H. Koops
Bundesforschungsanstalt für Fischerei,
Institut für Küsten- und Binnenfischerei, Hamburg, Federal Republic of Germany
ABSTRACT
The paper shortly summarises the information available on rearing conditions, feeds and feeding and stages of development for the European and Japanese eel.
RESUME
Un extrait est donné de l'expérience actuelle en ce qui concerne: conditions d'élevage, aliments et stades de developement des anguilles européennes et japonaises.
Young eels are needed for stocking inland waters not open to natural immigration and for farming. Two types of ‘seed’ material can be used i.e. the so called glass eels (surface migrating eel fry caught with different types of gauze nets in lower parts of coastal rivers) and fingerlings (undersized eels caught in stowe nets and traps of small mesh size at or near the bottom in estuaries or rivers). Both types of seeds (fry and fingerlings) are used for stocking rivers and lakes and for aquaculture. Farmed fry stages may be an alternative for some reasons (reduced mortality and reduced migration behaviour when stocked in inland waters.
Freshly caught glass eels have a strong migration behaviour and may escape even over vertical walls, if the tanks are not especially designed to prevent escape. This tendency may last for 2–3 months (Bohl, 1968; Meske, 1969).
The highest mortality among glass eels occurs within the first 2–3 weeks of adaptation, weak animals - not adapting or by other reasons - die during this time. Later on the mortality can be kept low, diseases can be treated by bathes (malachite green, furazolidon) or with medicated feeds (antibiotics, sulfa drugs).
Quite intensive studies on the influence of rearing conditions have been carried out by Kuhlmann (1975, 1979) for Anguilla anguilla. For any period during 283 days glass eels showed the best length and weight increment at 26.5°C within a range from 17–30°C. The feeding rate increased with temperature, the feed conversion improved up to 26°C. The higher food quantity taken at 29°C was more than counter balanced by an inferior feed conversion. Possibly the temperature also influences the sex ratio. At least a high percentage of males and females obviously is influenced by external factors. The highest percentage of females was observed in the 26°C group. The origin of elvers may be of importance. Mediterranean glass eels grew better than North-Sea glass eels both on industrial and natural (cod roe) food. Males grew faster (not bigger) than females.
Similar figures are given for the growth of glass eels in the first summer by several authors:
Müller (1964): 5–11 g; Koops (1965): 7.8 g; Matsui (1952); 9 g; Honma (1971) reports a weight increase
up to 25 g average weight from April (
g ca. 0.2 g) to November as valid for the modern eel farming in
Japan.
Industrial feeds for eels are characterized by a high protein and fat and low crude fibre content. Elver feeds are higher in energy protein and fat than feeds for larger eels. High quality fish meal is used as protein source, special starches and other binders allow to mix a water stable paste from meal, water and fish oil.
Honma (1971) gives the ingredients of eel feeds as follows:
| Meal from white meat fish | 55 – 62% |
| α-Starch | 15 – 23% |
| β-Starch | 10 – 15% |
| small amounts of fish liver powder, minerals and vitamins | |
| 10 – 15% oil is added when mixing with water. | |
The amino acid requirements of the European and the Japanese Eel have been studied by Hashimoto and others (1972).
Extreme growth variation within material of the same origin has been observed by all studying the growth of eels. Several size gradings are necessary already in the first year of culture (from glass eel to fingerling) to maintain growth, feed conversion and health status.
The production of glass eels - from egg to larvae - is still a matter of scientific interest and will probably never be realized on an economic basis.
Induced maturation and induced spawning of adult eels has been achieved by several scientists (for example Fontaine, 1936; Fontaine et al., 1964; Oliverau, 1961; Beëtius and Boëtius, 1967; Meske and Cellarius, 1973; Yamamoto and Yamauchi, 1974; Edel, 1975) by applying hormone treatment.
Fertilization and incubation of eggs has succeeded only with the Japanese eel. Reportedly the incubation took 38–45 hrs at 23°C. Five larvae survived for 5 days after hatching (Yamamoto and Yamauchi, 1974).
Culture methods from egg to glass eel are still unknown, thus culture has to rely on the catch of glass eel from natural resources.
In the following the term glass eel is used for immigrating elvers in different stage of pigmentation but still transparent. Eel fry is used for fully pigmentated elvers up to few g of weight and eels between about 5 and 50 g are called fingerlings. The Japanese eel culture starts with the culture of glass eel to fry and fingerlings.
Outdoor or indoor ponds up to about 150 m2 or tanks (half a meter deep or less) are stocked at a density of 500–600 g/m2. Before stocking the glass eels are treated with malachite green against ectoparasites. The first feeding is done with minced worms 2–3 days after stocking. Gradually worms are replaced by minced fish and after about 10 days only fish is fed. Compound dry feed (meal mixture to be mixed with water and fish oil) is used nowadays in the Japanese eel culture also for elver feeding. After feeding wet food for a few weeks many farmers change to compound feeds by gradually replacing the wet food. Two times feeding is recommended (morning and early afternoon) at darkened places. The food is offered on meshwire frames below or at the surface. The daily quantity may be in the order of 30% (for wet food) of body weight per day at higher temperature for glass eels and decreases with the increase of the average weight of the eels. The Japanese eel culture is a standing water culture where high phytoplankton densities, aeration and water exchange help to overcome critical periods of water quality (Honma, 1971). Climatic conditions are the reason why at least in parts of Europe glass eel culture and eel culture in general tend more to running water systems - open system, where waste heat is available or partial recycling of water where extra heating is necessary. Glass eels obviously are much more sensitive to water quality and diseases than larger eels and running water culture is recommended especially for high stocking densities. Eel fry has to be kept free from bottom layers of waste food and faeces otherwise severe gill infections may occur. Good results have been ontained by using small boxes with a perforated bottom, which are placed a few cm above a trout fry trough with seperate water supply for each unit (see fig. 1).
Since Japan is short of Japanese elvers, glass eels are imported from Europe. In 1971 the amount of elvers imported from France was about 23.5 ton and 4.5 ton from U.K. (Honma, 1971); even larger amounts have been imported in the following years. A tendency to specialisation can be observed: Some farmers concentrate on the production of fingerlings.
Intensification has led to smaller pond sizes and increasing demand for industrial feeds. Running water methods are used now where industrial cooling water (at power stations) is available.
In Europe the first eel culture enterprises have been established in Italy. The management mostly is similar to the traditional Japanese methods using standing water and relative low stocking densities. Stocking materials are glass eels or fingerlinges.
In the U.K. a few commercial farms are in operation or are built in connection with heated effluents. The farming starts with glass eels.
In the Federal Republic of Germany some eel farming activities are going on private or experimental basis. Methods used comprise running water units supplied with natural or heated water, standing water in natural ponds or tanks and open or (partially) recycling systems. Feeds used are industrial feeds or wet food. Farming starts with glass eels or fingerlings. Glass eels reared to 1–10 g are offered for stocking or farming. Experimental work is going on in other countries like Denmark, DDR, Poland.
SUMMARY
Relative few publications deal with the rearing of elvers. The paper shortly summarizes the information available for the European and the Japanese eel concerning rearing conditions, feeds and feeding, and stage of development. Further informations may be drawn from the literature cited.
Bohl, M., 1968 Vorläufiger Versuch über Aal-Fütterungsversuche in Wielenbach. Wasser- und Abwasserforschung 6: 235 – 239.
Boëtius, J. and I. Boëtius, 1967 Studies on the European eel, Anguilla anguilla (L.). Experimental induction of the male sexual cycle, its relation to temperature and other factors. Medd. Danmarks Fisk. Havundersøgelser N.S. 4: 339 – 405.
Fontaine, M., 1936 Sur la maturation complète des organes génitaux de l'anguille male et l'emission spontanée. C.r. hebd. Séauc. Acad. Sci., Paris 202: 1312 – 1314.
Fontaine, M., E. Bertrand, E. Lopez and O. Callamand, 1964 Sur la maturation des organes génitaux de l'Anguille femelle (Anguilla anguilla L.) et l'emission spontanée des oeufs en aquarium. C.r. hebd. Séanc. Acad. Sci., Paris 259: 2907 – 2910.
Hashimoto, Y., S. Arai and T. Nose, 1972 Amino acids essential for the growth of eels, Anguilla anguilla and Anguilla japonicus. Bull. Jap. Soc. Scient. Fish. 38: 753 – 759.
Honma, A., 1971 Aquiculture in Japan. Japan FAO Association, Tokyo.
Koops, H., 1965 Fütterung von Aalen in Teichen. Arch. FischWiss. 16: 33 – 38.
Matsui, I., 1965 Studies on the morphology, ecology and pondculture of the Japanese Eel (Anguilla japonica Temmick u. Schlegel). J. Shimonoseki Coll. Fish 2 (2): 1 – 245.
Meske, Ch., 1969 Die Gewichtsentwicklung von Aalen in der Warmwasserhaltung. Fischwirt 19 (12): 226 – 272.
Meske, Ch. and O. Cellarius, 1973 Artificially produced sexual maturity and observation of sexual differentiation in eels reared in warm water. Arch. FischWiss. 24: 191 – 197.
Müller, H., 1964 Wachstum, Fütterung, Markierung und Fang von Aalen in kleinen Teichen. 2. Fisch. 12. N.F.: 295 – 306.
Olivereau, M., 1961 Maturation sexuelle de l'anguille male en eau douce. C.r. hebd. Séanc. Soc. Acad. Sci., Paris, 252: 3660 – 3662.
Yamamoto, J. and K. Yamauchi, 1974 Sexual maturation of Japanese eel and production of eel larvae in the aquarium. Naure, London., 251: 220 – 222.
Edel, 1975 No reference given.
Kuhlman, 1975 No reference given.
Kuhlman, 1979 No reference given.
Fig. 1: Mortality (no. of dead glass eels per day) of cultured glass eels after stocking
23°C well water, open system
23°C 50% salt water (baltic sea, ca. 15% salinity)
50% well water, after 20 days well water only
95% recycling, temp. increasing 13 – 19°C
Initial quantities 3 kg = ca. 10 000 glass eels each
