Eel Aquaculture
Officers of the Agency for Food and Fibre Sciences, Fisheries and Aquaculture Business Unit, revised by Clive Jones, Freshwater Fisheries and Aquaculture Centre, Walkamin
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Two freshwater species of eels found in Queensland waters have been assessed for their suitability for aquaculture. They are the shortfin eel, Anguilla australis and the longfin eel, Anguilla reinhardtii. Shortfin eels occur in the coastal rivers of southeast Queensland while the longfin eels occur along the entire coast of Queensland.
Culture environment
Eels grow rapidly in a tropical climate preferring temperatures between 23° C and 28° C. In ideal conditions they grow to marketable size (150 to 200g) in 12 to 18 months, although growth rate can be extremely variable. For pond-based aquaculture, the best site is one with constant water supply, and which is not susceptible to flooding. Bore water is suitable as long as it is free from pathogens and chemical residues and has a pH of 7.0 to 8.0. Highly acidic water is not acceptable. A gently sloping site is advantageous to maximise use of gravity for filling and draining ponds.
For intensive tank-based eel aquaculture, site selection criteria are less restrictive, although a source of good quality water is still essential.
Breeding and life cycle
Shortfin and longfin eels have similar life cycles (Figure 1). Both species spawn in the Coral Sea in waters deeper than 300 m. Each female produces 5 to 10 million pelagic eggs, which hatch into larvae called ‘leptocephali’ in 2 to 10 days. Ocean currents carry the leptocephali to the continental shelf, where they develop into ‘glass eels’. The glass eels, which are toothless and unpigmented, move into estuaries with the assistance of currents and tides. Shortfin glass eels move into estuaries between late autumn and spring, while longfin glass eels enter estuaries mainly in summer and autumn.
When glass eels develop pigment they are called ‘elvers’. These elvers then move from the estuaries into lakes, swamps, and the freshwater reaches of rivers and creeks and other accessible water bodies (for example, farm dams). For shortfin eels this migration usually occurs during winter while longfin eels usually move during summer and spring. In general, males are found in the lower reaches, while females move further inland.
When freshwater eels reach maturity, they move downstream to the entrances of rivers and creeks, before beginning their spawning migration to the Coral Sea. In preparation for this, they undergo several physical and physiological changes. They become silvery, their eyes enlarge, their stomach degenerates and is absorbed and their gonads enlarge. Mature eels are referred to as silver eels and do not feed. The age at maturity varies considerably in the wild, with estimates of 8 to 12 years for males and 10 to 30 years for females. It is thought that all mature eels die after spawning.
For aquaculture, glass eels or elvers must be caught from the wild for stocking to the culture facilities. These animals are caught using nets in the estuaries and lower reaches of coastal rivers. Licences are required to catch them, and the quantity allowed to be caught is related to the size of the production facilities. Prospects for closing the life-cycle to enable captive spawning and production of glass eels are considered longterm (20 years +).
Growout
Glass eels and elvers are best cultured in tanks before being moved into growout facilities. Growout of elvers to market size can be achieved in either tank systems or earthen ponds. Tank culture can be managed in tanks of 1,000 to 20,000 L, on flow-through or recirculating water supply. Recirculation requires application of sophisticated treatment technologies to maintain suitable water quality. Earthen ponds for eel growout should be constructed on non-porous soils. It is possible to line ponds so that they do not leak however costs of construction will be higher. Ponds range in size from 0.1 ha to 0.5 ha, and are 1 to 1.5 metres deep.
Eels require large amounts of oxygen if they are to remain active and grow quickly. Aeration can be supplied using paddle wheels or aspirators. Blooms of phytoplanktonic algae are encouraged as they produce oxygen and shield the eels from direct sunlight. Water is exchanged as necessary to maintain water quality. The pH, dissolved oxygen, temperature, total nitrogen and nitrite levels need to be checked on a regular basis. If water quality deteriorates corrective measures need to be applied quickly to ensure no stock are lost.
Eels should be quickly weaned onto artificial feed from the time they enter the farm. A range of commercial fish diets (pellets) are available in Australia which are suitable for short and longfin eels. They are fed several times per day, which ensures that they are healthy, and grow rapidly.
Harvesting
Eels are harvested at a weight varying from 150 g to several kilograms, depending on the target market. Harvesting can be carried out by draining the pond using a net attached to the outlet pipe, using a seine net or at feeding time using a scoop net. The eels are then sorted into different sizes using a grading tray. The eels are then placed in holding tanks for several days without feed to purge their stomachs. They are chilled then packed into strong plastic bags with just enough water to ensure that their skin remains moist. Eels can breathe through their skin. The bags are then filled with oxygen and transported to the market.
Marketing
The largest market for farmed eels is the Japanese ‘kabayaki’ market. The Japanese consume more than 110 000 tonnes of eels a year but domestic production is only about 30 000 t. The kabayaki markets prefer eels weighing 200 g. Australian shortfin and longfin eels can be grown to this size in 12 to 18 months. A. australis closely resembles the Japanese eel, A. japonica, in both appearance and taste. The Japanese prefer eels that are uniform in colour so the potential for acceptance of A. australis into the Japanese market is high. A. reinhardtii’s mottled appearance detracts from its market appeal in Japan. This may be overcome as the kabayaki cooking process effectively disguises the mottled look and the taste is similar to the Japanese eel. Longfin eels are readily accepted throughout other parts of Asia and Europe, and particularly for larger size grades. Despite the high price paid for kabayaki eels, marketing of large eels up to 5kg each into alternative markets may be equally if not more economic.
Potential
Eel aquaculture relies on the availability and sustainability of its seed stock resource. Studies to date suggest a limited availability of glass eel resources in Queensland, although their full extent may take many years to determine. To ensure the available resource is not over exploited the Queensland Government manages the collection of glass eels, and does not permit their export. Heavy penalties apply. Without such controls the development of a sustainable industry would be severely impaired. Assessment of the acceptance of eels cultured in Queensland by the overseas markets suggests good market potential exists.
Queensland offers a good environment in which to culture eels. There is plenty of clean water and land available for aquaculture. The government has expressed its desire to help foster this industry through promotion abroad and locally and by funding research into eel aquaculture.
References
Gooley, G.J. and Ingram, B.A. (Editors) (2001). Assessment of Eastern Australian Glass Eel Stocks and Associated Eel Aquaculture. Final Report Project 97/312 and 99/300. Fisheries Research and Development Corporation, Canberra.
Russell, B.J., (1995). A Survey on the Distribution and Composition of Glass Eel Populations Anguilla spp. in south-east Queensland, April to August 1995.
Usui, A., (1991). Eel Culture. Fishing New Books, Oxford, U.K.
WBM Oceanics., (1992). Feasibility Study for Eel Aquaculture in Queensland. Prepared for Department of The Premier, Queensland.
Zeller, B.M., & Beumer, J.P., (1996). Eel harvesting and Culture in Queensland. DPI Publication, QL95021.
Additional fisheries information
Check FISHWEB at: www.dpi.qld.gov.au/fishweb/ or contact: Department of Primary Industries Call Centre: Telephone: 13 25 23 (cost of local call anywhere in Queensland).
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
