In a preliminary discussion, general agreement existed on the need of a simple nomenclature for the stages to be discussed in the meeting. Morphological or physiological criteria apart, a zootechnical nomenclature based on the different stages in management was preferred. It was agreed to distinguish between yolk-sac-fry (feeding), fry and fingerlings, and it was stressed to report both size and weight in connexion with these terms.
The opening addresses underlined and illustrated the importance of the mass production of fry and fingerlings for continuing fish-culture development, for to rely on natural fish stocks (with their natural fluctuations) for recruitment results in discontinuous availability of fish seed, loss of pond or hatchery exploitation time and possibly also the decline of the natural stocks.
According to the country reports, in the EIFAC region there are now more than 31 species in culture of 14 different families. For 19 species more or less dependable rearing procedures are available either in farm ponds or in the hatchery. For 12 species no predictive techniques are established yet (Appendix VI). For all the different species the rearing techniques can be further improved.
If it is accepted that a regular, even out-of-season supply of eggs is important, it follows that a regular possibly out-of-season technique for fry rearing is just as important. The requirement is an integrated system of egg production and fry and fingerling rearing which is independent of seasonal constraints and of climatic factors.
In this context it should be realized that we are dealing with the fish in their most fragile, vulnerable stages where they are required to accomplish very high specific growth rates which means that their feeding requirements are critical and rapidly changing.
Pond production of fry and fingerlings is the oldest established and, for many fish species, still the most important way of fish cultivation.
It requires the management of the pond ecosystem to be appropriate for the growth of fish and of their food organisms. This in some respects entails a compromise over conflicting needs of food organisms on the one hand and fish on the other.
Basic factors in pond cultivation as illustrated by the rearing of fry and fingerlings of Cyprinids were reviewed, as well as for other fish species. With the establishment of hypo-physation and artificial incubation techniques, even fish cultivation in ponds nowadays may have an in-door stage during yolk-sac absorption and first feeding.
Depending on the temperature and the availability of suitable food organisms, as for instance, Rotatoria, the fry are transferred to well-prepared, protected, small nursery ponds (a few days) after yolk-sac absorption. The success of the rearing in nursery ponds depends on two major factors:
- availability of suitable food;
- prevention of predation and diseases.
It was illustrated that for intensive stocking of fry in nursery ponds (100–600 fry/m2), high quantities of micro and mesoplankton (Rotatoria) must be present during the first days (3–5 ml/100 1 pond water). The use of supplementary feeding using small particle size (50–200 micrometres) was generally recognized, and the use of soya-bean meal, wheat meal, fish meal, blood meal, etc. was reported with emphasis to optimal feeding techniques and rations administered.
Prevention measurements to control predators as for instance the use of organic-phosphorous-esthers in the control of predacious plancton were thoroughly discussed as well as the prevention and curing of diseases, mainly caused by ectoparasites.
The nursery ponds are harvested after about 1 month and on the average result in 50 percent (30–70 percent) survival.
In contrast to the pond culture situation where management involves reacting to natural circumstances, the organization of hatchery systems can be designed in a more predictive or beit artificial fashion, resulting in more precise control of the production circumstances.
The concentrated rearing in hatcheries embraces the concept that food is not produced in situ but has to be provided. This means that the environment can be designed to be appropriate for the fish alone. In practical terms this means maintaining correct dissolved oxygen levels for the fish kept at high densities. This is provided for by water and/or gas exchange which also provides for the elimination of metabolic wastes. Additional measures can be taken in respect of temperature control or modification, and because of the simplified environmental requirements, prophylactic and therapeutic disease control measures can easily be applied (see Section 2.5).
Production management in the hatchery facilities is based on the oxygen consumption of the fish vis-à-vis the oxygen provision of the water. A large part of the oxygen consumed by the fish is used to convert food into fish flesh. It was elaborated that at optimal feeding levels, taking into consideration species, a fixed amount of oxygen is needed to metabolize 1 kg of feed. This permits optimization of production on the basis of available flow, i.e., the oxygen. More rational design of the rearing unit then is the next logical step.
It was clearly illustrated that intensive indoor rearing, in the case of Esocids at least, resulted in more economical use of the available water. With water becoming more and more scarce as a resource this was an important consideration.
In the pond farm rearing system, the problems pertaining to feeding tend to be quantitative, because natural food organisms provide in the diversity of nutritional requirements. Supplementary feeds therefore can be very useful in a quantitative way but need not be specified very precisely. In the hatchery situation where all the feed is provided, the specific nutritional requirements must be met by that feed.
For Salmonids, almost a century of research has yielded basic information for acceptable feed formulation. Where attempts are now being made to develop cultivation techniques or to move from pond cultivation to high intensity hatchery production for other species, the problems of feed formulation are still critical.
In a first approach, diets used and tested with Salmonids have not worked well with other fish fry such as Cyprinids. A further approach has been to design feeds which resemble in content and in size the natural live food appropriate for the species under consideration. Important features of such feeds to be studied are the biophysical characteristics in relation to feeding behaviour and the biochemical composition in relation to nutritional requirements and nutritional physiology.
The health of fry and fingerlings in rearing can be adversely affected by a number of factors. Even when starting with fry that are genetically and phenotypically (inbreeding, malformations, etc.) sound, the environmental and pathogenic circumstances may be such that health is not maintained. Adverse water quality and poor rearing conditions are of importance because they interfere with the homeostasis and resistance of the fish increasing its susceptability to diseases. Particularly in pond farm rearing water quality can be a problem, when the ponds are managed too much in favour of feed organisms (see Section 2.2). In the hatchery situation the feeds (see Section 2.4) and particularly the feeding management can play a similar role and cause a suboptimal condition of the fish. Mechanical injuries and inproper handling of the tiny fish constitute other environmental factors of importance in this context. The complexity of microbial and parasitic infestations already receives specific attention by EIFAC Cooperative Programme of Research on Aquaculture (COPRAQ) - Fish Diseases, and will not be dwelled on in this report.
To protect the health of the fish, the factors mentioned are best approached at the preventive level. Environmental stress of the fish has to be minimized at all times and the zootechnical conditions should be optimal. The next step is to proceed to eliminate or reduce the number of pathogenic organisms in contact with the fish or to increase the fish resistance to diseases (vaccination, etc.).
On an international level, proposals have already been made and are pursued to examine the health of fish by constant monitoring and to limit the spreading of fish diseases.
