Culture systems - eel

Eel farming around the world employs a variety of reliable, well established systems from relatively low-density (<5-10 kg/m3) flow-through pond culture under ambient conditions, semi intensive (10-100 kg/m3) pond and tank culture under semi-controlled conditions, to super high density (>100 kg/m3) in closed loop (re-circulation) 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 the use of surface waters at ambient temperatures to heated industrial effluent and geothermal artesian aquifers.

Traditionally, in Europe freshwater eels are cultured in ponds and are generally stocked at 100- 350/m2. When they reach marketable size they are transferred to larger ponds (1 000-1 500 m2). Water can be still or running, though the latter is preferred for intensive stocking and high production (Pillay 1995). According to Shepherd and Bromage (1990) the optimal temperature of 18-25°C is achievable for an average of 250 days every year, making it a very good location for eel culture. European eels grow slowly, even under the optimal conditions of 22-24°C, as cited by Grandi et al. (2000); at these temperatures, and feeding ad libitum, the rearing of elvers to a market size requires more than 2 years.

Indoor systems, where the culture water is treated and re-circulated, are a special development of the land-based intensive culture system. The development started in the 1950s in Japan and, experimentally, in Europe in the 1970s. Commercial utilization started in northern Europe, mainly in the Netherlands, Denmark and Germany in the early 1980s. Instead of being released, the waste water is recycled after mechanical and biological filtration and oxygen injection. Most eel farming in Europe, apart from the traditional forms of eel culture in Italy, is done in re-circulating systems where a combination of good water quality, appropriate temperature and oxygenation can allow stocking densities of more than 50 kg/m3 (i.e. 5 percent of the volume consists of eels) without compromising the health or welfare of the fish. In the Netherlands, eels are farmed in re-circulating systems at 23-26°C (Eding and Kamstra 2001).
All fish emit waste products after metabolism of their feed; ammonia is the most important toxic waste. With a better feed utilization by the eels, waste production can be minimized. The trend is that environmental rules are forcing re-circulation eel farms to reuse a higher percentage of water, thereby minimising the amount of discharge water, especially in Denmark and Holland (www.danafeed.dk/upl/doc/3.doc).
Eels are also cultured in marine and brackish waters. In the Mediterranean lagoons, or “valli”, Anguilla anguilla elvers enter with tidal flows into “lavorieri” traps. In this rich environment, with large quantities of organic food available, they grow very rapidly. In these environments, when oxygen concentration and pH are low, the system can produce hydrogen sulphide, which is toxic for animals and can cause negative impacts on the aquaculture system. The advantages of this type of culture include a reduction in pond construction and water supply costs; some culture centres are located close to thermal stations where the water is warmer and eels grow better than in colder waters. In “valli”, mainly in the North Adriatic, elvers of 15-35 g are grown at stocking densities of 4 to 15 kg/ha. These elvers are imported mainly from France; in 1999 the cost was € 1,000/kg. About 300 tonnes of elvers are imported each year, but in 1995 about 100 tonnes (50-100 g each) were also imported from Denmark, as there was a reduced supply from France (200 tonnes). For this reason, many Italian eel farmers have imported elvers from Denmark, Holland and Sweden (10 g weaned in re-circulating systems) since 1992 (Ciccotti, Busilacchi and Cataudella 1999).
A wide range of culture methods have been investigated by Japanese eel farmers; from outdoor ponds to closed systems, and from seawater to freshwater culture techniques. From these, four methods are currently used, which can be categorized by their main features: outdoor ponds, basic greenhouse ponds, ponds with a sedimentation unit, and ponds with a bio-filtration unit. Japanese eel farms are usually small (20 to 40 tonnes annual production). They are mainly located in Aichi, Shizuoka, Kochi and Kagoshima Prefectures, and most of them are supplied with water from boreholes. 120 tonnes of glass eels are required every year to satisfy the needs of these producers, whilst annual wild catches fluctuate from 50 to 140 tonnes. Farmers stock Japanese eels (Anguilla japonica) at high densities (500-600 g/m2); elvers are stocked in ponds (165 m2, 40 cm depth) where they are on-grown before being transferred to ponds of about 200 m2 surface area, at lower densities. Eels are mostly stocked in warm water ponds which have an average water temperature of 25°C throughout the year. Heating systems may be used for cold periods during the winter months. However, owing to the cold climate, Japanese eel farms have shifted from outdoor ponds to greenhouse ponds, in order to shorten the rearing period (Kobayashi, Shiino and Miyazaki 1999).
In Taiwan Province of China, eel farming is at the top of the fishery industry in terms of export value. The first experimental eel farm was established in 1952. Small-scale commercial eel farming on the island began in 1958 and the first large-scale expansion of eel farming took place in 1964, in the form of a nursery operation, raising glass eels to stocking size fingerlings for Japanese eel farms. Throughout the history of eel farming in Taiwan Province of China, the industry has mainly been developed for exploiting the market in Japan; the first exports of market-size eels to Japan began in 1970. In 30 years, eel farming has developed into one of the largest sectors of the aquaculture industry on the island. Japanese eels are a high-value product and suitable for intensive pond culture. Japanese eel (Anguilla japonica) ponds in Taiwan Province of China are all located outdoors and have earth bottoms. These ponds can be divided into two types: “hard ponds” (with concrete, stone or brick dikes) and “soft ponds” (with earth dikes). Hard ponds are mainly used to culture elvers to market size, while soft ponds are used for juveniles and have lower production costs. The technology for eel culture in Taiwan Province of China is well established and has reached a high degree of specialization. In recent years,
A. japonica is the only species cultured in Taiwan Province of China due to performance, temperature preferences and the serious disease problems encountered with other species. Its main export market is Japan, which is the destination of 90 percent of its eel exports (Wu 1999).
The capture-based aquaculture of eels in China is traditionally based, scale-dependent, low-tech and natural resource consuming. Both European and Japanese eels are cultured in earth pond systems, usually for the commoner or cheaper animals and products. A small percentage of production is from land-based tank systems and net-cage systems (placed in lakes and reservoirs) but, since these systems have higher investment costs, they are used only for those fetching higher prices (Mai and Tan 2000).
Japanese eels are farmed in Thailand in freshwater ponds. Its warm climate is suitable for raising eels, shortening the growth period. The offspring grow from a length of 4 cm to a weight between 250 and 500 g in five to six months.
Most of the research in Australia has focused on pond production. Short-fin eels grow rapidly in a tropical climate, preferring temperatures between 23-28°C. It has been suggested that the northern coast of NSW could provide the ideal climate, while temperature-controlled intensive tank systems could possibly be located anywhere. In these ideal conditions short-fin eels grow to marketable size (150 to 200g) in 12 to 18 months and 200-300g in 18-24 months. Growth in extensive pond systems is usually much slower. Eel farming also requires access to large volumes of water, due to the very high stocking densities and messy feeding behaviour of the eels; regular water exchange is usually a necessity. For the intensive pond culture of eels,
NSW Fisheries recommends a water budget of at least 60 million litres/ha/year (www.fisheries.nsw.gov.au/aquaculture/freshwater/eels.htm).
The best sites for the pond-based culture of short-fin eels are those with a constant water supply and which are not susceptible to flooding. Borehole water is suitable as long as it is free from pathogens, chemical residues and has a pH of 7.0 to 8.0. Highly acidic water is not acceptable and dissolved oxygen levels should be no lower than 3 mg/l. Free ammonia levels should be less than 0.2 mg/l. A gently sloping site is advantageous, to maximize the use of gravity for filling and draining ponds. 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 exceed 80 kg/m3, while in super intensive pond systems, they can exceed 20 tonnes/ha. Short-fin eels will usually tolerate conditions 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.
For intensive tank-based eel aquaculture, site selection criteria are less restrictive, although a source of good quality water is still essential. Choosing a potential site for tank culture is generally easier because less land area is required and the volumes of intake water are lower. Factors such as topography, soil quality and climate are not so much of an issue. A basic intensive re-circulation system should consist of a number of tanks (usually 1-13 tonnes), either independent or in groups, filtered by mechanical and bio-filters, which are used to strip nitrogenous waste and nutrients from the water. Re-circulation 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 recycled back to the tanks. The entire system should be contained within a vermin-proof, climate-controlled housing. Specialized technical advice should be sought to determine the best set-up (www.fisheries.nsw.gov.au/aquaculture/freshwater/eels.htm). There are many companies throughout the world that are capable of providing consultancy and equipment for these intensive re-circulation systems.