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INTENSIVE REARING OF SEA-BASS (Dicentrarchus labrax)




1.1. Warm waters-Cold waters

The growth of Mediterranean marine species by which we are concerned depends greatly on the water temperature in the rearing and is divided into three categories:

- Optimal at temperature around 25° C

- Reduced at temperature below 18° C

- blocked at temperature below 12° C

Risks of mortality appear at below 10° C while at 30° C pathological and oxygen consumption problems are remarked.

The great influence that the temperature has on the rearing: leads to two types of farm.

- The following farms with heated waters:

The sea water supplying these must be maintained at a temperature of 18 – 20° C the whole year.

The continuous optimal growth of the fish is obtained this way and the fish reach commercial size in 18 months.

- The fattening farms with sea-water at natural temperature:

This solution is only suitable in sites where the temperature of the water never drops below 14° C in the winter time.

It is characterised by two growth interruptions in winter per rearing cycle and the fish reach commercial size after 2 to 3 years.

1.2. The three phases in rearing

1.2.1. The nursery

Fattening starts when the weaned fry are transferred from the hatchery. They then under a lot of handling before being placed into a special unit, known as the nursery.

This is the unit contains small volume tanks (25 to 50 m3) characterised by:

- a very god accessibility (better control of the rearing, reduction of manual labour charges)

- the possibility of programming removable partitions while not interfering with the hydraulic system of the tanks.

The fry remain in this part for 6 months, until they reach juvenile stage with an average weight of around 20 – 25 g (in a rearing which has controlled water temperature).

1.2.2. First fattening -fattening

When taken out of he nursery the juveniles have reached a sufficient size to be transferred into tanks of more than 60 to 100 m3 in volume. If all the rearing tanks have a unitary volume of more or iess100 m3, no distinction is made between first fattening and fattening. If this is not so, the average size tanks are left for first fattening while the others (100 – 300 m3) are kept for fattening.

Juveniles having received first fattening reach around 70 g. before being placed into fattening. The fish leave the farm at a optimal weight for commercialization between 300 and 500 g.

1.3. The different types of tanks

1.3.1. Design

The tanks from an enclosed space for the fish and their rearing environment:

a) Hydraulic characteristics

It must be always possible to drain the tanks completely in a short time (half an hour to one hour). They must be designed in such a way so that they can create internal currents leaving no dead zones and maintaining a homogenous rearing environment.

Their outlets must be equipped with fish trap systems which do not clog up easily. They must also be located at S.M. concentration zone levels. The water level must be adjustable and constant no matter what the water renewal supply flow may be.

The design of the tanks must permit easy access and the use of all the equipment necessary to carry out the rearing. The suitable depth is below 1,20 m so to permit an easy and efficient supervision of the tanks.

b) Accessibility

The outside circumference around the tanks must permit, easy access to all the important parts, so as to permit the performance of all the rearing operations properly and the ensurance of efficient supervision.

The partitions must be smooth and non-porous allowing sterilization and quick cleaning.

The partitions are generally rigid panels (concrete - masonry - polyester).

1.3.2. Raceways tanks

These tanks are characterized by a length/width ratio of more than 5. They have an upstream water supply and a downstream outlet flow, this permits maintaining a quick continuous longitudinal water current supply on the slant. These tanks make use of the water supply very efficiently and can be cleaned out automatically, the S.M. is transported with the current).

They also permit to have high ratios of “water surface/farm land”. They are very well suited to adjoining tanks which have reoxygenation between each.

1.3.3. FOSTER-LUCAS tanks

These tanks don't differ from the raceways apart from the fact that they have a longitudinal partition and rounded ends. This solution allows the optimal use of the surface while having very rapid interior currents as seen with circular tanks. These currents are produced by the arrival of the renewal water and the air lift systems. These tanks are automatically cleaned out and are characterised by a more homogenous rearing environment than that of raceways

1.3.4. Other types of tanks

The circular or earth tanks are not often employed in fattening for work reasons:

- Complexity of the rearing operations

- Bad use of the surface available

On the contrary to the above, it is very advantages to carry out the fattening phase in floating cages located in the sea or in lagoons when the conditions of the site at right permit so.


2.1. Rearing programme-stock management

2.1.1. Nursery

The weaned fry of 1 to 5 g on arrival at the hatchery farms, are too delicat and are at a too rapid growth stage to be placed directly into the final fattening structures. They are therefore transferred into a special unit, known as the nursery for the reasons here following:

a) Frequency of manipulations

The nursery must allow quick and easy finishing and grading operations while not causing any stress. These operations are indeed often necessary so to avoid the problem of cannibalism. The great growth speed of the fry causes remarkable difference in size quite quickly. As the sea-bases attacks fry of the same size or inferior to his own size, frequent grading is necessary. This reduces the mortality rate while maintaining a more regular distribution of the population. The manipulations are therefore carried out every 2 to 4 weeks.

b) Extreme sensitiveness to the quality and regularity of the physico-chemical parameters of the rearing environment.

The principal parameter to have under control is the temperature. it must be as stable as possible (variations ≤ 2 to 4° C per day) and kept at around 25° C.

For this, it is advisable to have two water supplies, permitting to increase the temperature of each tank when necessary. The other parameters to be checked carefully are the oxygen ammonia tenor, pH and turbidity.

2.1.2. First fattening-Fattening

When the fry are taken out of the nursery they are transferred into enclosures of greater volume. If the optimal loads of 12 kg/m3 were maintained there would be overcrowding and this should entail:

- Management problems of the loads (heterogeneity)

- more rearing risks should be incurred (fishing, accidents)

- manipulations should be more complicated (stress)

After a duration of 5 (in heated water) to 10 months (after wintering) pre-fattened fry of a unitary average weight of 70 g. are obtained and these are then transferred into the final fattening tanks.

During this phase the manipulations are reduced to every 4 to 8 weeks because of the decrease in the speed of growth and the loads being more homogenous.

Loads at the end of fattening: 15 kg/m3

Average weight: 300 to 500 g.

2.2. Water management

The determination of the water supply level depends on the principal parameters here following :

2.2.1. Oxygen consumption in the tanks

Taking into account the loads admitted in intensive fattening (15 kg/m3) the oxygen consumption in the rearing tanks can be very high. This depends principally on :

- The age of the fish. The younger they are the more oxygen they consume per kg of live weight.

- The stress activity rate

- The feed rate

- The temperature: the higher the temperature the greater the metabolic activity and therefore an increase in the oxygen consumption is remarked.

The oxygen supplied through the water renewal must be equal to the difference between the consumption in the tank and the supply by the aerators, this itself depending on :

- The temperature and the salinity. The two factors are inversely proportional to the oxygen tenor of the water at saturation.

- The supply flow

There must be a sufficient supply of oxygen so as to maintain a minimum rate of 4 mg/l of oxygen at the outlet of the tank.

2.2.2. The gassy ammonia rate

When most of the oxygen supply is ensured by the aerator systems, the factor determining the minimal renewal rate of water can be the gassy ammonia rate. This rate itself depends on :

- The metabolic activity rate of the population. The more intensive it is (stress, feeding period) the greater the production of ammonia.

- Tank loads-feed rate

- Other physico-chemical factors

Ammonia is discharged in three forms in the rearing environment :

- Solid nitrogenous products

- Ionized ammonia (ions NH4+): Ammonium ion

- Non-ionised ammonia (NH3): dissolved gas

Only the third form is highly toxic for fish. The toxicity threshold can however vary depending on the age of the fish (proportional) or the temperature and the level of stress (inversely proportional).

Ammonia in rearing environments tends to evolute from the first form to the two other forms (solid solubility) and the report between the two other forms is balanced at a variable level by the following transformation :

(NH4+) (OH-)===NH3 + H2O
ionized form gassy form

The tendency of this balance varies depending on the temperature and the pH (Fig. below)

Effects of pH and temperature on the distribution of ammonia and ammonium ion in water. Data from LIAO et al. (1972)

2.2.3. Other physico-chemical parameters Temperature : Rearing technique employed in heated water :

We have observed in 1.1. that the temperature is an essential parameter for the management and competitivity of a rearing. In the case of rearing with controlled temperature, a minimum supply flow must be maintained in such a way so that the thermic loss can be counterbalanced in the tanks.

The rearing water supplied to the farms may be heated by means of :

a) Industrial thermic systems :

With this option which is wide spead, the water flows more or less directly from the cooling circuits of industrial plants (petrochemical, power stations).

This solution which is on general rather complex leads to heavy investments. The complexity is caused by the quantitative and qualitative irregularity of the water (temperature, physico-chemical characteristics).

This the water may be employed directly when it has been mixed with “natural” sea-water or when treated, or as warm water in the heat exchangers if the physico-chemical characteristics are not suitable for use in the rearing.

b) Geothermal drilling :

This solution, limited to vary precise geographical areas, is very advantageous when carried out in shallow water with high temperature (above 25° C) as the drilling in the case is not very expensive and there is a constant hot water supply.

The water is obtained from artesian wells or by means of pumps and it is employed as a hot fluid in the fresh water-sea water heat exchangers.

This solution usually leads to an increase of investment concerning :

The limited choice for the site which could lead to over expenditure for land or the development of sites which are not very suitable (too narrow, too elevated unsuitable edaphic characteristics).

Hydraulic systems are more complex (double or triple circuits) involving treatments, recycling, pumping stations and the facilities must be covered so as to limit the loss of heat in regions which bad climates. Suspended matter tenor

A too high tenor of S.M. is harmful as it is can cause asphixia (clogging of the gills) and feeding problems (decease in the viability) pH.

This is a very important parameter, as it conditions the balance between toxic ammonia and non toxic ammonia. It is advisable to maintain the pH above a level of 7,5 and below a level of 8,5.

In the rearing tanks, the pH varies inversely to the tank load. If the pH should vary, it will be necessary to raise the renewal rates.

2.2.4. Hydraulic aspects

The renewal rates in the tanks which are calculated depending on the loads in the tanks and the limiting physico-chemical parameters (NH4 - O2 S.M. tenor).

To define the limit design of the hydraulic structures it is necessary to take into the account :

- The average and maximum hourly renewal rates (maximum needs)

- The minimum time - for filling and drainage the tanks

- The balance coefficient of the needs for all the tanks.

Finally, the design of hydraulic systems in rearing farms must taken into account, so as to enable the rational management of the water :

- Possibility to increase or decrease the renewal flows to maximum or minimum without causing any perturbation.

- An isolation as complete as possible between the systems of the different rearing units so as to limit the contamination risks and to adapt the water qualities to each rearing phase.

2.2.5. Usual valves

The average value of the renewal rates practised with intensive rearing is as following :

- 1 to 15 renewals per hour

- Drainage of the tank within 15 to 30 minutes

- Filling of the tank in 30 minutes

2.3. Feeding

Feeding is the most important element in the running of a fattening unit.

2.3.1. Generalities

The principal points to be taken into the account for the management of feed in a rearing unit are as following : Calculation of the food rations

The food rations distributed daily must be calculated very precisely, for each category to ensure optimal growth, while applying managerial plans for the rearing. These calculations are carried out while taking into account :

- the size of the fish

- the type of food

- the t° of the water

- the control and follow up the rearing.

The daily food ration varies on general between 1 and 3 % of the total biomass for sea-bass and gilthead sea-bream in fattening.

To optimalize the distribution of the daily ration, 4 or 5 feeds will be programmed during the day, divided out evenly from sunrise to sunset.

The distribution of the feeds is one of the principal jobs of the manual labourers. Therefore it is advisable to employ automatic distributors although they may demand heavy investments. The food formulation balance-Appetency

Special attention must be given to the food formulations and the stocking of this food so as to obtain a perfectly balanced diet.

A food unbalance, even slight, can have heavy consequences, deteriorating the sanitary state of the population, causing risks of epidermic and a transformation coefficient increase of the food.

On parallel, the appetency of the food must be checked, and this principally when dry feed is distributed. Unappetizing food leads to a decrease in consumption, losses and the rapid pollution of the tanks.

2.3.2. Dry and wet food

Presently two principal types of food can be distinguished : Dry feed :

At the present time this is the most renown formula employed for the fattening of sea-bass and gilt-head sea-bream.

The food is regularly distributed in the form of dry pellets which is supplied by manufacturers.

This solution, although the simplest and the more easily mechanised one has two major disadvantages :

- high costing (average transformation coefficient, high cost per kilo of feed).

- Still average adaptation of the food distributed to the needs of the fish. (appetency problems, deficiencies, osmotic exchange).

- It has nevertheless the advantage of lowering the manual labour (distribution) and stocking costs. Wet food

Wet food is still not widely employed and it is badly controlled in the rearing of the sea-beas and gilthead sea-beam.

It is however quite advantageous as it brings a remedy to the disadvantages of the dry food described here above. It is manufactured in wet food pellet form in the farm from minced waste fish combined with “premix” type of meal which is supplied by the food manufacturers.

The development of this food is blocked by :

- problems of technological order in the mechanization of its manifacture and its distribution.

- the definition of formulations depending on the components available and their biochemical composition.

- higher investment costs.


3.1. Introduction

The first rearing unit in CRAVELINES by which this report is concerned, was the first industrial marine aquaculture unit to be set up in France and the first aquaculture centre unit. Different fattening techniques have been developed over these past years on the Mediterranean coasts and many difficulties have been encountered, the principal one being of economic order. The length of time that the rearing cycle demands, which for sea-beas reared in natural conditions is 30 months seriously limits the profitability of the operations. The valorization of the thermic effluents was the principal reason for the implementation of this farm on this site, located in the immediate vicinity of the power plant and the SEPIA International, this was sanctioned following the international meeting organised in 1982 by SERAG(Syndicate for the study of an aquaculture network system in GRAVELINES)to define the aquaculture design of this farm.

The principal aim of this implementation was to demonstrate, on one hand, the fiability of the rearing techniques and on the other hand, the profitability of these marine fish rearings.

3.2. Design of the pilot farm

The passage of the water in the cooling circuits of the power plant raises the temperature of the sea-water by 13° C which during the year evolves between 15° C and 29/30° C.

So as to ensure these temperatures in the rearing environments, even when exiting temperature is much lower, it has been decided to cover the entire farm(3000 m2) with agricultural green houses. This cover prevents evaporation caused by wind, cooling caused by the difference of the temperature at the water/air interface, the decrease in the temperature due to snowfalls or even the decrease in salinity caused by heavy rainfall.

The farm has also been designed to limit exterior agressions (pollution from the inner port in DUNKEROUB, dredgings, chlorination in the cooling circuits of the power plants). A recycling circuit permits the isolation of the farm from the outside for about 8 days in “protected” rearing conditions.

The circuit comprises of :

For the nursery:

- a special discharge circuit

- a decantation unit

- a pump cover and a pumping station

- a purification unit (biofiltration of the waste water)

For the fattening :

This system is simpler as it doesn't need a real depuration unit.

The temperature of the rearing environment can be optimised by mixing a heated water supply (450 1/s) and a sea-water supply at natural temperature. Thus the highest possible temperatures can be maintained in winter (using only heated water) while in summer, the highest temperature in the rearing environment is limited to 25 – 26° C which seems ideal for sea-bass and gilthead sea-beam

The water management complex allows this manipulation. It must be remarked that the possibility of two independent circuits with different temperature exists :

- one for the nursery

- the other for the fattening units

The nursery and fattening tanks have been designed while taking into account the following points :

- Working facilities (flow regulation - observation - feeding the fishdrainage)

- Automatic cleaning.

The outlet is placed at the bottom of the tank near the end and the water level is regulated by an overflow. The water discharged is channeled at a normal flow rate through the evolution pipe or flushed through this channel when the tank is being cleaned out.

3.3. Description of the pilot farm

3.3.1. The water circuits

The farm has at disposal three types of water supply :

- heated water

- “natural” sea water

- recycled water

These different sources of water are mixed proportionally depending on what has been decided at water management level, so as to ensure the best conditions possible to the fish, from a temperature viewpoint as well as a physico-chemical one.

The water supply overflows are evacuated by means of drain sluices.

Heated sea-water

This water is pumped downstream from 3 structures of the EDF power plant at a maximum flow of 400 1/s.

Its temperature varies between 15 and 32° C.

This heated water is used entirely as soon as its temperature reaches 25° C. In Summer it is mixed with cold sea-water.

Cold sea-water

This water is pumped up stream from the EDF power plant into a seawater supply canal which joints up with the inner port of DUNKERQUE. It permits the temperature of the rearing water to be lowered during Summer.

Recycled sea-water

This recycling permits, the recovery of water which has already been used in the rearing, making it compatible once by means of successive treatments, for the rearing of fish and then it is distributed throughout all the tanks. This method not only permits a complete isolation of the structure from the outside environment (power plant stoppage, pollution), but it also ensures a greater overall flow in the tanks when necessary (temporary overflow of the structure).

The quality of the water is verified daily by means of physico-chemical and the steady flow, in the tanks is optimised according to the fish load contained in the latter.

3.3.2. The rearing units

They are two in number: the nursery and fattening.

The nursery

It has FOSTER LUCAS type of tanks of around 35 m3 each in volume. Each tank has a water supply by means of either two circuits (the model employed can be regulated from 5 to 30 l/s).

- the nursery circuit

- the fattening circuit

A low-pressure aerator, type air lift, permits to reinforce the speed of the current and to increase the supply of dissolved oxygen in the rearing environment. An outlet located at the bottom, equipped with an anti-vortex system, permits the elimination of wastes (faces - uneaten food) deposited on the bottom of the tank.

Automatic pressure feeders completes the equipment of the tanks.

The fattening unit

It has two series of 14 adjoining tanks, having a unitary volume of around 65 m3. These tanks are identical, apart from their size to those in the nursery.

All the rearing units get their water supply by means reinforced concrete channels which are placed crosswise in the tanks and are located on the lateral walls of the latter.

A series of platforms situated on top of the walls of the tanks permit the usual work (feeding, observation, flow regulation, treatments, etc…).

3.3.3. Annex equipment

Along with the rearing unit, the farm has the following facilities :

- a wet food manufacture workshop with a cold room.

- a technical room which is divided into two parts :

- a generating plant

- componants (decanters - pumping station - biofilters of the recycling circuits

3.4. The running of the farm - First results

3.4.1. General characteristics

The general characterisitics of this rearing are as following :

- The annual production when in full operation is 65 to 70 t. (around 35 t in 1985).

- The production staff comprises 4 people: 2 technicians and 2 workers, one of which a fisherman.

- The number of fry placed into rearing each year is around 300 000 divided up between sea-bass and gilthead sea-bream.

- The global surface area of the tanks is around 2 200 m2 for a total surface area of 3 200 m2(60 % ratio).

- The warm water flow is 1600 m3/hour, the cool water flow is 800 m3/h.

3.4.3. First results

The management of this form was placed in the charge of the Aquanord Maritime Cooperative which regroups specialists in aquaculture, professional fishermen, and experts on the commercialization of sea-products.

After a first trial carried out in the provisonary facilities in 1983 and whose objective was the appreciation of certain characteristics of the site, the production was started in June of 1984 with the introduction of 200 000 fry :

170 000sea-bass fry
20 000gilthead sea-bream-fry
10 000sole fry(trial)

The growth rate remarked was on the whole comparable to the initial previsions. The survival rate however did not come up to expectations, this is quite normal when we realize that this operation began before the works had been completed.

After a year's work, the first sales took place, which confirmed on one hand the technical hypothesis and on the other hand the heavy commercial demand and the high sale prices.

The quality of the fish was unanimously appreciated by french and foreign experts. since the first trials, demands have increased and the demands exceed our production possibilities (5 to 6 ton per week).

3.5. Future production

This pilot farm is the first element of aquaculture site in GRAVE-LINES. A hatchery will be built in the coming months which will give autonomy to the site and a rapid possibility of expansion.

This “aquaculture zone”created as a traditional development zone will regroup finally :

- around 22 production farms (22“lot”)

- common circuits: • supply - heated water

- cool water

- Annex service units

If this realization is success, France will pass from experimental and small scale production stage which it is at present doing, to large scale production stage of marine fish, GRAVELINES will finally permit more than 1 200 t/year.

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