Shortfin and longfin eel aquaculture

Longfin eels are the primary species used for aquaculture in Queensland, but small quantities of short–finned eel are still farmed. Almost all eels produced in Queensland are prepared for live marketing and export.

Due to their complex life cycle, eels are not bred in hatcheries. As a result, the eel aquaculture industry has relied on harvesting juvenile eels from the wild for stocking of aquaculture ponds. As such, seasonal changes in wild stocks can limit expansion in the eel industry.

Marketing and industry

The largest market for farmed eels is the Japanese kabayaki market.

All farmed eels are currently exported and markets as live product; however, as Australia's multicultural population increases, opportunities may emerge for a small domestic market.

Shortfin eel

The shortfin eel (Anguilla 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.

Longfin eel

The longfin eel (A. reinhardtii) has a mottled appearance that 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, particularly in larger size grades. Despite the high prices paid for kabayaki eels, marketing of large eels up to 5kg each into alternative markets may be equally, if not more, profitable.

Culture and production systems

Eels can be grown in ponds and recirculating tank systems. For pond-based aquaculture, research has determined the best site is has a reliable, constant water supply, and is not susceptible to flooding. A gently sloping site will make it easier to fill and drain ponds. Eel farming requires volumes of water and a water exchange is due to the high stocking densities and messy feeding behaviour of the eels.

Bore water is suitable for ponds, if it is free from pathogens and chemical residues and has a pH level of 7.0–8.0. Highly acidic water is not acceptable.

Tank systems

For intensive tank-based eel aquaculture, site selection criteria such as climate and soil quality are less restrictive, although a source of good-quality water is still essential.

Stocking density

They can tolerate a high stocking density, which means a large number can be grown in a relatively small area. Stocking rates in tank and pond systems depend on the capacity of the system and the intensity of the operation. Biomass in ponds should not exceed 10t per hectare, and in tanks it should not exceed 50kg per cubic metre.

Water quality

Eels require large amounts of oxygen if they are to remain active and grow at an optimal rate. Aeration can be supplied using paddlewheels or aspirators. Blooms of phytoplanktonic algae are encouraged as they produce oxygen and shield the eels from direct sunlight.

Regular water exchange is necessary to maintain water quality due to the often high stocking densities. The pH, dissolved oxygen, temperature, total nitrogen and nitrite levels need to be checked regularly. Ideally, the water should have a pH of around 7, dissolved oxygen levels no lower than 3 parts per million and free ammonia levels of less than 0.2mg per litre. If water quality deteriorates, corrective measures need to be applied quickly to ensure minimal stock loss.

Temperature

Eels grow rapidly in a tropical climate, preferring temperatures between 23°C and 28°C. In ideal conditions they grow to marketable size (150–200g) in 12–18 months, although growth rate can be extremely variable.

Collecting juvenile eels

For aquaculture, juvenile eels are caught from the wild and grown in culture facilities. Eel aquaculture relies on the availability and sustainability of juvenile eel resources. Access to juvenile eel resources in Queensland vary, depending environmental conditions.

Juvenile eels are caught using nets in river basins associated with 23 rivers along the east coast of Queensland. Queensland commercial juvenile eel licences are limited by number (12), but with no quota limit.

To ensure this resource is not over-exploited, the Queensland Government manages the collection of glass eels and does not permit their export. Heavy penalties apply. These controls are required for a sustainable industry.

Feeding and growing

Tank systems

Glass eels and elvers are best stocked into tanks before being moved into grow out facilities. Grow out of elvers to market size can be achieved in either tank systems or earthen ponds. Tank culture can be managed in tanks of 1000–20,000L, with flow through or recirculating water supply. Recirculation requires sophisticated biofiltration treatment technologies to maintain suitable water quality.

Pond systems

Earthen ponds for eel grow out should be constructed on non–porous soils. Sandy clay soils are optimal to prevent pond leakage. It is possible to line ponds so that they do not leak, however construction costs will be higher. Ponds range in size from 0.2–2ha and are 1–1.5m deep.

Feeding

Eels should be quickly weaned onto artificial feed from the time they enter the farm. A range of commercial fish diets (pellets) is available in Australia, which are suitable for shortfin and longfin eels. Eels are fed several times a day, which ensures they are healthy and grow rapidly.

Food conversion ratios (FCR) for both shortfin and longfin eels should be in the range of 1.5–2:1 (kilogram of food: weight growth).

Harvesting

Eels are harvested at a weight varying from 150g to several kilograms, depending on the market. Harvesting can be carried out by draining the pond using a net attached to the outlet pipe, a seine net or a scoop net (at feeding time).

The eels are then sorted into different sizes using a grading tray, and placed in holding tanks for several days without feed to purge their stomachs. They are then chilled and packed into strong plastic bags with just enough water to ensure that their skin remains moist. The bags are then filled with oxygen and transported to the market.