Eels - aquaculture prospects

Introduction

There are four species of eel found along the east coast of Australia, however, the two species most suitable to aquaculture in NSW are the longfin eel (Anguilla reinhardtii) and the shortfin eel (Anguilla australis). Both these species have a natural distribution from Tasmania, northerly through Victoria, and up the east coast to Queensland (QLD). The distribution of the longfin eel extends to northern Queensland whereas the shortfin eel is found only as far north as south-east Queensland.

It is thought that the Great Dividing Range acts as a natural barrier to the distribution of eels from the eastern drainage basin into the western drainage basin. However, both species have been recorded in parts of the Murray-Darling basin and are thought to have entered via the Murray River. Both species are also native to New Zealand.

Biology of eels

Eels have a complex reproductive life-cycle. Eels are known as catadromous - that is, they live in freshwater but migrate to the ocean to breed. Every year adult eels (known otherwise as silver eels) migrate from the east coast of Australia and New Zealand to the Coral Sea, where it is thought that they spawn at depths of around 300m. Eels spawn only once in their lifetime, and after spawning it is presumed that the adult eels perish.

Once the eggs hatch, ocean currents carry the tiny larval eels, known as leptocephalia, back to the continental shelf. At around eighteen months of age they metamorphose into 'glass eels'. At this stage they are transparent and undeveloped.

The glass eels are carried by tides and currents back to shore and into coastal estuaries, where they undergo further development. Colour pigmentation forms and the glass eels metamorphose into 'elvers'. Elvers are an exact copy of the adult eel but are much smaller ranging from 8-20cm, and 1-3 years of age.

The elvers migrate upstream to occupy all estuarine and river environments where they mature into adult eels and apparently live for an average of 15 - 20 years. During high rainfall or flood events, populations of eels are further dispersed into off-river storages such as farm dams.

Populating land-locked waterways is also assisted by their ability to cover considerable distances over land in damp conditions.

NSW industry status

While a significant commercial fishery for eels exists in NSW (both from dams/impoundments and estuaries), and export markets are available, the eel aquaculture industry in NSW is small. For the production figures see the NSW DPI aquaculture production reports.

The world aquaculture production of eels is estimated to be in excess of 130,000 tonne/annum, worth over $1.3 billion. Most of this production occurs in Asia, with China, Japan and Taiwan the biggest producers.

Impediments to eel aquaculture in NSW

Due to their extremely complex life cycle, eels have not been bred in hatcheries. As a result the eel aquaculture industry worldwide has so far had to rely on the harvesting of glass eels from the wild for the stocking of aquaculture ponds. Overseas, annual eel production from aquaculture has begun to decrease, due to a number of reasons. One of the overriding factors has been the decrease in glass eel supply. This has in part been attributed to over harvesting of glass eels causing recruitment or reproduction to fall.

With limited knowledge of our eel stocks and in an effort to avoid these problems, NSW DPI is taking a cautious approach to the harvesting of glass eels and the development of eel aquaculture in this State. While there is huge potential for a viable industry to develop, we must avoid those problems experienced overseas.

Policy has been developed by NSW DPI regulating the eel aquaculture industry in conjunction with the harvesting of glass eels.

Growth and production

Eels can be grown in both pond and recirculating tank systems. Most of the research in Australia has focused on pond production, however, most production in places like Europe is undertaken through the use of indoor recirculating tank systems.

Stocking rates in tank systems and intensive pond systems vary, depending on the capacity of the system and the intensity of the operation. In well-developed tank systems, stocking rates can reach upwards of 80kg/m3, while in super intensive pond systems, stocking rates can exceed 20 tonne/ha.

The ideal temperature range for eels to remain healthy and to efficiently convert feed into growth is 23-28°C. Temperatures above optimal will result in lowered feeding and growth rates, stress and sometimes death.

For pond based systems, it is suggested that the north coast of NSW would provide an ideal climate, while intensive tank systems that are temperature controlled could possibly be located anywhere. Under a climate controlled environment it is expected that eels can be raised to market size (200-300g) after 18-24 months. Growth rates to market size in 9-12 months have been reported overseas. Growth in extensive pond systems is usually much slower.

Eels require high protein diets for efficient growth. Commercial diets are mostly imported from overseas and consist of a paste like substance for glass eels, to an extruded hard pellet for older eels. Food conversion ratios (FCR) for Australian eel species should be in the range of 1.5-2:1 (kg of food: weight growth), however FCR's as low as 0.9:1 have been recorded overseas.

Eel farming also requires access to large volumes of water. Due to the often very high stocking densities and messy feeding behaviour of the eels, regular water exchange is usually a necessity. For intensive pond culture of eels, NSW DPI recommends a water budget of at least 40ML/ha/yr.

Behaviour

Although suitable for highly intensive culture, the growth of eels in intensive systems is generally variable. Growth variation will to some extent be a natural phenomenon. Larger eels will tend to dominate food resources and in order to prevent a pecking order becoming established and to ensure sufficient amounts of feed reach smaller eels, regular size grading should be practiced. Eels are top order freshwater predators and without regular grading, cannibalism will occur.

This will also assist in marketing and harvesting by providing uniform growth to market size.

Site selection and system design

If pond production is the chosen method for the culture of eels, a number of factors should be taken into account. A potential site should have the following characteristics:

• Above the 1/100 year flood heights.
• Good clay soil capable of holding water without leakage.
• Away from areas of acid-sulphate soils and high water table.
• Access to abundant supply of good quality water (at least 60ML/ha/yr).
• Suitable climate for pond culture of eels.
• Adequate area for expansion and irrigation of effluent.
• Suitable topography (ie gentle slope to allow for gravity drainage of ponds).
• Close to markets and infrastructure.

Choosing a site for tank culture is generally easier because less land area and access to water volume is needed, while factors such as topography, soil quality and climate are not so much of an issue.

A basic intensive recirculation system should consist of a number of tanks (usually 1000-13000L in size) independently or group filtered by large biofilters, which are used to strip nitrogenous waste and nutrients from the water. Recirculation systems can also incorporate a number of other units including UV and ozonation systems to disinfect water and protein skimmers to remove protein based wastes etc.

After passing through the filters, the water is then recycled back to the tanks. The entire system should be contained within a vermin proof, climate controlled housing. Specialised technical advice should be sought to determine the best set-up for your needs.

Water quality

Eels can be raised in both freshwater and saltwater. Eels by nature are also quite hardy, hence their ability to do well in highly intensive culture conditions. This is not to say that water quality can be neglected. The high intensity nature of eel farming combined with eel's messy feeding habits and high protein diet requirements can mean that water quality may suffer.

As a general rule biomass in ponds should not exceed 10 tonne/ha, or 50m3 in tanks.  High water quality should be maintained  a pH of around 7, alkalinity in excess of 20ppm, free of pesticides and other contaminants, dissolved oxygen levels no lower than 3ppm and free ammonia levels of less than 0.2mg/L.

Eels will usually tolerate levels worse than those specified, for very short periods, however this will stress the fish and leave them susceptible to various bacterial, fungal and viral infections, leading to higher mortality rates and possible transmission of infection to other ponds.

Diseases and parasites

Because glass eels are sourced from the wild, they may already be infested with various parasites or diseases, or alternatively act as disease carriers. However, the acclimatisation to freshwater that usually occurs after capture eliminates most parasitic infections through osmotic forces. Bacterial and fungal infections on eels raised in freshwater can be usually treated by simple salt baths at a rate of around 10ppt (=10g/L salt).

Eels, like most fish, are particularly susceptible to infection at times of high stress, for example when water quality has deteriorated, there is constant presence of predators in ponds, or at times of handling. While eels are generally quite tough, they still need to be handled with care. As in most forms of aquaculture, the best cure is prevention and a combination of good husbandry and management techniques will ensure that stock remains relatively disease free.

The maintenance of disease free stock involves the quarantining of all new eels to the site and regular salt bathing of eels in quarantine facilities. Water quality should be maintained to relieve stress and regular inspection of eels or elvers should be undertaken to monitor health and condition.

At the first sign of a disease outbreak a sample of eels should be inspected and water quality checked. If a source of infection or disease cannot be identified, a professional fish veterinarian should be consulted.

Marketability

More than 130,000 tonne/yr of eels is produced worldwide; the main producers being China, Japan and Taiwan. Over 70% of this product is produced for the Japanese 'Kabayaki' market.

Kabayaki is a style of serving eels, where eels of around 150-200g are butterflied, placed on skewers, basted in a thick soy based sauce, and steamed or grilled. More than 90% of eels consumed in Japan are served this way, with eel being the most widely consumed freshwater fish in Japan. However, the quality of eel for this market is highly prescriptive and investigation should be undertaken to determine the potential of this market.

The Australian shortfin eel is very similar in appearance to the species of eel favoured by the Japanese market (Anguilla japonica). As such the shortfin eel is well accepted in Japan and attracts similar prices to (A. japonica) There is potential for Australian producers to export shortfin eels to Japan for this market.

The longfin eel is different in appearance to the species (A. japonica) and as such is not as well accepted in Japan, leading to lower prices. However, the longfin eel is similar to the species favoured in China (Anguilla mamorata), and as such there is some potential to export this species.

For eels, it is hard to evaluate the size of any potential markets for Australian producers. Certainly the export markets are present and active, however the very nature of eel farming (i.e. reliance on glass eels for seedstock, seasonal nature of glass eel collection, limited technical know-how etc) has restricted development.

Declining eels stocks overseas may work to the advantage of Australian producers. Australia and in particular NSW are fortunate to have avoided most of the problems experienced by other countries, such as waterway pollution, subsidence of land from excess bore water extraction, disease and particularly excessive over-harvesting of glass eels.

There is excellent potential to promote the 'clean green' image of Australian aquaculture produce, while we ensure that the activity is conducted in a sustainable manner. It is anticipated that there will be great potential for Australian producers to step in and replace some of the shortfall in supply that may arise overseas.

Furthermore, as Australia's multicultural population increases, there may be opportunity for a small domestic market for eels to develop.

Further information

For further information, call NSW DPI Port Stephens Fisheries Centre: on 02 4916 3900.

You can obtain copies of licensing and policy requirements for eels from here. Further information on the farming of eels can also be obtained from the following references;

References

Beumer, J (1983). 'Suitability of Anguilla australis for Intensive Culture'. Aust Aqua, (4)2:13-18.

Forteath, N (1994). 'Eel Culture In Australia'. Austasia Aquaculture, 8(5):54-55.

Forteath, N (1990). A Handbook on Recirculating Systems for Aquatic Organisms. Fishing Industry Training Board of Tasmania Inc, Australia.

Hart, P and O'Sullivan, D (eds.) (1993). Recirculation Systems: Design, Construction and Management. Aquaculture Sourcebook, Tasmania.

Gooley, G (1998). 'Eels'. The New Rural Industries - A Handbook for Farmers and Investors. Rural Industries Research and Development Corporation.

Reeve, J (1998). 'Eel Farming'. Proceedings from the Queensland Warmwater Aquaculture Conference (Status and Potential) 1998. Aquaculture Information Technologies (ed).

WBM Oceanics Australia (1995). 'Feasibility Study for Eel Aquaculture in Queensland. Queensland Department of the Premier.

Wray, T (1995). 'Eels in Japan'. Fish Farming International. 22(10):10-11.

NSW Fisheries (1998). 'Eel Aquaculture Policy'