by
H. KOOPS
Bundesforschungsanstalt für Fischerei
Institut für Küsten- und Binnenfischerei
Hamburg, Federal Republic of Germany
Abstract
It is possible to feed elvers and eel fingerlings in ponds with minced fish and shrimps or fresh meat.
The food quotient for elvers is lower than 10 and for fingerlings also it may be about the same.
The results of experiments described in this report show that commercial pond feeding of eels is feasible.
L'ALIMENTATION DES ANGUILLES (ANGUILLA ANGUILLA) EN ETANG
Résumé
On peut nourrir les civelles et les jeunes anguilles en étang avec du poisson et des crevettes hâchés ou de la viande fraîche.
Le quotient alimentaire des civelles est inférieur à 10 et il en est probablement de même pour les jeunes anguilles (fingerlings). Les résultats des expériences décrites dans cette communication montrent que l'alimentation commerciale des anguilles en étang est réalisable.
ALIMENTACION DE ANGUILAS (ANGUILLA ANGUILLA) EN ESTANQUES
Extracto
Es posible alimentar angulas y alevines de anguila en estanques con pescado y mariscos o carne fresca picados.
Los cocientes de alimentos para las angulas es menor de 10, y también para los alevines puede que sea aproximadamente el mismo. Los resultados de los experimentos descritos en este informe muestran que es factible la alimentación comercial de las anguilas en estanques.
Feeding experiments with elvers and small eels (fingerlings) (Anguilla anguilla) in ponds were made by the Institut für Küsten- und Binnenfischerei, Hamburg, in the years 1963–1965, to determine whether and under what conditions pond culture of eels is possible. A short paper about the results known so far has already been published (Koops, 1965).
An analysis of the growth of elvers in ponds is of some interest for several reasons. Elvers are the cheapest stocking material for eels (price for elvers in Germany is 0.01 – 0.02 DM each). Since the growth rate of individual eels differs greatly from the economic point of view the stocking of elvers permits sorting out the faster growing specimens for further feeding. Lastly, it is possible to learn more about the sexual development of the eel. Japanese investigations on Anguilla japonica have shown that the percentage of females in the total stock is 50–60 percent in natural waters, but only 0.5 – 9 percent in pond culture (Matsui, 1952). In the case of the European eel, some observations indicate that in most of the elvers the sex ratio is not constant.
For investigations on the growth of elvers, in May 1963, one 100 m2 pond in Gorleben/Elbe was stocked with 3 kg (= 7,500)elvers (experiment 1). Because of unfavourable conditions (renewal of water was not possible) the results obtained at harvest on the growth rate represent minimum values.
Food was given in the form of minced coarse fish on a gauze frame. For control measurements samples of eels were taken at the feeding place. However it was revealed at the end of the experiment that the average weights of eels, as estimated from these samples were too high, and so these samples have to be considered as considerably biased (Table I).
Table I
The growth of elvers as determined in a pond experiment at Gorleben/Elbe
| Date | No. | Length in mm | Weight in g | k factor average | ||||
| min. | average | max. | min. | average | max. | |||
| 15- 5-63 | 58 | 68 | 77 | 83 | 0.26 | 0.40 | 0.46 | 0.0800 |
| 4-10-63 | 48 | 86 | 112 | 140 | 0.64 | 1.77 | 3.61 | 0.1196 |
| 29- 5-64 | 49 | 83 | 116 | 143 | 0.50 | 2.20 | 4.04 | 0.1320 |
| 10- 6-64 | 67 | 87 | 122 | 176 | 0.74 | 3.05 | 10.41 | 0.1522 |
| 11- 8-64 | 337 | 104 | 145 | 211 | 1.34 | 5.24 | 16.94 | 0.1601 |
| 9-10-64 | 39 | 107 | 167 | 245 | 1.71 | 7.80 | 26.10 | 0.1518 |
| Total autumn | ||||||||
| 64 | 5151 | 78 | 136 | 307 | 0.50 | 4.20 | 57.66 | - |
; G = weight in g; L = Length in cm)
It is obvious that the k factor increases with increasing length of the eels. Table I shows further by how much the growth rate of eels differs. From May, 1963 until autumn 1964 the weight of slowest growing eels had at least doubled. Average weight had increased by ten times and a few eels weighed 120 times that at the beginning. In an aquarium experiment (experiment 2) the difference of growth was similar, but did not vary so much. Probably the insufficient feeding of the elvers is the reason for the high growth difference in the pond. For culturing eels in a pond several sortings into size-groups will be necessary. Table II shows the size composition of the total pond stock in autumn 1964, compared with the size composition of elvers kept and fed in the aquarium during the same period.
Table II
Size composition of elvers reared and fed for two summers in a pond and in an aquarium
| Length in cm | Weight in g | Pond (n=4958) percent | Aquarium (n=79) percent |
| 7 – 10 | 0.5 – 1.5 | 7 | 4 |
| 11 – 15 | 2.1 – 5.6 | 78 | 46 |
| 16 – 20 | 6.9 – 13.7 | 13 | 46 |
| 21 – 25 | 15.6 – 27.0 | 1 | 5 |
| 26 – 30 | 30.0 – 57.7 | 1 | 0 |
The average weight of cultured eels - the k factor increases from 0.1200 in eels of 10 cm to ca. 0.1700 in eels of 20 cm - is considerably higher than the weight of eels of comparable size living in natural waters. The food-quotient could not be determined in the pond experiment. In the aquarium experiment, it was between 5 and 10, for water temperatures from 14–20°C. The total mortality in the pond experiment was 31 percent from May 1963 to autumn 1964. The total weight of the eels stocked increased from 3 kg at the time of stocking to 20.8 kg at the time of fishing.
Small eels from 15 – 35 cm are referred to as “fingerlings” here, and they are caught in the tidal area of the river Elbe and other rivers for stocking inland waters. Unlike elvers these eels belong to several age groups. The percentage of males is higher (up to 80 percent). Males are slow-growing (maximum length about 42 cm) and therefore undesirable for pond culture.
In May 1964 a 100 m2 pond in Gorleben/Elbe was stocked with 210 kg (= 15,000) eel fingerlings (average weight 13.6 g).
The pond was supplied with ground water of 9°C and the temperature in the pond did not exceed 16°C. The renewal of water was limited to about 1 litre per second. The quantity of food differed and depended on the quantity of coarse fish caught in the nearby river fishery. On an average it was not possible to feed more than 30 percent of the quantity needed. In general, at a maximum more than 10 percent of the total weight of the eels were fed.
At the time of stocking the eels were extremely lean and the average weight was only two-thirds of the weight normally to be expected. About 15 percent of the eels suffered from the “cauliflower” 1 disease and in the first six weeks there was a loss of 12 percent by the “red disease” 2.
Since the eels hide in the substratum, special “Eternit” plates of 1 m2 size were placed on stones at the bottom of the pond. A 1 m2 plate is sufficient as a hiding place for 25 kg of eels.
Minced coarse fish were used as feed. Apart from the coarse fish obtained from the river fishery, deep frozen sea fish and shrimps and slaughter house refuse were also used. The pond was rectangular in shape. A long and narrow pond has been found best suited for feeding eels because the eel when searching for food orientates by its sense of smell. So the current of water inside the pond aids the eel to detect the food. The feed was given on a gauze frame in order to be able to remove feed residues, such as scales and bones. The data on the growth of the fingerlings is given in Table III.
Table III
The growth of eel fingerlings in a pond
| Date | min. | Length average | max. | min. | Weight average | max. | k factor average |
| 8- 5-64 | 15 | 22.2 | 31 | 4.2 | 13.6 | 36.5 | 0.1094 |
| 8- 7-64 | 17 | 23.7 | 33 | 0.7 | 19.1 | 48.5 | 0.1363 |
| 3-11-64 | 16 | 25.5 | 37 | 4.7 | 27.0 | 108.3 | 0.1471 |
| 7- 7-65 | 17 | 26.6 | 37 | 5.3 | 33.4 | 109.5 | 0.1665 |
The average weight doubled between May 1964 and November 1964. However, it must be stressed that the growth rate could only be followed by taking sub-samples. The pond had not been fished out completely, and the bias of sampling which has entered these calculations cannot be estimated. Another experiment (experiment 4) carried out at Müden/Mosel has shown that here the average weight calculated from the sub-samples was lower than the overall average weight.
In the Gorleben experiment the increase of the k factor was found to be higher than normally expected, for at the beginning the eels were extremely lean. Table IV demonstrates to what extent the k factor can rise in such a case. In the table the data on two control samples collected on 12 August 1964, three months after stocking, and on 2 July 1965, 14 months after stocking, are compared.
Table IV
Increase of the k factors in the course of experiment 3
| Size group cm | n | 12-8-64 average k | n | 2-7-65 average k |
| 16 – 20 | 42 | 0.1300 | 6 | 0.1507 |
| 21 – 25 | 50 | 0.1319 | 52 | 0.1555 |
| 26 – 30 | 34 | 0.1387 | 49 | 0.1682 |
| 31 – 35 | 9 | 0.1503 | 41 | 0.1777 |
| 36 – 40 | - | - | 4 | 0.2029 |
It can also be concluded that the k factor of fingerlings increases with length. Eels with “cauliflower” disease were lower in weight than healthy eels and the condition factor decreased with increase in this disease.
In aquarium tests it has been proved that the “cauliflower” grows relatively quickly in fresh water. Such eels are hindered very much in taking food and finally may die. The aquarium tests showed also that new tumours appeared on eels which at first seemed to be free from “cauliflower”; an increase of the infection rate also could be observed in the pond experiment.
1 Ulcers resembling cauliflower in appearance on the jaws and nostrils and occasionally on the fins and other parts of the body.
2 Highly infectious bacterial disease caused by Pseudomonas punctata forma Sacrowiensis, resulting in red colouration of the body and fins.
On 30 April 1965, a pond of 700 m2 at Müden/Mosel was stocked with 350 kg eel fingerlings with an average weight of 16.6 g. This corresponds to a density of 30 eels per m2. In this experiment 25 percent of the eels had the “cauliflower” disease at the time of stocking.
Similar to experiment 3 (Gorleben) the eels were fed with minced coarse fish. However there was not sufficient feed because of the poor results of the river fishery. Only about 20–25 percent of the needed feed was available. The result of the total fishing on 25 October 1965, six months after stocking, was:
| 15,570 small eels | = | 428.5 kg | average weight | 27.5 g |
| 1,012 medium eels | = | 79.5 kg | average weight | 78.6 g |
| 68 silver eels | = | 5.5 kg | average weight | 80.9 g |
| 16,650 total | = | 513.5 kg | average weight | 30.8 g |
This experiment permitted the calculation of the food quotient (total quantity of food supplied divided by the difference between the total weight of eels at the commencement and the end of the experiment) for eel fingerlings. This amounted to 14.5. Because of the small quantities of feed supplied, the food quotient is relatively high, since a considerable portion of it was necessary for maintenance and only a small amount was available for further growth.
Another factor responsible for the high food quotient is the high mortality (some 25 percent) as caused mainly by the diseases observed. It is likely that the food quotient will be lower than 10 if more feed can be supplied and if the natural mortality could be considerably lowered. Sorting out of diseased eels prior to stocking is highly recommended.
Koops, H., 1965 Fütterung von Aalen in Teichen. Arch.FischWiss., 16(1):33–8
Matsui, I., 1952 Studies on the morphology, ecology and pond culture of the Japanese eel (Anguilla japonica Temminck and Schlegel). J.Shimonoseki Coll.Fish., 2(2):1–245
