Station de Recherches des Eaux et Forets, Groenendaal, Belgium
The subject of yolk sac fry culture is dealt with in the first part. Subsequently the different methods of culturing pike fingerlings are discussed with emphasis on the production of pike in intensive culture in artificial tanks with live plankton feeding. This type of culture requires constant and intensive care for the artificial environment. As far as the replacement of live feed by artificial feeds is concerned little progress has been made thusfar. The importance of studying the efficiency of pike stocking in relation to the production policy is emphasized.
L'élevage d'alevins de brochet est exposé dans un premier chapitre. Ensuite sont discutées les principales méthodes d'élevage de brochetons et plus particulièrement l'élevage intensif de petits brochetons en bassins artificiels avec nourrissage de plancton vivant. Cet élevage nécessite un contrôle constant du milieu artificiel. La situation actuelle concernant le remplacement de la nourriture vivante par une nourriture artificielle est discutée. Peu de progrès semblent avoir été faits dans ce domaine. Finalement on insiste sur la nécessité d'étudier l'efficacité des repeuplements en brochets afin de déterminer la méthode d'élevage la plus avantageuse.
The ecological function of the pike in natural waters is well known. The deterioration of the natural environment and the often excessive angling pressure have necessitated the culture of pike for stocking purposes. Since cannibalism is already apparent at an early stage, the culture at high densities cannot extend over more than a few weeks. Stocking is sometimes carried out with fry but generally with fingerlings under 6 weeks.
For a more detailed description of the culture of pike the reader may be referred to Huet (1973), who made an extensive compilation on the subject.
Fry culture starts with the transfer of the eggs from the incubation jars to the hatching trays just prior to hatching. This stage is reached about 110 day-degrees after fertilization e.g. 11 days at a water temperature of 10°C. The hatching in the incubation jars is not to be recommended. The eggs can be put on the trays at a density of 12 cm-2. The same trays can be used as the ones used in the culture of salmonids.
After hatching the empty egg shells are siphoned off. The removal of the egg shells is made easier, when the eggs are placed on perforated trays that retain the shells but allow the fry to pass. In yet an other method the eggs are placed on floating screens that consist of a wooden frame with fine mesh material stretched over it. After hatching, the empty shells and the larvae are separated by gentle movements of the screen.
After hatching there follows a hanging stage. In order to increase the substrate, supporting materials are placed in the tanks: synthetic cloth, aquatic weeks, fir branches, unplaned wood, etc. Since the hygiene in the tanks is very important, no easily rotting materials are used for support. The tanks are protected from the direct sun light.
The water temperature can be from 7 to 19°C and the optimal temperature should be about 12°C (Hokanson et al., 1973).
At a water temperature of 12°C hanging continues for about 9 days. The fry then swim to the surface in order to fill their gas bladder and adopt a horizontal swimming position. At this stage the yolk resorption is almost complete and the fry are stocked either in the natural water or in the culture facilities to grow up to fingerlings.
Three culture methods can be discerned: a) extensive culture in ponds - sometimes with submerged aquatic vegetation - or flooded grass land; b) semi-extensive culture in small ponds with plankton feeding; c) intensive culture in artificial tanks or in floating installations with intensive feeding of live zooplankton.
This type of pike culture is based on the natural feed and is carried out in ponds - sometimes with submerged vegetation -, or on grass land that is temporatily flooded. These waters range in surface from 0.5 to 5.0 ha and rarely are over 0.5 m deep. The production per unit of surface area is higher in the smaller bodies of water, which is probably linked to the higher productivity of the area adjacent to the banks.
Because the submerged vegetation is important for the production, normal fish ponds are less productive than inundated grass land with vegetated ponds in an intermediate position.
The stocking density and the production period may vary considerably (2 to 80 m-2; 2 to 7 weeks), primarily depending on the productivity of the water and the required size of the pike fingerling at harvest. Presently, the attention is focussed on the production of 4 to 5 cm fingerlings, which under normal circumstances at a density of 10 to 15 m-2 can be obtained in a production period of about 3 weeks. This relatively small size of the fingerlings is indicated by the fact that pike of this size become cannibalistic especially when forage fish are absent and the water temperature approaches 14°C.
The survival rate varies from 50 to 5% or less, but average survival is about 20%. There is a negative correlation between the survival rate and the size of the pond, the stocking density and the length of the production period. The variability of the mortality is due primarily to climatic variations, which have an influence on the production of the necessary plankton. This is the reason that it is preferred to stock with fry late in the season and that sun exposed and wind protected waters give better results.
A different method of extensive culture is occasionally practised in the USA (Fago, 1977). A pond is stocked with brood fish and when they have spawned, 9 cm fingerlings are obtained after 8 weeks. After spawning it is attempted to catch the brood fish with fyke-nets or by angling.
The advantages in the extensive culture are its simplicity and the possibility to obtain relatively large and good quality fingerlings. There are, however, also disadvantages: a) the fluctuating results forestall an estimate of the harvest; b) draining the ponds is time consuming and cumbersome, especially when they are many, and cannibalism may take a toll if draining is a few days late; c) harvesting the pike fingerlings is difficult especially when they are small and when there are filamentous algae in the pond; d) the production per unit of surface area is low.
If early as well as late fry are available, it is possible to realize 2 consecutive crops of fingerlings, the second usually being somewhat inferior. Ponds that have been used in pike culture can be put to further use in the production of 1 summer carp. It is selfexplenatory that in this case the ponds should be completely drained.
Steffens (1976) has cited an experiment in which pike fingerlings were produced in brackish water with a salt content of 3 to 4‰.
In this type of culture the pike are reared in small ponds with a surface area under 0.2 ha, where plankton is provided daily. This plankton is collected either in big ponds with a plankton net mounted on a boat (Netherlands) or in small ponds that are reserved and specially fertilized for this purpose (France). In the latter case the plankton net is drawn through the pond with the aid of a winch mounted on the pond bank. Another possibility is the automatic provision of plankton as cited by Kriegsman (1970) in Switzerland.
As for the extensive culture the stocking density and the production period in semi-extensive culture may vary (12 to 120 fish m-2; 2.5 to 6 weeks). The survival rate is also variable but centers on about 20%. The production per unit of surface area is increased as compared to the extensive culture, but the same disadvantages pertain albeit to a lesser extent. However, an additional risk of pollution and development of filamentous algae is introduced when excess plankton is administered.
3.3.1 In artificial tanks
Increasingly the culture of pike fingerlings is carried out in artificial tanks with intensive feeding of zooplankton because the production is more economical and less fluctuating than with the preceding methods. Intensive culture was first used by Einsele (1949) in Austria but has been widely adopted since especially in the Netherlands, Germany and Switzerland.
Rectangular or round ranks are used, composed of different materials but mostly fiberglass. This enables easy and rapid cleaning which is very important in this hygiene sensitive culture. Rectangular tanks have the advantage that more rational use can be made of the available surface area than with round ones. Most often the rectangular tanks are 4 m × 1 m × 0.45 m deep.
Sometimes a longitudinal division is installed in order to improve the water flow and the utilization of the distributed food. Round tanks mostly are about 2 m in diameter.
The stocking rates vary from 500 to 30 000 m-2, but it is recommended to not exceed 9 000 m-2. In practice a density of about 3 000 m-2 is usual. The production period may vary from 2.5 to 5 weeks, but since the aim is to produce 3 to 5 cm fingerlings, 3 weeks is most usual. The survival on the average is about 75% but may vary from 50 to 90%. Often it is possible to produce 2 consecutive batches per tank.
The flow of the water allows for 1 renewal every 8 hours, with or without additional aeration at the point of entry into the tanks. The discharge water should still contain 5 mg of oxygen 1-1 at least (Steffens, 1976). The temperature should be between 15 and 20°C. When the temperature of the water can be controlled, large fluctuations of the water temperature can be avoided and it becomes possible to stimulate the feed intake and the growth of the pike fingerlings. When, in addition, the water is recirculated the energy input can be economized. In the framework of the prevention of diseases it is helpful to sterilize the water by means of U.V. or ozonization or alternatively to use non-chlorinated tap water.
The tanks are to be cleaned daily and it is advisable to regularly disinfect the equipment with quaternary ammonium compounds. All in all the labour requirement is about 1 man per 12 tanks.
For different diseases Steffens (1976) recommends the following treatments:
- Trichodina: bath in kitchen salt (0.4% - 10 hours).
- Costia: bath in formalin (200 ml m-3 - 30 minutes).
- Chilodonella: bath in formalin (200 ml m-3 - 30 minutes).
- Ichthyophthirius: bath in malachite green (0.15 g m-3 - 2 hours).
- Myxobacteria: bath in Trypaflavine 3 × ( 3 g m-3 - 10 hours).
A preventive treatment with malachite green is given weekly.
In the Netherlands (O.V.B. Annual Report, 1972–1977) the eggs are disinfected with iodine (WescodyneR) as a preventive treatment against a disease caused by a Rhabdovirus. Gérard (1974) recommends a dose of 50 mg active agent 1-1 of water during 10 minutes.
At first the pike fry are fed once a day with live plankton (especially copepods). This is increased later on to 2 or 3 times per day. The plankton can be collected using different methods as described previously (Kriegsmann, 1952).
According to Lillelund (1958) the daily ration of a 22 mm pike fingerling is 600 copepods. Einsele (1953) makes mention of the ingestion of 50 000 – 60 000 planktonic crustaceans by a pike of 5 cm. It then follows that for a large scale operation a considerable quantity of plankton is required. Since the presence of plankton depends largely on the weather conditions, the time required per day to obtain the required amounts fluctuates widely.
3.3.2 In floating installations
The floating installations comprise cages or fine mesh nets that are placed at the discharge of a water body that is rich in plankton. Alternatively concentrated plankton is released upstream of the installations.
It would be a great improvement if the live zooplankton could be substituted by a non-living feed, preferably dry. As yet this is only in an experimental stage. Smisek (1968) succeeded in feeding the pike fry with a mixture of plankton and ground spleen. Graff (1968) as well as Graff and Sörensen (1970) were able to rear fingerlings up to a size of 7.5 cm with a dry trout feed in aquaria. Similar experiments were successfully carried out by Van Drimmelen (1973, pers. com.). Thusfar these interesting results have not been applied on a practical scale.
At present research is under way in France to develop artificial feeds suitable for the production of pike fingerlings. Based on the first results of this research (C.S.P. - I.N.R.A., 1977), the following conclusions can be drawn.
- Theoretically it is possible to feed the fry up to the stage of fingerling with artificial feeds, but mortality is considerable especially during the dirst 10 days.
- When the fry are fed with live zooplankton during the first 10 days, they can be changed onto frozen plankton and later artificial feeds with little difficulty.
- Up to the age of about a month the young pike only take moving objects in their direct proximity. Later on the fingerlings actively go after the feed when this is administered.
- Four cm pike fingerlings that have been adapted to artificial feed do not show cannibalistic tendencies, provided the density does not exceed 0.5 fish 1-1 of water. However, these fingerlings will resort back to their predatory instincts as soon as the artificial feeding is suspended.
A series of trials (1970–1977) have been set up in the U.S.A. to test feeding formulated feeds to esocids (Graff, 1978; Orme, 1978). The following conclusions can be drawn:
- Most of the attempts to start fry on dry feeds, have resulted in failure.
- The greatest success with dry feeds has been achieved only when the fish are started as fingerlings.
- The tiger muskellunge (sterile hybrid: northern pike × muskellunge) is the most adaptable to a formulated feed. These feeds now are fed to tiger muskellunge on a production scale.
- There has been moderate success with northern pike. This species had a conversion factor of 1,9.
- The autimatic mechanical feeders are generally set to feed small amounts at frequent intervals during daylight hours. There is however a big problem of waste accumulation.
It is thus apparent that in pike culture we are still far removed from a completely artificial feeding as it is practised in the rearing of salmonids. Probably, extensive and involved research will be necessary to advance in this direction. An alternative could be the mass culture of live organisms suitable for feeding pike fingerlings.
The mention of this aspect of pike culture probably is beyond the bounds of this paper, because the mass production of summer pike (10 cm or more) is utopian. The pike of 7 to 8 cm becomes indeed cary cannibalistic even when an abundant quantity of forage fish is present. The production of fish of this size therefore is very costly. Moreover, even if we accept that the stocking value of pike increases with its size, it is questionable if it then is more advantageous to stock 1 bigger pike instead of more small ones. According to a recent study carried out by the O.V.B. in the Netherlands (O.V.B. Reports 1972–1977) stocking of 4 to 5 cm pike should be more economical at least in the waters that were studied. This type of study concerning the efficiency of pike stocking is essential and should be carried out on a wider scale, although it is difficult work especially since the characteristics of the water to be stocked are quite variable. When however, more conclusive knowledge is obtained about the stocking efficiency it will be possible to develop a harmonious stocking and production policy for pike.
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