MEANING And EXPLANATION OF The WORD "WEANING"

The rearing system

Weaning takes place in a dedicated section of the hatchery where the metamorphosed fish (about 45 days old) will grow until a size of 2-3 grams. At this stage they are named fingerlings or juveniles, and must have assumed the adult aspect.
The layout of the weaning sector recalls that of an enlarged larval sector equipped with a number of larger round or rectangular tanks. Their capacity is typically between ten and twenty-five m³ since a larger size become more difficult to handle due to the many operations of grading and thinning out of the fish population. Because fish biomass becomes important at this stage as it can reach up to 20 kg/m³, the weaning stage represents a true intensive rearing system. This is also conceptually far from the larval rearing stage. It can also be considered an intermediate step between the culture of small delicate post-larvae and the much stronger juveniles.
Where environmental conditions are favourable, i.e. in particular for temperature and water quality, a simple flow-through water system is sufficient to feed the unit, provided that the incoming water goes through of coarse filtration (50-100 µm) and passes through UV-light sterilizers. Cold winters or poor environmental conditions require a semi-closed system, in which water is partially recirculated, heated and passed through a biofilter. The latter is a more complex and costly situation, whose management requires additional expertise.
Heating and equipment do not differ significantly from their analogues in the larval rearing section. Light is provided by neon daylight tubes, controlled by a timer. One tube of 58 W for each 10 to 20 m³ tank is considered appropriate. The mesh size in the tank outlet screens should increase to 1, 2 and 3- mm according to fish size. At least one complete set of filters is needed and, to be on the safe side some additional screens should be kept as spare parts. Outlet screens are changed every evening or even more frequently if they are quickly clogged by suspended solids or others debris. They should be flushed with tap water and soaked in hypochlorite solution overnight.

Fig.57.01-02 Modern weaning units with round and rectangular tanks (photo STM Aquatrade)

Water circulation is a key parameter for keeping good rearing conditions: good circulation eliminates dead zones and water stratification, helps in accumulating faeces and debris close to the outlet, provides current for a healthy swimming activity, distributes oxygen evenly in the water and
attracts fish under the automatic feeders. An optimal water circulation is related to the shape of the tank and to its water depth. The position of the water inlets and of the aeration should be adjusted accordingly.
High fish densities can be maintained only if pure oxygen is added, thus contributing to reduce the water exchange rate and therefore pumping costs. However, if oxygen bottles are used, they are comparatively expensive and their management and replacement with new ones is not so efficient, in particular in case of a big hatchery. In cases where large oxygen demand has to be supplied, it may be advisable to use liquid oxygen instead. Its storage in large containers assures long term autonomy. Its consumption can be optimised by a dedicated water circuit, under pressure, supplying supersaturated water to the rearing units with higher efficiency (above 70% solubility) and reduced oxygen waste. Moreover this specific circuit can also be linked to a computer-assisted monitoring and regulating system that, through a feedback regulation, can automatically adjust oxygen levels to the actual rearing conditions and demand, with significant savings and an increase in safety and optimisation of growth performances.

Preparation of the weaning unit

Before any rearing activity can start, the entire system (tanks, water pipes, recirculating pumps and sumps) Large incubation facilities in Nuova Azzurro hatchery should undergo the same preparation process which has been previously described for the larval rearing unit. (photo STM Aquatrade) The same procedure also applies whenever a harvested tank has to be restocked.
If a biofilter is installed, its conditioning process should start at least one month before the first production cycle starts. Annex 25 describes the more important cleaning procedures needed in the weaning unit.

Fry culture

Rearing parameters

After metamorphosis, fish fry are more tolerant to small environmental variations than post-larval fish, but the rearing parameters in the weaning/nursery sector still require close monitoring. The values for the major environmental parameters in the weaning unit are described in Annex 13.
A complete control of water quality should be carried out if possible every day, whereas the main chemical and physical parameters such as dissolved oxygen, pH and temperature should be frequently monitored during the day.

Fig.58.01 Small liquid oxygen reservoir sized for hatchery need (photo STM Aquatrade)

• Temperature: should be kept within an 18-22°C range. Higher values result in better growth, but also increase bacterial growth and fish metabolic rate, which in turn increase oxygen consumption and total ammonia nitrogen production. In addition high temperature ranges also induces negative forms of fish behaviour such as cannibalism. Without adequate solutions to keep key environmental parameters within safe margins, such as, for example, the possibility to increase water renewal and oxygen supply, to raise rearing temperature usually is equivalent to increasing management problems.
• Salinity: better feed conversion and survival rates can be achieved by using slightly brackish water at 20-25 ppt salinity, but because of the large quantity of fresh water required to lower salinity in full strength seawater, this operation becomes economically feasible only when a natural brackish-water supply or a fresh water well with adequate capacity is readily at hand.
• Total Ammonia Nitrogen (TAN): it is expressed as ppm of ammonia nitrogen (the sum of NH₃ plus NH₄⁺ expressed as -N) and should preferably remain below 1 ppm and never exceed 2 plus NH₄ ppm. The toxic fraction of TAN, the Unionised Ammonia Nitrogen (UAN, expressed as ppm of N-NH₃) can be calculated from appropriate tables, knowing the pH and salinity of water sample. Pure oxygen injection can partly convert toxic unionised ammonia N-NH₃ into non toxic nitrites (NO₂^-)and nitrates (NO₃^-). Decreasing water pH values is also effective in reducing the fraction of toxic ammonia.
• Dissolved oxygen (DO): the relevant biomass of the fry in the weaning tanks and their high metabolic rates creates a very high demand for oxygen. DO values should be kept as close to saturation as possible, a safe range being 80-100% saturation measured at the water outlets. Oxygen supply should also be able to cope with the daily fluctuations that are associated with feeding. Modern extruded pellets require higher DO levels in the water (100-120% saturation) than traditional pellets. DO levels should be checked all day round and should be plotted by a computerised monitoring system. In case that this ideal solution could not be implemented, DO levels should be checked every hour for a couple of days in order to be able to plot its evolution in the 24 hours and identify the peak demand times. Then, it would be enough to check DO at least during these peak times and whenever fish behaviour seems to be altered. For practical purposes this means after the morning first feeding, twice in the afternoon and twice in the evening. At night, and in particular at the beginning of the weaning period, a few measurements are advisable, although this is a less critical period because at night fish activity is greatly reduced.
• Photoperiod should be shortened by two hours in relation to the larval sector, i.e. 14 hours light (8 - 22h) and 10 hours dark, to avoid excessive activity in late hours that may increase pollution and cannibalism. At 80-90 days of age the natural photoperiod can be set.
• Light intensity should be lower than in larval rearing tanks and can be set at 1,000 lux. As the twilight effect is considered not so relevant, the elimination of a dimmer switch allows the stallation of common daylight neon tubes. A timer to switch on/off automatically may beuseful, but it is not strictly necessary.
• Water renewal rates should be increased with respect to those of the larval rearing unit, as the fish biomass increases considerably and dry pellets are provided in large amounts. A peak renewal rate of at least one complete tank volume every two hours should be foreseen in the design of the weaning sector. However this rate may vary according to the water system used: the less efficient recirculating semi-open circuits should contemplate a higher renewal rate (one volume per hour) than in the case of the flow-through circuit.
• Bottom aeration during weaning it is no longer intended to keep fish dispersed in the water, but its importance should not be underestimated. A properly placed aeration does not only add oxygen to the water, but contributes to create optimal hydrodynamic conditions. Square or rectangular tanks require a particularly careful placement of the aeration to prevent the formation of dead corners, where uneaten feed and faeces would accumulate and spoil the water quality.
• Screen mesh size should be set at 1 mm at stocking, to be replaced as soon as possible with 2 and 3 mm screens to stand the increased water flow and the associated debris.

Fig.59.01 Oxygen monitoring devices are often installed to check automatically DO level in the weaning sector (photo STM Aquatrade)

Fig.60.01 Hand made outlet screen with 2 mm plastic net (photo STM Aquatrade)

Transferring fish from larval to weaning section

Seabass and gilthead seabream can be moved to the weaning section at about 45 days of age when the rearing water temperature is 18°C. To anticipate the transfer is not advisable because the young postlarval stages are very fragile when their metamorphosis is in progress.
As the transfer operation is clearly very stressful to fish, a few precautions should be adopted:

• prepare all equipment, well cleaned and disinfected, the day before,
• train staff beforehand: everybody should know what to do, when and why,
• if possible, organize the transfer early in the morning,
• choose harvesting and transporting methods which are gentle with the young fish (see below),
• three-four days in advance fish should receive an increased amount of vitamin C (up to 10,000 mg/kg of feed) with their diet, as it has anti-stress properties
• do not feed fish before transfer,
• clean thoroughly the bottom of the larval tank to avoid polluting the transfer medium,
• receiving water quality parameters should match those of the larval rearing tank in terms of salinity and temperature,
• have an emergency oxygen supply at hand in case of some unforeseeable delay,
• try to never touch fish, and never let them jump or stay out of the water,
• never concentrate too many fish in a small container,
• never let dirty water enter the weaning tanks, and avoid that fry containers touch the floor before being immersed in the receiving weaning tank;
• try to keep the period in which fish are without aeration or water renewal to the shortest possible,
• feed fish as soon as possible when placed in the new tank in order to avoid cannibalism,
• immediately after transfer feed plenty of live feeds (artemia metanauplii) to favour a prompt recovery.

Preventive bacteriostatic treatments before and after transfer are sometimes applied to reduce the risk of disease outbreak on fish stressed by the transfer. However, since this practice can induce the appearance of drug resistant bacterial strains, it is recommended to replace this practice in favour of an anti-stress diet and a carefully managed operation.

Fig.61.01 Gilthead seabream fry ready for transfer in larger tanks (photo STM Aquatrade)
In some cases where it would be necessary to minimize the risk of bacterial infections caused by excessive handling, or by weak larvae, a preventive treatment lasting five days is advisable. In this case the larval tanks can be treated with Furazolidone, at a concentration of 30 ppm during two to three hours, a treatment which is repeated for three days before harvesting and two days after. The larvae should not be fed during the treatment. In order to apply the treatment:

1. dissolve the required amount of Furazolidone in a beaker (some drops of formalin will help);
2. increase DO levels in the tank to 130% saturation and then stop water exchange for the time of the treatment;
3. distribute evenly the Furazolidone solution;
4. once the time established for the treatment has expired, flush out the water, renewing it on a flow-through basis during a suitable time to eliminate the product;
5. use a skimmer to retain floating Furazolidone foam and clean the tank walls with a sponge.

Depending on the design of the larval tank, fry can be harvested either by netting them or by draining the tank through the bottom outlet while concentrating fish in a screened container (a procedure similar to the harvest of rotifers and brine shrimps, already described). The key points in the last case are the position and the dimensions of the tank outlet, as it should be sufficiently large (at least 1½”) and should be placed at a minimum distance of 40 cm above the floor.
Fry transfer from in-floor larval tanks:

1. fill an adequate number of weaning tanks with water at the same temperature and salinity as that of the larval tanks; the volume of the weaning tanks will be related to the desired final fry density. Make sure that the weaning tanks have an adequate water inlet, proper lighting and aeration and a 1000-µm water outlet screen;
2. place a soft nylon seine net with 2-mm knotless mesh inside the tank where fry have to be transferred and gently encircle part of the fry stock. Do not trap too many fish at once to avoid excessive overcrowding;
3. close the net and lift its two ends to the water surface creating a sort of bag hanging from the tank rim. Adjust the net to keep fish inside a submersed pouch;
4. dip a plastic bucket in the pouch and gently fill it with water and fry in a way that the fish do not remain exposed to air;
5. pour the bucket directly into the weaning tank or, if it is far away, into a wheeled and aerated plastic container which will be used for the transfer;
6. repeat steps 3 to 6 till completion of the harvest;
7. the very last fish which escaped capture with the seine net, typically the strongest animals, can be collected through the bottom drain or by means of a dip net
8. empty and clean the empty tank before it gets completely dry.

Fig.62.01-02-03-04-05-06-07 Harvesting steps from in floor fibreglass tanks (photo STM Aquatrade)

Larval transfer for tanks placed above the floor (typically FRP round tanks with conical bottom):

1. repeat steps 1 and 2 of the previous protocol;
2. fit in the weaning tanks the outlet screen of 1 mm;
3. dip the harvesting filter inside a large wheeled container placed near the tank to be harvested; inside it place a diffuser connected to an oxygen bottle or to the liquid oxygen distribution line;
4. connect a flexible hose, not collapsible, to the drain placed at the tank bottom and place the opposite end into the harvesting filter; to work properly, the drain must have a PVC ball valve of the same diameter of the hose;
5. siphon the bottom dirt out;
6. stop water inflow in the tank, but keep the aeration on;
7. place a screened siphon into the tank and start draining the water into the filter container; the screen in the siphon will prevent fish from escaping;
8. once the water level inside the tank has reaches the upper level of the conical bottom, open the drain valve and the fry will move with the water into the filter container through the hose;
9. always keep the end of the hose in the filter container under water, and avoid a strong outflowing current, if necessary adjust the differences in water level to reduce the current;
10. as soon as the larval tank is empty, flush the hose with water taken from the water inlet valve of the tank to help the last fish to get out of it;
11. close the valve and disconnect from the bottom, lift it with the attached hose over the filter level and re-open it so as to drop the last fry into the filter. A bucket with some water placed under the larval tank bottom will ensure that no fish will fall on the floor;
12. dip the filter container into the tank and let fish get out. If the filter is too small to contain all the fry of the larval tank, repeat the procedure for a phased harvest;
13. clean the empty filter tank before it gets dry.

Feeding

Feeding procedures in the weaning section differs from those of the larval rearing unit. Main changes are the end of the live feed supply and the setting up of a truly intensive rearing system based on automatic distribution of dry feed. The feeding protocol that follows applies to both species and is based on the following assumptions:

• initial fry density 10-20 fish/litre;
• water temperature of 18°C;
• salinity range 35-37 ppt;
• feed quantities refer to those supplied to one m³ of rearing tank volume.

Refer to Annex 17 and Annex 18 for an indicative feeding regime during weaning, considering that in any case figures refer to a specific diet. Quantities should be adjusted during the first days of weaning according to fish behaviour and feed demand.

Feeding live-food

Artemia is supplied to minimize stress and cannibalism, usually associated with the early weaning stages. At an initial fry density of 10-20 fish/litre 4,000 metanauplii/litre are distributed three times a day at 11, 16 and 21 hours.

Feeding moist food

Even if today there is a tendency to replace moist food with by more advanced dry feeds, it still remains a useful resource to supply additional nutritional integrators and, in some cases, drugs at low cost. It can as well replace dry feed in an emergency. In this case its ration is determined according to fish size: for fry up to 1 g it is 25% of the total biomass (wet weight), between 1 and 10 g it is gradually reduced to 5%. As its distribution is not mandatory but supplementary, it is not normally included in the feeding protocols (Annexes 17 and 18). If distributed as supplement (5% of biomass), the quoted dry feed rations should be reduced by 20%.
Moist feed should be prepared fresh every day and should be totally consumed within the same day. If distributed as supplement of dry feed they should be given in three rations at 15, 17 and 19 hours. This pattern may change according to local conditions, in particular when moist feed is also a vehicle to deliver drugs.
The composition and texture should ensure its stability when placed in water, as well as assure its buoyancy for a better control of feed uptake by fish. For this purpose it can be smeared on a framed mosquito net that is kept submerged just below the water surface. Preparation and distribution have already been discussed in the previous section and can also be found in Annex 24.

Feeding dry feed

Strictly speaking, weaning (in the sense of shifting from live to artificial feed) commences during the larval rearing. The young fish actually receive the first feeding with inert feed at the very early age of 17-19 days, but it is much later, after the transfer into the weaning sector, that dry compounded feed become their only nutritional source. Live feed distribution is discontinued when they reach an age of sixty days.

Fig.63.01 Belt feeders are frequently used for the distribution of small crumble (photo STM Aquatrade)
A strict control of feeding behaviour is necessary to prevent both under and overfeeding. Underfeeding triggers cannibalism and broadens the range of fish sizes. Overfeeding causes health problems and rapidly increases water ammonia levels in the tanks. Feeding is first determined theoretically and is later fine-tuned depending on fish behaviour and growth performance. It is then adjusted to the actual biomass after assessing the average individual weight and the number of fry in the tanks. This is normally done in connection with grading and thinning out operation

Feed distribution

Feeding should be carried out at regular intervals, spreading feed over the whole water surface with a plastic spoon and always checking the actual feed consumption rate, the presence of any feed leftover on the tank bottom, and the general fish behaviour. To reduce pollution and cannibalism, as well as to better adapt fish to dry feed, 60% of the dry feed daily ration should be distributed in the morning at 8, 9, 10 and 12 hours, the remaining 40% in the afternoon and early evening at 14, 16, 18 and 20 hours. To avoid mistakes and to keep records of the feeding rates, each tank should have its own plastic container to hold the daily ration of dry feed. Any feed leftover has to be noted down on the tank file (see Annex 26). The same staff should be responsible for feeding fish in order to acquire experience in detecting fish behaviour and needs.

Fig.63.02 Demand feeders are used with bigger size pellets (photo STM Aquatrade)

Management of the weaning section

The management of the weaning section requires the same procedures indicated for the larval rearing unit, with special attention for new characteristics of older fish such as cannibalism, pollution, greater DO needs and fluctuation, and disease outbreaks.

Staff

The weaning unit should be staffed with a head of the unit and with sufficient skilled workers to cover the whole period of light hours. The personnel should also be recruited according to the increased workload as fry are transferred from the larval unit to this sector. Typically, staff moves gradually from the larval rearing section to the weaning section at the end of three or four larval production cycles. The degree of automation can also marginally affect staff numbers.
As an example, a weaning sector with an overall capacity of four-five million fry to be produced in three cycles is adequately staffed by a team of six workers plus the unit head. The same watchman in charge of the larval unit takes care of the night controls.
Staff of the weaning section should be well trained and should always know what to do, when and why. Working protocols have to be distributed to all workers, and written instructions for the work to be carried out the next day must be prepared by the unit head. Every duty has to be performed properly and has to be recorded in its file. Nothing has to be hurriedly done, and everything has to be carefully planned.

Fig.63.03 Large weaning/pre-growing unit in Ittica Mediterranea (photo STM Aquatrade)

Daily operations

Weaning requires the same carefully planned and implemented working protocol as set for the larval rearing sector. Annex 27 summarises the activities to be performed during working hours. The use of all consumables should be recorded on a specific file and their replacement must be ordered well in advance. Proper hygienic conditions are mandatory (see below), as well as a complete separation among the sections of the hatchery to avoid possible contamination.

Control of environmental and biological parameters

The weaning procedures require a close monitoring of environmental conditions (abiotic parameters) and of the fish population (biological parameters). The first ones, to be checked as a routine daily, have already been described before and their frequency is indicated in Annex 27. The biological parameters are detailed below in terms of their operating procedures and frequency of monitoring.
As weaning fry are much sturdier than post-larval stages, they can be periodically sampled and checked for a closer control on the population.
These observations should focus on:

• fish behaviour,
• growth and food conversion rate (leading to grading),
• deformity rate (selection),
• swim bladder presence
• mortality and final survival rate.

Fish behaviour

As a general rule, any trouble with the environmental conditions in the tanks directly affects fish behaviour before the onset of unequivocal signs of stress such as a large mortality. A routine watching by experienced personnel immediately reveals if something is going wrong. A healthy fish displays the following signs of normal behaviour:

• complete control of the swimming activity
• successful feeding/preying activity (even some cannibalism),
• fast response to sudden stimuli (typically a hand waved over the tank),
• proper colour (silver grey instead of black),
• mass concentration under feeders and artemia buckets,
• all water volume occupied by actively swimming fish,
• absence of mass concentration at the water inlet (which may reveal oxygen deficiencies).

Fig.64.01 Fry sampling nets (photo STM Aquatrade)
Whereas observation of fish behaviour has to take place on a daily basis, other controls require a representative sample of the population. To avoid excessive stress to fish and the bias of inaccurate samples, these controls usually take place when weaning tanks are periodically harvested to grade and thin out their fish population. Moreover, as counting and weighing are done in each size batch, accuracy is higher.
As a rule, these periodical checks of the fish population should take place at 80, 100 and 120 days of age. Till 80 days of age 100 fish represent a suitable sample, whereas in older populations a larger sample of 200 fish is suggested to cover the population variability. Fry must be anaesthetised as described below before checks are conducted, in order to avoid mortality and pathologies.

Fig.64.02 Healthy gilthead sea bream population (photo STM Aquatrade)

Controlling growth and deformity rate

Weaning growing performance should be assessed fortnightly, and possibly in coincidence with fish grading. If that is not possible, weight and length are measured on a limited sample of the population as follows:

1. prepare one 1-l beaker filled with 300 ml of water from the rearing tank. Put it on the balance and tare;
2. harvest some fish by means of a hand-net and place them in the beaker, trying to avoid adding water to the beaker in the operation (the use of a tea strainer may help);
3. for greater accuracy, the amount of water added could be calculated in advance by weighing the difference between a wet and a dry strainer, and this difference can then be subtracted from the final weight;
4. weigh the beaker with the fry and record the weight;
5. return fish to a bucket and count them;
6. repeat the previous steps for every new batch taken with the hand-net from the tank till a sample of 100 fish is obtained;
7. calculate the average individual weight by dividing the sum of weights by the total number of fish;

To measure body length and perform other biological controls proceed as follows:

1. anaesthetise with 2-phenoxyethanol the sampled animals (200 to 400 ppm solution depending on fish size), place them on a clean glass and measure length (in mm) with a small piece of millimetric paper placed under the glass;
2 .for each individual record its total length (TL) and any morphological abnormality (see below for details);
3. by placing a strong light source below the glass, the presence of the swim bladder can be easily detected in seabass, while it is more difficult to visualize for gilthead seabream;
4. anaesthetised animals should be returned as quickly as possible to a bucket with clean aerated seawater, but they should be returned to the rearing tank only when they have totally recovered, to avoid aggression by the other fish.

Fig.64.03 Hand made device for an accurate measurement of small fry (photo STM Aquatrade)
This procedure is fast and does not harm the sampled fish if properly done. However, the hand net does not offer a completely representative sample of the population, as the biggest sizes usually remain close to the bottom. A better solution is sampling the fish obtained by means of a small seine net hauled through half tank.

Fry grading

Even when coming from the same egg batch, post-larval fish and fry of both species do not exhibit a uniform growth pattern. As a result, small fish co-exist with fish twice as big in the same tank. The aggressive behaviour of larger fish and the unnatural crowding condition of the rearing tanks quickly result in cannibalism. In seabass populations it develops in a typical way: at the beginning larger fish gulp smaller specimens from their tail, leaving their heads protruding out of the predator mouth (“double-headed fish”), later on, with the full development of the scales, the gulping sequence is reversed from head to tail, as in the adult fish. Gilthead seabream does not gulp, but bites their unlucky mates first in the eyes then in the belly and in the caudal fin.

Fig.65.01-02 Small and large fry graders (photo STM Aquatrade)

If left uncontrolled, cannibalism may become a major cause of mortality. Reducing its incidence can be achieved by diminishing light levels over the tank, by reducing water transparency with the addition of microalgae and by increasing the food ration both in terms of quantity and frequency. The best solution is, however, to grade frequently fish into homogeneous size groups.
Fry grading is achieved by letting them pass through a series of sorters of different calibre. A common model is represented by a floating PVC tray (length 45 x width 20 x height 15 cm), whose base is equipped with a grid of stainless steel bars whose distance is calibrated. When floating in the tank, the base remains submerged.
The space between bars is set according to the fry thickness at various ages. A complete set of graders, covering the various ages, should then include the following measures: 2 - 2.5 - 3 - 3.5 - 4 - 4.5 - 5 and 6 mm. To accelerate the grading operation, it is advisable to use three boxes per graded size.
An acceptable size variability of the sorted batch should remain within 10%, i.e. the difference in weight between the smallest and the largest fry should not be more than 10%. When the periodical population weight check shows a larger variability, a grading should be performed. To select the adequate sorters, a preliminary test should be made with a sample of about 100 fish and the size dispersion should be determined after grading.
Grading operations require the following equipment:

• three new weaning tanks ready to host fish,
• at least two graders of the two selected sizes,
• three floating cages placed in the tank to be sorted,
• a couple of hand nets,
• plastic buckets.

The small floating cages used to keep for a short while the graded fish during the operation are simply made with a floating square frame (a PVC pipe of 50 mm diameter) to which a net bag of soft mosquito net is glued (70x70 cm, 80 cm deep).
The grading operation proceeds as follows:

• harvest all fish and keep them in a floating cage (or in a submersed bag made with the harvesting seine net as described in the harvesting procedure for fry transfer),
• place the empty floating cages in the tank and place the larger grader stacked on top of the smallest one, with the piled graders inside a floating cage,
• collect the trapped fish with the hand net and pour gently into the piled sorters,
• move gently the graders to help fish moving through the bars,
• place the fish remaining in the upper grader into one cage or directly into the receiving tank,
• repeat for the underlying grader with the intermediate sized animals,
• the smallest fish which have not been retained by both graders concentrate into the cage from which are then moved to their tank,
• repeat the above mentioned procedure till all fish have been graded,
• when handling fish adopt the same precautions adopted for their transfer.

Sorting fry with skeletal deformities

A significantly higher incidence of anatomical abnormalities may be observed in seabass and gilthead seabream produced at industrial hatcheries than in wild caught animals. Deformed specimens should not be utilised due to their slow growth rate, and also because of they are prone to get diseases and have poor marketability. Moreover, they compete for food and space with healthy fish. Therefore they should be detected as early as possible and eliminated.
Skeletal deformities in seabass and gilthead seabream fry typically affect snout, opercula and backbone. The snout may show a deformed mouth in a variety of shapes, with deformities affecting both upper and lower jaw, as well as cranial bones. One or both opercula may not develop completely, leaving part of the gills exposed. The most severe deformities affect the backbone in the form of kyphosis, lordosis or a mix of them. Swimbladder in part of such animals is missing, and its absence is probably at the origin of such deformity, although this is not, however, the only possible cause. In most cases swimbladder is present and the cause of spinal deformity has to be researched in other directions. Such skeletal deformities have been associated to nutritional deficiencies, gas supersaturation in the water, something possible when heating is used, genetic disorders and generally unsuitable rearing conditions (abnormally strong water currents in the rearing tank). Of all deformities, the less damaging one is that of the opercula, for which an incidence rate below 10%, if it is the only deformity present, is usually considered acceptable. The presence of other deformities should not be accepted, and batches affected by several deformities have to be screened and animals that carry the deformities must be eliminated.

Fig.66.01-02-03 Gilthead seabream grading (photo STM Aquatrade)

A visual examination is the most common method to detect abnormal fry. Not only their shape, but also their unusual swimming behaviour may reveal an abnormal development. Other methods include an analysis of morphometry, stereoscopic observations and soft X-rays, the latter usually carried out on fingerlings being sold, as a proof of their quality. A major drawback is that skeletal deformities will fully develop to be visually identified only when fish are over 0.5 g in size.
A search for deformities is routine work on the fry samples collected to assess size. If it is desired to return the animals to the tank the control can be easily carried out on anaesthetised fish. A careful examination should be done on both sides of each fish. All findings must be recorded on a dedicated file for each tank population.
If the percentage of deformities in a given fish population exceeds the quality standards set above, the deformed animals should be sorted out. The only effective technique to remove such fry is to sort them by hand. To reduce the workload, fish should first be checked for the presence of swim-bladder (see below) and then the selection should be carried out only on the fraction with swim-bladder.
In any case, the entire population has to be fished out and anaesthetized. It is then spread over a smooth surface, such as a PVC or stainless steel table maintained constantly wet, where fry are sorted by hand and the deformed fish are discarded. Only small groups of 2-300 fish should be caught and anaesthetized to avoid a long permanence in the tub where they are treated and on the sorting table, which could be damaging.

Fig.67.01-02-03 Fry quality sorting (photo STM Aquatrade)

Swim-bladder control

The importance of a proper swim-bladder development and the precautions to be adopted for its activation in young post-larvae have already been mentioned (see previous section). However, a variable percentage remains usually without a functional swim bladder. The importance of this organ for a normal development and growth (see above) requires the sorting and elimination of those specimens without swim-bladder. As handling represents a considerable stress for fish, they should be sorted only in coincidence with other grading or measuring controls.
Marine fish fry without a functional swim-bladder can be easily separated from normal fish. The method to separate them is based in the difference in buoyancy in hypersaline water. Anaesthetized animals with functional swim-bladder will float while the others will sink to the bottom. The procedure requires one fry sieve, three 50 l plastic tubs, buckets and the usual harvesting tools such as a fry seine net and hand nets. The sieve can easily made with a 40 cm long piece of a 20 cm PVC pipe with mosquito nylon net glued on one bottom.
Proceed as follows:

1. fill one tub with tank water and dissolve enough sea salt to obtain a salinity of 50 ppt; place in it the fry sieve (with an aeration line inside);
2. water supply in the other two tubs should flow through them. To avoid changes in environmental parameters water should come from the weaning circuit;
3. add 10 to 20 ml of 2-phenoxyethanol to the first tub (200 to 400 ppm solution depending from fish size) and mix well;
4. harvest the fish population as usual and keep it inside the net at the water surface;
5. with the hand net take 100 to 200 fish at time and place them into the screened cylinder;
6. wait a couple of minutes until they separate completely and the entire water surface is covered with floating fry;
7. collect carefully all floating fry with a hand net and stock them in the other two large tubs, ensuring that water is renewed, until complete recovery from the anaesthesia;
8. take the sieve out and discard the fish without swim bladder that have sunk to the bottom of the sieve;
9. repeat this procedure until the whole population of the tank has been checked. The use of several sieves speeds up sorting, provided that enough staff is available to take care.

Remarks:

• before starting the procedure, test anaesthetic at different concentrations with a dozen fish to determine the optimal concentration under your particular working conditions (as the effect will vary according to water quality and fish size);
• check DO frequently and in case supply pure oxygen to maintain DO levels above 80% saturation;
• check the effectiveness of the separation by examining a few fry of each haul by transparency to verify the presence of a normal swim-bladder;
• use the recovery tubs alternatively and never mix recovered fish with anaesthetised ones as the first will attack those that have not yet recovered;
• when treating fry older than 70 days, increase water salinity up to 60 ppt in the tub where the anaesthetic is being used to speed up separation;

Cleaning

The same cleaning procedures described for the larval rearing section apply here. Due to the much larger organic load and higher water temperature, the sanitary conditions of the rearing environment require a close monitoring and thorough cleaning. Every day, when siphoning the entire tank bottom, the presence and the number of dead fish has to be recorded on the tank file. A complete description of the cleaning routines in the weaning sector is given in Annex 25.

Fig.68.01 Well cleaned biofiltration unit (photo STM Aquatrade)

Hygiene and sanitary conditions in the rearing environment

When dealing with rearing fishes at high density, the danger of a disease outbreak is always present. This danger is greater when fish of various age groups are reared close to each other, thus increasing the danger of spreading diseases. Hatchery staff should be made aware of such danger and be fully trained, both mentally and technically, to maintain rigorous hygiene criteria as a matter of routine. The following general rules can be adopted to minimise contamination in a modern industrial hatchery:

• identify clearly four major sectors for: live feed, larval rearing, fry weaning and broodstock;
• do not exchange or mix water, equipment and instruments among these sectors and apply this rule strictly;
• establish a routine for daily and weekly cleaning procedures to ensure that all equipment and material are kept under the best hygienic conditions;
• adopt disinfecting procedures and identify a disinfectant sufficiently efficient for the cleaning routines;
• select equipment and instruments and establish working plans which can coexist with the rule of maximum hygiene in the simplest way.

If in spite of these preventive measures, disease symptoms are noticed in one of the hatchery sectors, then it becomes urgent:

• to be able to make an early diagnosis identifying symptoms and their origin;
• to initiate rapidly the appropriate therapeutic treatment to cure the fish showing the symptoms.

An efficient veterinary service and a regular sanitary survey are therefore essential to ensure the success of intensive hatchery operations.
A number of chemical treatments are now commonly applied and have become standards as routine practices during the rearing period:

• treatments of broodstock against bacteriosis and parasites;
• disinfection of fertilised eggs;
• prevention of stress-related diseases after handling.

The last one is particularly relevant in the case of transfer and grading of fry. Since these are operations periodically involving all individuals reared in the hatchery, it is of extreme importance to standardize them into routine practices which prevent the occurrence of stress-induced bacteriosis. A record after each treatement (Annex 28) must be kept. The suggested routine practice is to treat all groups of fish being transferred from larval to weaning tanks and from weaning to pre-growing tanks, as well as all fish being graded.
The best chemicals for treatment appear to be those belonging to the family of the furanics, such as nitrofurazone, furazolidone and furaltadone. The treatment consists in a 6-h bath at 30 to 50 ppm, repeated for three days before and two days after fish handling.
In case of evidence of lesions caused by the handling operations the antibiotics of the quinolones family are quite effective, such as flumequine and oxolinic acid. The treatment consists either in a 1 hour bath at 2 to 50 ppm repeated for two days or in administration through treated feed at doses of 12 to 50 mg/kg fish for seven days. Since these antibiotics are actually the most efficient therapeutics for some of the most dangerous and widely distributed bacterial infections (vibriosis and pasteurellosis), they should never be used for prophylaxys, but only for a therapeutical action against a detected pathogen infection.
In Annex 29, a number of chemicals are listed with their most common dosages. A more complete list of diseases affecting cultured seabass and gilthead seabream can be found in Annex 30.
Note: All chemicals for prophylaxys and therapy should only be handled by officially appointed personnel following the advice of a veterinarian; the choice of chemical and its dosage for treatment should strictly abide to current national regulations.