Rural Industries Research & Development Corporation

The New Rural Industries
A handbook for Farmers and Investors

Eels

Introduction

There are four species of freshwater eels endemic to Australian coastal catchments, all belonging to the Anguillidae family. The two species with most potential for commercial farming are the shortfin eel, Anguilla australis Richardson and the longfin eel, A. reinhardtii Steindachner.

The shortfin eel is typically a temperate species but with a natural range which extends from south-east Queensland through to Victoria, Tasmania and the Murray River in South Australia.

In contrast, the longfin eel is typically a more sub-tropical species but also has a broad natural distribution extending from northern Queensland through to eastern Victoria and north-eastern Tasmania.

Anguillid eels, including Australian shortfins and longfins, have a unique life-cycle in which the sexually mature adult eels migrate out to sea to spawn at depths of >300 m. The tiny larval eels, or `leptocephali', are then thought to be carried in large numbers (hundreds of millions) by oceanic currents back to the continental shelf before they metamorphose into the next developmental stage known as `glass' eels (up to 12-18 months of age).

The glass eels are carried by tides into the estuaries of coastal rivers where they undergo further development to become `elvers' (up to 1-3 years of age), which have adopted the adult form in all respects other than size. The elvers then undertake a more active secondary migration into the freshwater, upper reaches of the catchment where they grow and develop into sexually mature adults before returning to the sea to spawn (average 10-25 years of age, although this varies with species and location).

All Anguillid eels are thought to spawn only once. Australian shortfin and longfin eels are thought to spawn in the Coral sea, and the return trip from spawning grounds to fresh water and back can cover several thousands of kilometres.

Because Anguillid eels cannot be successfully bred artificially, all aquaculture seedstock is obtained by harvesting natural stocks of glass eels and brown elvers annually during their respective migrations.

Markets and marketing issues

The world aquaculture production of freshwater Anguillid eels is currently estimated at more than 130,000 t/year, worth over US$1.3 billion. The bulk of this production occurs in Asia - with China producing approximately 50,000 t/year (mostly farmed), Japan 35,000 t/year and Taiwan 34,000 t/year - and to a lesser extent Europe (10,000 t/year). Approximately 70% of the Asian production is for the Kabayaki market. Kabayaki is a steamed, grilled, spiced eel product served on skewers with a specially prepared sauce. It is considered a delicacy by Japanese consumers and is in huge demand. Cultured Kabayaki eels are typically about 150-250 g in weight (average 200 g) and sold live, although frozen eels are also used.

Glass eels are the seedstock of choice for Asian and European commercial eel aquaculture which relies on the associated wild glass-eel fishery for A. anguilla in Europe (250-1000 t/year) and A. japonica in Asia (100-150 t/year).

Glass eels range from approximately 5000-8000 individuals or `heads/pieces' per kg at harvest. Market prices for these glass eels ranges from US$750-10,000/kg, with A. japonicus being the preferred and therefore higher-priced species in the Asian market.

In contrast to the increasing global demand for cultured eels, worldwide eel production is diminishing. The main reason is a decline in Asian and European glass eel stocks because of the combined effects of overfishing (for both glass eels and adult stocks) and environmental degradation.

Production of Australian shortfin and longfin eels is currently around 500-600 t/year worth approximately AUS$4-6 million. The vast majority of this production comes from `stock enhanced' wild fisheries (also referred to in the industry as `cultured' eels), in which wild elvers and sub-adult eels are taken from coastal rivers to lakes, swamps, wetlands etc., where they are left to grow to marketable size under natural conditions. Elvers are the preferred seedstock for such practices, and at present most such elvers come from Tasmania. When available, Tasmanian elver prices typically range from AUS$250 to 350/kg. Victoria has the largest annual production based on this practice, estimated to be approximately 250-400 t/year.

There is at present no commercial supply of Australian glass eels, nor are eels produced in commercial quantities in Australia by intensive aquaculture. An assessment of Australian glass-eel stocks and the development of intensive-culture technology is being undertaken for shortfin and longfin eels as part of a collaborative R&D project between Queensland, NSW, Victoria and Tasmanian state fisheries with additional funding from the Fisheries Research and Development Corporation.

Pilot-scale commercial production is also under way in some states, with state agencies in Victoria, Queensland and NSW providing access to limited quantities of glass eels (from 50 to 200 kg/State) and intensively cultured elvers (Victoria only) on a trial basis.

The sustainable yield of Australian glass eels for aquaculture seedstock is not known. There appears to be little commercial potential for exporting Australian glass eels or elvers as a commercially valuable and relatively limited natural resource,

Internationally, market prices for cultured eels and eel products vary with species, country and product type and quality. Most Australian production is exported to European (Germany, The Netherlands) and Asian markets (Hong Kong, Taiwan and Japan) as adult size (> 1 kg), fresh, chilled or frozen whole fish. Larger longfins are sometimes exported live into selected Asian markets (Taiwan and Hong Kong), and some value-added smoked products are also supplied to the local market.

The range of existing and potential `cultured' and wild eel products from Australia, together with nominal/estimated wholesale unit prices (delivered) are given in the table above.

Because there is no Australian intensive eel industry, the marketability and benchmark prices of cultured shortfin and longfin eels are unknown.

The greatest potential appears to be in the production of smaller eels for the Japanese Kabayaki market. Value-adding through local processing is another means by which profitability and marketability can be enhanced. At present few eels are processed in Australia, although the appropriate infrastructure and expertise exist for other seafood products.

Also, 5-10 g elvers from glass eel seedstock could be intensively produced for commercial stock enhancement purposes in existing wild fisheries such as in Victoria.

Production requirements

In the face of unsatisfied market demand and declining world production, increased attention is being paid to the development of intensive culture technology and the use of glass-eel seedstock.

Australian eel production is only likely to exceed current levels substantially if intensive aquaculture practices are adopted based on the sustainable use of wild glass-eel seedstock. Ultimately, the limited availability of such seedstock will probably also constrain any further development of the Australian industry.

Eel farming around the world employs a variety of reliable, well established systems and technologies for intensive production, ranging from relatively low-density
(< 5-10 kg/m³), flow through pond culture under ambient conditions, to semi-intensive (10-100 kg/m³) pond and tank culture under semi-controlled conditions, and super high-density (>100 kg/m³) in closed loop (recirculation), tank culture in a completely controlled environment.

Culture tanks and ponds vary in size from small nursery tanks (e.g. 1-10 m³ capacity) to large grow-out ponds (e.g. 0.05-0.2 ha surface area). Water supplies for culture systems also vary from fresh to brackish, and from surface waters at ambient temperatures, to heated industrial effluent and geothermal artesian aquifers.

The use of greenhouses is common in Japan, primarily as a cost-effective means of increasing water temperature and therefore growth rates.

Preliminary trials with shortfin eels suggest that water temperatures for optimal growth and survival range between 23 and 28°C (average 25°C), with feeding becoming irregular at temperatures below 15 to 20°C.

In Australia, suitable production temperatures can be found under ambient conditions in eastern Australia at more northerly latitudes (e.g. northern NSW and Queensland). At more southerly Closed-loop, or recirculation systems, are becoming increasingly popular as a means of providing controlled environment conditions for year-round production, for reducing demand for fresh water and discharge of nutrient-rich effluent, and for securing the system to ensure the minimal escape of exotic stocks and disease pathogens to natural waterways.

The major components of such systems include mechanical filtration (e.g. sand filters, drum/micro-screen filters, hydrocyclone filters, plated settlement tanks etc.) to remove suspended solids, biofilters (e.g. rotating drum/disc filters, trickling filters, upwelling filters, fluidised bed filters etc.) to remove dissolved metabolites, mechanical aeration or oxygenation to increase dissolved oxygen levels, sterilisation units (e.g. UV filters, ozone generators etc.) to destroy pathogens, and miscellaneous pumps, pipes and valves to reticulate the water through the system.

The need for increasing levels of filtration, including solids removal and biofiltration, and supplementary aeration/oxygenation increases with the increasing percentage of recirculation, stocking density, feed rates and water temperature.

The degree of effluent treatment and re-use, and options for nutrient-rich effluent disposal vary with the environmental licensing conditions, the quality and quantity of effluent, the system's location and the efficiency of design.

Government licensing requirements and associated costs vary from state to state, but in most cases include some form of culture permit to enable commercial production and sale of aquaculture produce, local government planning and health regulatory permits, and water pumping and/or discharge permits.

Varieties/breeds

The Anguillidae form a relatively cosmopolitan family of predominantly freshwater eels, which contains the one genus with 16 species distributed throughout the world. Of these, A. japonica and A. anguilla are the two most commonly cultured species.

In addition to shortfin and longfin eels in Australia, commercial aquaculture of A. rostrata, endemic to the coastal waters of North America, is also under development.

Longfin eels are sometimes mistakenly referred to as `conger' eels. True conger eels are in fact mostly marine and belong to a different family, the Congridae. Other Australian Anguillid species which have not been evaluated for their aquaculture potential are the northern eel, A. bicolour, and the South Pacific eel, A. obscura, both of which have a fully tropical range in northern Australia.

Husbandry

Almost all forms of intensive eel farming around the world rely on artificial foods: mostly high-energy, protein-rich, compound diets in the form of a moist paste for glass eels, and steam-pressed or extruded pellets of varying sizes for later developmental stages.

Natural feeds such as aquatic worms (e.g. Tubifex spp.), brine shrimp and minced fish have been used successfully to wean Australian glass eels onto artificial foods. Artificial pastes and pelleted diets imported from Taiwan have also proved suitable and these products are now being formulated and manufactured in Australia for shortfin eels.

Food conversion ratios (FCRs) for Asian and European intensive-culture systems vary between 0.9:1and 1.9:1 (kg food per kg fish weight). Industry standard FCRs have yet to be determined for Australian eel culture systems, but likewise will vary with species, size, system design, food type, water temperature and quality.

Growth rates of Anguillid eels, including the Australian species, are inherently variable and frequent mechanical grading throughout the production cycle is necessary (usually every 4-8 weeks).

Eels are also naturally aggressive, highly carnivorous, top-order predators with a relatively large mouth. In high density aquaculture, such grading will minimise the risk of cannibalism and encourage efficient feeding behaviour and food conversion within the system.

The maintenance of optimal water quality conditions is critical to ensure adequate growth and survival under intensive aquaculture. Enhanced production through increased stocking densities and growth require increased feed rates, leading to a further increased, organic, loading on the systems. Water quality is managed through appropriate water exchange and/or water filtration, together with the application of appropriate husbandry (use of high quality feeds, optimal feeding regimes, routine maintenance and cleaning of the culture system etc.). Key water quality variables such as temperature, dissolved oxygen, pH and total ammonia should be monitored regularly.

Pest and disease control

Glass eels collected in salt water are routinely acclimatised to fresh water for intensive grow- out almost immediately, thereby eliminating the resident, natural, marine, parasitic fauna of the eels. In Australian systems, subsequent outbreaks of mostly common bacterial, fungal, protozoan and larger ectoparasitic disease pathogens within freshwater culture systems are routinely treated with short to long term salt baths at concentrations of 5-10 g salt/L of water.

Harvest and postharvest handling requirements

Cultured Australian eels need to be handled and processed in much the same way as for the commercial fishery. This includes freshwater purging of the fish in tanks or cages, followed by cleaning (evisceration) for some products, packaging and chilling (ice slurry), smoking or snap freezing before delivery to markets. All the commercial fish-processing facilities and procedures designed for human consumption must meet local and state government health standards. Processing facilities approved by the Australian Quarantine Inspection Service (AQIS) are required for the export of Australian seafoods, including eels. Cultured Kabayaki-size eels exported live to Asian markets would need to be purged in cold fresh water, chilled (to 4-5°C) and packaged in oxygen-filled polyethylene bags transported inside insulated polystyrene containers.

In general, cultured eels intended for export markets should be purged to remove any foreign residues or off-flavour, should be of uniform size, quality (particularly fat content and skin texture) and appearance (not damaged), and should be appropriately packaged (depending on whether live, chilled or frozen) to prescribed quality standards.

Economics of production

With intensive aquaculture, Australian shortfin and longfin eels could be produced from glass- eel seedstock to a marketable Kabayaki-size product in approximately 18-24 months. Refinement of this technology through R&D and identification of more optimal production parameters may lead to production of such a product in 12-18 months in the short to medium term.

Asian industry standards for A. japonica stipulate 600-900 kg of marketable eel biomass (Kabayaki product; based on up to 90-95% survival) per kg of glass eels within 9-12 months from the glass eel stage.

By contrast, total survival for A. anguilla is reported to be as low as 10-45% for the same period, with conversion rates as low as 110-130 kg of marketable eels for every kg of glass eels.

Generally, larger glass eels collected earlier in the season tend to exhibit higher survival and growth throughout the production cycle, in comparison with smaller glass eels collected later in the season.

Conversion rates for intensively cultured Australian eels will ultimately depend on the development of customised and efficient, intensive, culture technology and the inherent suitability of the Australian species to adapt to such high-density systems.

The capital costs of eel-production systems vary directly with capacity and the degree of intensification and technical sophistication: i.e. the bigger, more intensive the system, the more complex the design and higher the purchase cost.

Operating costs depend primarily on the efficiency (kg fish/m³ of water) and capacity of the system design. Major recurrent production costs include depreciation on capital infrastructure, interest on borrowed funds, seedstock (glass eels, elvers), water, labour, feed, energy (for recirculated, heated systems), system maintenance, processing, packaging and freight.

As for Asian and European intensive eel farms, economies of scale will apply in establishing overall profitability for Australian eel farms - costs per unit of production will decrease with increasing system size and capacity.

As a starting point, Australian producers should expect production costs to be about AUS$10-15/kg during the initial research and developmental phase of the industry.

Longer-term production costs and profitability ratios are unknown at this stage. However it is likely that with adequate glass-eel supplies, more efficient culture systems and feeding regimes, and improved value-adding and marketing strategies, intensive freshwater eel farming in Australia could well prove to be an economically viable industry.

.

Key contacts

Key references

Beumer, J. P. 1996 `Freshwater Eels' In Freshwater Fishes of South-Eastern Australia Ed. R. M. McDowall, Reed Books, Sydney

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

Heinsbroek, L. T. N. 1991 `A review of eel culture in Japan and Europe' Aquaculture and Fisheries Management, 22:57-72

Ingram, B., Gooley, G., Larkin, B., De Silva, S. and Collins, R. 1996 `Intensive tank and pond culture of Australian short finned glass eels in Victoria' Austasia Aquaculture, 10(4): 74-75

Kailola, P. J., Williams, M. J., Stewart, P. C., Reichelt, R. E., McNee, A. and Grieve, C. 1993 Australian Fisheries Resources Bureau of Resource Sciences, Department of Primary Industries and Energy, and Fisheries Research and Development Corporation. Canberra, Australia.

Usui, A. 1991 Eel Culture (2nd Edition). Fishing News Books, Blackwell Scientific Publications Ltd, Oxford, England

*About the author