Feeds and feeding regimes - eel

Formulated eel diets are in use in many farms (Pillay 1995). Almost all forms of intensive eel farming now rely on artificial feed, that is high-energy, protein-rich, compound diets in the form of a moist paste for glass eels, and steam-pressed or extruded pellets for later developmental stages.

Starter feeding for glass eels is considered the most difficult component of the rearing of eels. The non-acceptance of artificial feed can lead to mortalities and retarded growth. Elver feeding starts when the water temperature reaches about 15°C. The ideal temperature range for eels to remain healthy and to convert feed efficiently is 23-28°C. Temperatures above optimal result in reduced feeding and growth rates, stress and sometimes death. Temperatures below optimal result in decreased metabolism and growth. Small aquatic worms (Tubifex sp.) are considered a suitable first feed for elvers and, after two or three days, fish flesh is added in progressively increasing quantities until about the tenth day, when only a paste of minced fish is given (www.fisheries.nsw.gov.au/aquaculture/freshwater/eels.htm). Sometimes brine shrimp is used in conjunction with worms and minced fish to wean Australian glass eels onto artificial foods. Eels should be quickly weaned from the time they enter the farm. A range of commercial fish pellets are available in Australia that are suitable for short and long-fin eels. They are fed several times per day, which ensures that they are healthy, and grow rapidly. Artificial pastes and pelleted diets imported from Taiwan Province of China have also proved suitable and these products are now being formulated and manufactured in Australia for short-fin eels (www.rirdc.gov.au/pub/ handbook/eels.html).

Eels are naturally aggressive, highly carnivorous, top-order predators with a relatively large mouth. In high-density aquaculture, grading (usually every 4-8 weeks) minimizes the risk of cannibalism and encourages efficient feeding behaviour and food conversion. Eels are nocturnal feeders, so the feeding spot is usually covered with boards or other suitable materials, to make it as dark as possible. Food conversion ratios (FCRs) for Asian and European intensive-culture systems vary between 0.9:1 and 1.9:1. Industry standard FCRs have yet to be determined for
Australian eel culture systems, but it is expected that they will vary with species, size, system design, food type, water temperature and quality (www.fisheries.nsw.gov.au/aquaculture/ freshwater/eels.htm). The recommended protein level in practical diets for elvers is 50-60 percent, and 40-45 percent for sub-adults (Pillay 1995).
Taiwan Province of China produces a large amount of aquafeeds for eels. Table 15 shows data from Taiwan Province of China and Korea.
Table 15. Production of artificial eel feeds (tonnes) in the Republic of Korea and Taiwan Province of China (www.aquafeed.com/asia/pickup.html)

Fish cultured in earth pond systems in China, such as eels, are directly fed with raw ingredients, such as rice bran, wheat bran, rapeseed meal, peanut meal and soybean meal. 70 percent of the formulated aquafeeds annually produced in China are used in pond culture, with the balance being used in netcage and land-based tank systems. Extensive studies to develop artificial feeds started in the 1980s as the market demand increased; the freshwater species studied included Japanese eels. By 1999 there were several hundred aquafeed manufacturers in China (Mai and Tan 2000). One company producing special feeds for eels is the Zhongmao Feed Processing Co. All feeds are prepared based on scientific formulas, prepared by aquaculture specialists and researchers from knowledge gained from studies conducted with the various growth stages. The ingredients are imported and include high quality fishmeals, starch with concentrated vitamins, minerals, amino acids, proteins, fats, yeast and trace elements as the additives. The proximate composition of this feed is shown in Table 16 (www.zhongmao.com/english/sljg.htm).
Table 16. Composition of eel feeds produced by Zhongmao Feed Processing Co.

Even though the actual amount of feed consumed by glass eels depends on water and temperature conditions, the recommended initial daily ration is about 30 percent of the total weight of the released glass eels, distributed in several portions. Adequate feeding is important to reduce cannibalism. In about 4 months, glass eels grow to around 7 g and in another
4 months they reach about 100 times their stocking size.
During the early years of eel culture in Japan and Taiwan Province of China, adult eels were fed with silkworm pupae, but in recent years compound feeds have come into common use (Pillay 1995). Figure 31 shows the total amount of artificial feed used in Japan for feeding farmed eels.

Figure 31. Total eel feed production [solid (pellets) and powder] in Japan (Nakada 2000)
In Italy, eel farmers usually add cheap homogenized fish meat (according to the fishery season) to paste food, while northern European farmers usually employ cod roe (Grandi et al. 2000). Generally, adult eels are fed at about 10 percent of the total weight stocked per day, when fresh fish is used. If a formulated diet is employed, it is suggested that it should have 46-52 percent protein and 3-5 percent fat. The roe of Atlantic cod has been recently used in Europe, while in Australia, European carp roe is utilized. De Silva et al. (2001) examined four types of roe (European carp, mirror dory, orange roughy and warehou) and showed that European carp and warehou roes were preferred by eels in the latter stages of growth; these two types have significantly higher contents of arachidonic acid.
Changes in feeding affect growth rate and body composition. Garcia-Gallego and Akharbach (1998), monitoring the fatty acid content of body lipids in a fish farm during the production cycle of Anguilla anguilla from elvers to commercial size (>100 g), showed that the fat content in the body of eels notably increased in direct relationship with size. Changes in dietary lipid composition (through changing moist natural food to pelleted feeds, or changing from a low to a high fat content) influenced the fatty acid composition of body and muscle lipids. The composition of the three different food types is shown in Table 17. Elvers, initially weighing about 0.2g, doubled their weight in 2 months, even if they showed a certain delay during the transition from initial food (cod roe) to pellet I. Similarly, body-fat content also doubled in this interval. Eventually, the change from pellet I (lower fat) to pellet II (higher fat) resulted in the highest value of body fat accumulation. Some eels kept for 1 year in the farm multiplied their initial weight by a factor of 375 and others by a factor of 10: in any case, the transition from cod roe to pellet I and from this to pellet II encouraged growth rate increases, reflecting a progressively higher energy intake per unit of diet consumed.
It is observed that there is a high degree of fat accumulation in the muscles of cultured eels, compared to the average of wild eels (Sumner et al. 1984, Jover et al. 1990; Akharbach 1995). Graynoth and Taylor (2000) showed that the maximum growth rates of short-fin eels depends on high quality food, low competition for food and the close to optimum temperatures found in New Zealand. In addition, competition is reduced if eels are grown at high density in turbid water with a high abundance of phytoplankton (Jones et al. 1983; Seymour 1989; Degani and Gallagher 1995). Water temperature influences food consumption and the metabolic rate that controls growth (Brett 1979). Low temperatures (8-9°C) cause a cessation of growth, but food supplies, rather than temperature, control growth rate.

Table 17. Composition of feeds for eels (Garcia-Gallego and Akharbach 1998) Content (% dry matter)

Some studies have shown that the diet should be highly digestible, in order to minimize the release of nitrogen, phosphorus and solids to the aquatic environment. The optimum protein content should be 44.5 percent for Japanese eels (Arai et al. 1971, 1972; Nose and Arai 1973), and range from 30 to 48 percent for European eels (Spannhof and Kuhne 1977; Degani, Levanon and Trieger 1984, 1985; Arai, Mas Alvarez and Ogata 1986). Recent studies on the American eel show that optimum growth from 8 to 22 g requires 47 percent protein (Tibbetts, Lall and Anderson 2000). The protein/energy ratio has to be accurately balanced to obtain rapid growth, efficient feed utilization and to give a flesh rich in protein with a low fat content (Tibbetts, Lall and Anderson 2001); this optimization minimizes feeding costs (Tucker 1992). The best results for growth rate, feed efficiency and nutrient digestibility are obtained with a diet with about 22 g DP MJ DE-1 (Tibbetts, Lall and Anderson 2001).
The average weight increase of Anguilla anguilla fed zero or 10 percent soybean meal in their diet was significantly higher than that achieved when the soybean content was 20 percent (Degani 1987). The body protein (dry weight) of eels fed the highest soybean diet was lower compared to the other experimental groups. However, no significant differences were found in the body fat among the experimental groups. Tanimoto, Koike and Takahashi (1993) showed that feeding eels a commercial diet containing tochu (Eucommia ulmoides Oliver: a herb medicine) leaf powder improved cultured eel meat texture. A firmer texture was produced that contained more collagen than the control. American eel elvers are raised on pelleted fish feed to make sure their taste and colour are consistent.
All the feeding trials conducted with cultured eels indicate that the balance between proteins and lipids is critical, not only for growth but also for flesh quality. As seen in other capture-based aquaculture species that are cultured because of their meat quality, the development of better diets assists in the successful marketing of the product.