INTENSIVE REARING OF EEL

Mr. G. ARLATI

1. PRELIMINARY VIEWS

The following arguments must be taken into consideration :

- Rearing methods, stagnant or running water

- Choice of an ideal spot for the implantation of the eel fishculture.

1.1. Rearing methods: stagnant or running water It is possible schematize eel rearing from the following fundamental characteristics :

1. quantity and quality of the water;

2. type of “seed”(elver or small eel), (“ragano”);

3. feed;

4. pathology;

5. market.

1.1.1. Water

To develop and live, eels have an urgent requirement of pure water and if possible abundantly. There must always be a good amount of oxygen present; When the density per surface area is increased, it must be possible to guarantee the oxygenation by means of auxiliary equipment.

1.1.2. “Seed”

By eel rearing is meant, the rearing of very small eel oreel, at elver stage, which are taken from the sea, to commercial size. It is not yet possible to reproduce the complete biological cycle of eel in a laboratory: although the artificial insemination problem has been solved, it is still impossible to ensure the survival of the leptocephalus at larval stage.

1.1.3. Feed

Every type of intensive zootechnical rearing is based on the distribution of special food which is suited especially to the product reared.

With eel culture, it is not sufficient to employ a well balanced diet to ensure the result: the growth of the product depends, in fact, on the water temperature, on which depends the quantity of food that must be distributed, and the appetency of the food itself. It is also necessary that this latter, as it is very expensive, guarantees a good conversion index.

1.1.4. Pathology

Parasites and bacterial diseases must be foreseen, by respecting the qurantine period of the fish brought from outside and the scheduled prophylaxis treatment of the environment, and if these should appear, quick intervention is demanded by employing adequate therapeutic treatments.

1.1.5. Market

A breeder knowing how to sell is ell will obtain a better return than one having no commercial experience.

1.1.6. Eels are reared in earth or concrete tanks, in greater densities than that, which could be tolerated by the quantity of physical oxygen available in water. For this reason, two different fundamental methods of oxygen supply are employed, which in turn give rise to two rearing the models: in stagnant water and rearing in running water.

With the stagnant water model, which favour the growth of the phytoplankton green algae in the water of tank, the oxygen is furnished by the photosynthesis. A dense photosynthesis (20 cm optimal transparence) can not increase unless the water flow is very slow or inexistant. In fact, for these tanks, which have more or less stagnant water, the water exchange per day is around 5% of the total volume.

With the running water model, the oxygen requirement of the eels is furnished by a constant new water flow, as seen with the culture of trout. A continuous renewal of abundant running water is therefore required for this type of tank.

The temperature and the quantity of water at disposal bring about the definition of the choice between one model or the other :

- When there is a limited amount of water at disposal, it is possible to have stagnant water tanks or at the most, recycled water tanks having more or less running water.

- When there are great quantities of water as disposal, it is possible to have either stagnant water tanks and if the mild climate permits so, running water tanks.

The temperature at the water is a fundamental point: 19 – 28° C for the european eel. From an economic point of view, the rearing of eel is advantageous when sizes of 120 – 180 grammes are reached within two years or less, when starting off with elvers (0,2 – 0,3); this rapid growth is only possible in optimal temperatures (21 – 26° C).

- The european eel dose not eat in temperatures of under 8 – 9° C, but better results are obtained at temperatures of more than 17° C (22° C optimum).

1.2. Choice of the ideal spot for the implantation of an eel culture

The spot will be all the more suitable for rearing when the specific conditions here following are present :

1. Availability of a good water supply, channeled or pumped from a canal, or better still, coming from underground sources or wells. For instance, around 400 – 450 m³ is required per day to rear 40 tons of eel per year.

2. The water must not be contaminated(by insecticides, phytomedicaments, etc...) Alkaline or natural water is the best, while acid water (below 6,5 pH) is not very recommendable for rearing.

   The eventual presence of wild eel in the water which is to be employed indicates clearly that the water is of good quality and suitable for rearing.

3. The rearing bite must not receive sea inflows or be flooded.

4. The water exchange in the tanks must preferably take place by means of downflows. In other words, the energetic costs must be as limited as possible, the use of pumps for the distribution of water must be avoided when possible.

5. The soil must not be porous so as no water leakage is possible from the bottom or sides of the tanks. Clay mixed with sand is the most advisable.

6. The site must receive sunshine so as to favour the algae bloom, which by the chlorophyllian synthesis produces oxygen.

7. the site must be cleared so permitting the oxgenetion of the surface water by the wind.

8. Good roads and electricity must be available

9. The availability of important markets in proximity must be ensured.

2. INTENSIVE REARING

The technics employed for the intensive rearing of eel are based on three essential conditions: favourable temperatures, sufficient oxygen, artificial food.

Favourable temperature for european eel rearing vary from 8 to 28° C; The optimal temperature for the introduction and weaning of small wild eels varies from 18 to 24° C; for the weaning and growth of elvers, it is situated around 22 to 26° C. The further the temperature away from the optimal values, the longer the biological cycle of rearing will be.

The dissolved oxygen content required for the survival of the eel is from 2 – 3 mg/l for elvers and 0,5 mg/l for adult eels. In rearing, these oxygen requirements are greater and vary depending on the temperature (and the salinity), the size of the eels and their metabolic activity.

Generally, saturation values of 80 – 90 % are acceptable up to 20° C, and a value around saturation for temperatures of more than 20° C.

Normally, artificial feed for eels it made in the from of moistened paste balls, mixtures of meals, oils, water and having or not a certain quantity of minced fresh fish content.

1.1. Intensive rearing cycle

At present, intensive rearing can be subdivided into three parts :

- Elver weaning and rearing

- Small wild eel (“ragano”) weaning

- Eel rearing

The breeder must establish his own production with fish coming from outside: elver or “ragani”caught in sea or brackish waters, or with small eel which have grown from weaned elver or “ragani” size, and coming from other rearings. This method is employed so as to avoid the appearance of the “closed cycle” at biological level, and it gives diversified results to the breeder. Only the description of the different phases in the rearing cycle, and the most renown methods and technics shall be taken into account here.

2.1.1. Elver weaning and rearing

Elver weaning and rearing is carried out in both brackish and fresh water, in special tanks, built especially for this purpose. Generally concrete or polyester resine tanks are employed, of different shape (rectangular, square, or round) and of variable size (from 8 to 20 m²)and having the required equipment from a technological point of view at disposal, (independent hot and cold water circuits, premixed O² supply, air supply by means of different types of diffusers : blowers, venturi, etc… an outlet circute having partial or total feed back of the hot water and when required the possibility of discharging non reusable water, etc…) These tanks are located in enclosed green houses or sheds.

Elver rearing is known as the 1st stage and it is economically advantageous when there are higher temperatures than 21 – 22° C up to 25° C available. Elvers, coming from the sea, can be of variable weight (0,20 gr. to 0,40 gr) depending on the season and the area where caught, available from November to February the italian eel is smaller, while those found on the French or English Atlantic coast are bigger and are available in the spring, from March to May.

The “seed” density varies depending on the rearing method employed : if stagnant water tanks, having an exchange of 6 – 7 1/minute, are employed, the density is 0,4 kg per m²; if running water tanks, having an of 40 – 45 1/minute about are employed, it is 3 kg. per m².

At this point weaning takes place, by employing a minced mixture of worms and anchovy and/or sardines, blended with feed in powder from, or with a binder. Little by little, a meal of more or less compound food is added to the paste. It is a complete and self sufficient food which becomes less and less appetizing (anchovy, live worms, etc…). within 4 – 5 weeks, if the product is healthy and well adapted to the environment and has not suffered any stress for the latter, the importance of adapting the elvers gradually, within 24 – 36 hours, which have come from temperatures of 9 – 12° C, to ambiant rearing temperatures must remarked; it is weaned at 22 – 25° C; it is necessary to control the hygienic and sanitary aspects and carry out the required treatments when necessary.

At this stage, a first selection is made, as a uniform and homogeneous size facilitates growth speed: the smaller eels will be placed back into the weaning tanks, the others, when weighed, will be fed a moistened paste to fatten them and another selection will be made 3 or 4 weeks later. At this rythme, within 8 months the elvers reach the size of small eels (“raganello”) of 15 – 16 gr.

A typical cycle can be resumed schematically in the following way :

In stagnant water tanks, it is possible to have a maximum density of 1,6 – 1,8 k/gm² about, while in running water tanks, 5 kg/m² is obtained. In any case, the densities, apart from depending on the water exchange and the size, also depend on the oxygen supply technic employed.

Finally, if all the operations are to be profitable, it is necessary to obtain 20 – 25 kg. Off small eels within 9 months from 1 kg of elvers, and to reach 30 – 35 kg within a year. The above data is purely indicative, as the minimum percentage, less than 15 % will reach 15 gr within 5 – 6 months; the greater part, 45 to 65 %, will reach a play size in the scheduled time; and the rest, known as the “tails”, will take longer to fatten.

The present cost of elvers is 27 – 30 000 lira/kg; the cost of the finished product of this 1st rearing stage, which is but a rough estimate, as there exists no real market, is around 46 – 52 000 lira/kg.

Italy's future for eel culture lies in the rearing of elvers, but up to the present day, very few 1st stage rearing exist, which could provide a guaranteed production. This is explained by the fact that 5 or 6 years ago, breeders were encouraged to start of experimental elver rearings at industrial scale, but a more conclusive evaluation on the costs/benefits showed that this would only be of profit if low costing hot water was at disposal.

The secret of a good first stage rearing result is based on the selection, the feed and the sanitary operation programme.

2.1.2. Small wild eel weaning (“ragano”)

It is not necessary to have water of more than 22° C at disposal for small wild eels, good performances are also obtained at 18° C. Some breeders even employ water from sources having temperatures of 13° – 14° C, but this is only carried out when a small number of animals are to be weaned for rearing purpose, as the lenght of the weaning cycle is rather long (4 – 5 months) and not always conclusive. It must be remarked that the weaning of elvers as well as small eels must try to “domesticate” a good amount of animals in the shortest time possible. A good environment, efficient technical management of the facilities and a correct weaning method would ensured success.

The concrete or polyester resine tanks, located indoors, facilitate the weaning and sanitary controls; these tanks can be of variable size and form (from 24 to 200 m²).

As most of the small eels are taken from the Mediterranean French coast, it is necessary to carry out an antiparasite treatment on arrival, employing formaline and green malachite for two days before starting to wean them and keeping them under observation (quarantine) for at least 2–3 weeks before sorting them out; taking these precautions, it is possible to intervene immediately if a serious and contagious disease should occur among them. It is better to eliminate immediatly a certain number of contaminated elements than to run the risk of the whole rearing catching the disease: the consequences would be all the greater and costly in the end.

After this stage, weaning starts by distributing whole sardines threaded on to a wire through the head or a moistened paste mixed with fish (40 %) or with red worm (10 %).

During the weaning period, it is recommendable to carry out two antiparasite treatment each week as means of prevention, preferably each morning and in any case the fish must always be fasting.

After about one month, a first selection may be carried out: the smaller ones being kept in weaning while the others will receive less and less feed attractants in the paste. After about two weeks, a second selection is carried out followed by another, two are three weeks later on, at the third sorting out the product will be big enough to go into open air tanks for fattening purpose or 2nd stage growth. It is presumed when employing this method, that concrete tanks and the required structures, so to facilitate fishing and selection, be at disposal. Elver tanks are quite commonly employed to wean small eel during certain periods.

In all cases, if there are similar conditions present, the density of the product may be freely multiplied by three compared to that for elvers.

Most of the Italian eel cultures are second stage rearings employing concrete fisheries (RAVAGNAN model). This area, which is easily controlled and isolated, takes up about 1/10 of the total surface of the tank and it is possible to carry out, along with the management operations (collection and fishing of the fish), the weaning of small wild eels. However, this is only feasible under one condition; that during this period, the rest of the earth tank is not employed for rearing purposes, as this operation would be very risky and could have an effect on the sanitary situation in the tank, resulting in serious consequences.

Indeed, the more knowledageable breeders, employ the fishery to wean small wild eels, who have been accostumed to fattening feed, and set them free, after selection, into the other part of the tank. In this case, the “seed” density is 1, 5 to 2 kg/m² when the whole rearing tank is taken into consideration.

It is also very important not to recollect the outlet water, but to let is discharge into a small decantation lake (3 000 – 8 000 m²) before finally flowing out of the rearing.

During the weaning period, the average mortality varies from 10 to 30 %, depending on whether, enclosed structures or open air rearing, are employed by the production centres.

Small wild eel of 15 – 20 gr average size cost 5 000 to 5 500 lira/kg, (unfortunatly this price will rise, as there are fewer and fewer small wild eel available on the market today) and are sold when weaned at 2 – 3 times their original price.

2.1.3. Eel rearing

The fattening cycles of eel takes place outside, in either fresh water - which is preferable, as it quickens growth, or in brackish water in earth tanks, of square or rectangular shape and having variable surfaces (400 to 1 600 m²). It is not obligatory to have at disposal “hot” water 22° C, but temperatures of below 14 – 15° C do not give good results.

In the North of Italy, where the possibility of obtaining water at 12–14° C from well exists, the most commonly employed method, is that of using semicurrent water, which as a constant and very slow flow (6 – 8 1/sec. for a surface of 1 000 m², which roughty corresponds to the same number of cubic meters). All solutions are possible as long as the collection and fishing of the products is feasible, as the management is the most important operation. Tanks have been especially built so to permit the automatic selection of the products.

The average period for the rearing cycle is linked with the climatic situation and the seasons; in the North, to reach commercial size, when beginning at small eel size (25 g), takes 8 – 9 months; in the South, the same result is obtained in 5 – 6 months.

The fish already stored out and weaned, are placed into the fattening tanks, in the Spring. This way, part of the product of commercial size will be available for Christmas, a time when there is a great demand for this product. The “seed” densities can vary from 1 to 3 – 4 kg/m², depending on the method employed; the maximum density reached, is three time more. During this period, mortality reaches 15 – 30 % and for fresh water rearing it is necessary to carry out sanitary treatments against ichthyophtyriase, protozoan parasites which are very contageous and the cause of drastic economical consequences: these diseases are the cause of absense of growth and even death if widely spread throughout the product, when the proper controls are not carried out.

Here below is a production plan for a 2nd stage rearing, carried out in semi-current water flows having an average annual temperature of 18 – 19° C (Centre and North of Italy).

It is not obligatory to carry out all the selections recommended, but it is necessary to do three at least (the 1st, 3rd and 5th) so as to obtain an average size male of 120–180 gr, and a female of 300 – 350 gr. These selections are carried out in the aim of having fish of uniform and homogeneous size, this permits a shorter rearing period. The selection may be carried out manually, it is more difficult but more efficient also than the automatic option which is quicker but not as exact. At adult stage, the eel transforms sexually. When adult stage is almost reached, the eel is not profitable to rear because its feed conversion decreases greatly, becoming practically non-existant. This is why, breeders principally aim at producing as many females as possible: these being the only ones reaching a weight of more than 250 – 300 gr, as their maturity takes longer; on the contrary, males reach maturity from a sexual point of view when they reach a size of 120 – 150 gr, and must be commercialized (fig. 1).

Good breeders can obtain a percentage of 30 – 35 % of females which require 6 – 8 months rearing to reach this size.

Present market prices are 11 000 – 12 000 lira/kg (three fish weighing 300 – 350 grammes each) while for males of 120 – 150 gr, (7 – 8 fish per kilo), the prices range from 7 800 – 8 000 lira per kilo.

3. FEEDING

3.1. Feed requirements

The eel is a carnivorous and predatory fish having a tendency for cannibalism, it has a stomach. It preferes moistened feed. It's feed requirements have been thoroughly studied by the japanese researchers, but only concerning the species reared on site. The essential amino-acid requirements can be seen here following, after NOSE, ARAI and HASHIMOTO (1972).

Essential amino-acid requirements for 100 gr, of proteins

Also certain data concerning the composition of feed on sale in japan can be found in table 1.

The biological value of the protein, which means it's essential aminoacid content, is of particular importance in the artificial diets for eel. It must not differ too much from the concentration relations existing in the protein of the organism of the fish. Here finally, can be seen the nutrient requirement of warmwater fishes from the National Academy of Science-WASHINGTON, a mixture for the mineral integration :

and the composition of the vitamin mixture :

Two types of food exist in Italy, the first one, known as the starter, has high protein content (50 – 55 %), and poor lipid content (5 %); it is given to elvers and during the weaning of small wild eels, it costs around 1 100 lira/kg. The second one has a lower protein content and a greater lipid one, it is especially suited for growth and fattening, till the eels reach commercial size, it costs around 1 000 lira/kg.

3.2. Feed technics

A moistened paste is prepared just before being distributed to the eels, as shown in tables 2 and3. This paste is prepared with equal quantities of meal and water (the water being the same as that found in the tank). Some breeders add a fish oil percentage which can vary from 2 to 9%, depending on the water temperature and the size of the fish, otherwise a percentage of minced blue fish (5 – 10 %) can be added instead. Some breeders add a vitamin mixture to the diet every two months (1 – 2 % of dry matter) during 5 – 6 days so as to avoid deficiencies, due to lake of vitamins, occuring (Tab. 4),

The food is distributed daily by means of feeders, which are placed on the surface of the water, 4 – 6 times per day for elvers, 3 – 4 times for small eels in weaning and twice for eel in growth, respecting the quantities indicated in the daily food table. When the work must be carried out in lower temperatures, the paste is given out only once per day. A real and practical estimate, to verify the quality of the food, is made by watching how well the food is accepted and its conversion index: to enable this, the breeder must mark down the food consumption each tank. For elvers (0,2 – 12 gr), the conversion index is 1,4 – 1,8; for small eels (12 – 25 gr) it is 1,8 – 2,2; while for those in fattening (25–180 gr) it can vary from 2,3 to 3,2.

The percentages are given in dry matter by calculating the meal at + 1,7 of the amount of fish given and they are valid for optimal rearing temperatures (22 – 23° C). Fats must not be added when the temperature falls below 15° C.

4. GENERAL PROGRAMME PLAN

When a rearing is started up, principal aspects of the technical management must be taken into account. For this, let us examine a programme model, concerning the rearing cycle in a typical fresh water facility in the North of Italy which has water supplies from wells 15 – 16° C at disposal and when required an additional water supply from a source of non polluted water.

Although the optimal conditions of temperatures are not found here (Tab. 5) it is still feasible today, to produce eels, starting with small eels and not elvers, when a temperature of 18 – 20° C is present for 10 months of the year as shown in the theoretic plan (Table 6).

The fundamental problems, which shall be briefly confronted here, concern:

1. the possibilities of finding small eels;

2. food;

3. sorting out;

4. an adequate programme of the environmental prophylaxis, carried out while respecting the elementary sanitary norms.

4.1. Finding small eel (“ragano”)

The possibility of finding small eel is essential for the smooth running and success of the entreprise. The small eel should be lively, strong and healthy. Most of the small wild eel come from abroad at the present day (5 000 lira/kg, 50–60 fish per kg). Weaning will be all the easier when they are healthy. Above all, a good breeder must get the wild eel to adapt to artificial food, or rather to get the greatest amount of fish to “adjust”as quickly as possible. By employing the more recent and sophiscated methods, 85 to 90 %of the fish can be weaned in 3–4 weeks. For this phase, indoor concrete tanks, having a constant water exchange, water from sources or wells, must be employed, in preference.

4.2. Feed

Apart from the weaning period during which special substances bringing on a appetite, are added to the paste, the food diet must be autosufficient and well balanced. Indeed, a good breeder usually adds 5 to10 % of blue fish, which is minced just before the distribution of the feed takes place, special“integrators” in the appropriate quantities, depending on the requirement of the rearing, are added.

The nutrient function of the eel is linked with the temperature of the water and the dissolved oxygen content present in the tank (Fig. 2) and this latter depends on the weight of the eel (Fig. 3 and table 7). The food conversion indexes of the plan are those obtained with the food there presented, and integrated in the appropriate way to the above mentioned temperature: They are at optimum in these conditions, which must n't stop improvements being made in the near future.

4.3. Selection

Selections are very important in modern eel rearing. The aim of these, is to obtain a tank of fish of uniform and homogeneous size, therefore the greatest possible number is fed and a quick weight increase is obtained. In fact the rearing cycle is reduced and consequently the biological risk.

The selections shown in the plan, concern the principal selections, which means, those which are really necessary in any environment and which must take place immediatly after weaning and when commercial size is reached, for both males and females (big eels). Two additional selections are necessary for each fattening cycle, they are obligatory within the programmed management, as the equipment and methods of today are at disposal and permit not only the collection and fishing of the product more quickly (transfer by means of a siphon) but the real sorting out (semi-automatic sorter).

4.4. Prevention

The basis of sanitary protection, for every rearing, is prevention rather than treatment; this is why it is necessary to intervene adequatly, employing the precise programmed operations of prophylaxis on the environment, the parasites and the bacteria. This sanitary protection programme, includes the intervention methods, linked with the rearing environment and the particular situation in which the work is carried out. Some prevention normes, exist consisting very simply of keeping optimal conditions in the tanks. In this way, the pathological problem will not be as important, as there will be fewer occasions for “stress”. Therefore it is very important to follow certain methods of management;

- ensure that the rearing tanks have a sufficient supply of non-contaminated water every day.

- respect the quarantine period and disinfect adequately the environment every second day, at least over 2 – 3 weeks, (depending on the temperature) for those fish coming from outside before placing them into fattening tanks.

- dry the tank completely by removing organic residue at the end of each rearing cycle at least and by leaving it in the sun for 7 – 10 days: a better productivity will be obtained this way.

- distribute a well balanced and adequate food diet.

- Carry out good anti-bacterial prophylaxis, followed by polyvitamined treatments, at least 2 – 3 times per year, at the most critical moments.

- eliminate dead eels from the tank immediately and incinerate them.

5. CONCLUSION AND ECONOMIC CLACULATION

It must be understood at once, that the total use of the production possibilities of a rearing, is not possible before 2 – 3 years after it has been commenced.

Let us now analyse quickly some data on production, as seen in the plan.

Even in the non optimal conditions of the example given (18 – 20° C), productivity is quite good and still profitable, from an economic point of view : indeed, in 11–12 months, 16 % of the fish reached adult male stage (120 – 150 gr; 6 – 8 fish/kg), 12 – 13 months after their arrival at small eel stage; the remainin 40 % reached adult eel size after 22 – 23 months.

Until male stage is reached, mortality is from 22 – 28 % (average 27 %) and 35 %for the complete biennial cycle of the big eel; taking into consideration the mortality of the different rearing phases and the male/female relation of the small eels bought, it is possible to obtain 42 adult males (5 – 6 kg), and 23 b female eel (11,5 – 13,5 kg) when starting off with 100 small eel. These results are feasible for every good breeder who is prepared and capable of dealing with the problems in the proper way, which may arise in the course of the daily work.

Finally, taking as an example a rearing which has a water surface area of 15 000 m² and a annual production capacity of at least 40 tons of marketable heavy product, the yearly economic calculation (table 8) where the operational result (gain) is given.

TABLE 1

SOME DATA CONCERNING THE COMPOSITION OF JAPONESE COMMERCIAL FOOD

(from H. AOYE, reported by TOMIYAMA & HIBIYA, 1977)

1) In miscellanea: fish meal, Yeast meal, Liver meal, Linseed meal Soyabean meal.
Vitamins : Vit, A oil, calciferol, tocopherol acetate, thiamine nitrate, pyridoxine hydrochloride, nicotinic acid amine, calcium panthotenate, folic acid, cyanocobolamin, vit. K, riboflavin, biotine, inositol, chloride, ascorbic acid.
Minerals: Calcium carbonate, ferric acid, fumaric acid, potassium chloride, magnesium sulphate, calcium phosphate, manganese sulphate, cupric sulphate, zinc sulphate, calcium iodate, cobalt chloride.

TABLE 2

DAILY FOOD TABLE

Table 3

DAILY FEEDING SCHEDULE FOR ADULT EELS (25–120–300 gr.)

Healthy eels are voracious animals and consume the food given to them quickly. Consequently, it is important to study how eels eat and to only give them the quantity that they would eat.

Table 4

DEFICIENCY SYNDROMES OF WATER SOLUBLE VITAMINS IN A. japonica
(by HASHIMOTO, 1974)

Table 5

INITIAL CONDITIONS OF THE WATER TO BE EMPLOYED FOR EEL REARING

by OGAMI, 1974; translated by HOSHINA, 1978

Table 6

PROGRAMME PLAN (18 – 20° C)

○ = Necessary selection -

* = Advisable selection

TABLE 7

Eel: Individual and per kg oxygen consumption of body weight

TABLE 8

Annual Economic Calculation of a 15 000 m² of Water Rearing

(*) NB: eels: L 7 300–7 600/kg-Free rearing; Big eels: L 10 000–15 000/kg-Free rearing.

Fig. 1. Course of growth of Anguilla anguilla; female (○); male (+).

From Bulletin of the Japanese society of Scientific Fishers.

Fig. 2. The feeding activity for eels and the dissolved oxygen content depending on the temperature.
(MX: maximum; A: average; L: low; N: nil.
Here following as the subdivision of the temperature values concerning the feeding activity (L: mortality, Sub O: suboperative; OP: operative; OT: optimal; CR: critical; Semi O: semi operative)
(by RAVAGNAN)

Fig. 3. The oxygen consumption for eel depending on weight.
(by RAVAGNAN)