961) Observations of the effect of diet on shell coloration in the red abalone, Haliotis rufescens Swainson. Veliger. 4:29-32

lus Mercedes Lopez Acuna (1999) Growth and the energy budget of juveniles of the abalone Haliotis tuberculata. University of Southampton, Department of Oceanography, PhD thesis, 1-197

Morse, D.E., Duncan, H.. Hooker, N. & A.N.C. Morse. (1977) Hydrogen peroxide induces spawning in mollusks, with activation of prostaglandin endoperoxide synthetases. Science. 196:298-300

Newman, G.G. (1968) Growth of the South African abalone Haliotis midae. tnvestigational Report Division of the Sea Fisheries South Africa. 67:1-24

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