N. Fijan
Department for Biology and Pathology of Fish and Bees,
University of Zagreb, Zagreb, Yugoslavia
ABSTRACT
The health of young fish in rearing can be adversely affected by genotypic and physiological properties of parents, water quality and rearing conditions, feed and feeding, by mechanical injuries and especially by microbial and parasitic diseases.
The control of factors in these groups has to be primarily oriented towards prevention. The principles of prophylactic measures for microbial and parasitic diseases directed towards elimination or reduction of number of pathogenic organisms, increasing the fish resistance to diseases, adjustment of the environment, prophylactic medication, upgrading of the sanitary level of establishments and legal regulations are discussed and the importance of health monitoring is stressed.
It is argued that the efficient health protection of reared fry and fingerlings can be achieved in rearing establishments of high sanitary and hygienic level, where properly selected prophylactic measures permeate all steps and phases of fish cultural operations. The gaps in knowledge and economic constraints are factors limiting the success of health defense. Cooperation among specialists of various profiles, health monitoring and inspection, legal regulations and research on fish diseases should back up and secure further progress in health protection.
RESUME
La situation sanitaire en alevinage peut poser des problèmes à cause de: caractères génotipiques et physiologiques des parents, qualité de l'eau, conditions d'élevage, alimentation, blessures méchaniques et plus particulièrement maladies bactériennes et parasitaires. C'est tout d'abord la prévention qui permet d'apporter une solution aux différents problèmes.
Un rapport est fait des principes de la prophylaxie des organismes pathogènes: élimination, réduction de leurs nombres, augmentation de la résistance des poissons, modification et contrôle de l'environnement. L'importance d'une réglementation internationale est soulignée.
The knowledge on health protection of reared fry and fingerlings is in a state of constant and fast progress. However, the level that has been achieved in practice is in many cases not quite satisfactory: the mortality rates and the health of the young fish are still some of the main constraints for the desired increase of fish production by aquaculture. This is particularly the case in pond rearing of warmwater fishes and in developing systems for mass production of species that have earlier been reared only to a limited extent or not at all.
In this review we have summarized the factors that can affect the health of fish, the approaches that should be considered for the control of undesirable effects of these factors and the strategy of their incorporation into rearing technology for fry and fingerlings.
The health can be defined as the sound status of all bodily organs. This definition subsumes a normal growth rate of the young fish and ability to maintain health under slight to moderate loading of adaptation (pressure on) and defence mechanisms. In mass rearing operations, the loading of organisms is often multifactorial and increases proportionally with the degree of intensification. The factors that load the organism can have an additive or even a cumulative effect. The removal of any of such factors or release of its pressure therefore ameliorates the chances for avoidance of diseases.
High level of intensification, especially the industrialized mass production of fry and fingerlings alters the environment and the fish, thus changing the host - parasite relations as well. Such changes result in occurrence of microbial and parasitic diseases that do not exist or that are not as harmful to natural populations and populations in less intensive rearing systems. The lowered survival rates, impaired growth and often an inadequate sanitary quality for stocking and further rearing caused by these diseases has placed them into the focus of interest.
There is a general consensus that genetic factors may increase the incidence of congenital anomalies in fry and the predisposition to some infections diseases. In some inbred lines of carp the carp pox appeared in high percentage of offspring while the crossbreds in the same environment did not have the disease (Hines et al., 1974). Similar genetic influence was also reported for the occurrence of swimm bladder inflammation of carp (Hines et al., 1974; Kanaev, 1976).
Selection for increased heritable tolerance to diseases has so far not been explored sufficiently. Some examples of success in selection for tolerance to specific diseases (Ehlinger, 1977; Gjedrem and Aulenstad, 1974; McIntyre and Amend, 1978) have not found a wide application yet.
The offspring of broodstock with an optimal age for reproduction is more vigourous and has higher survival rates in early life stages than the offspring from too young or too old broodfish. Other physiological characteristics of parents that positively influence the health of progeny are adequate nutrition and environment for the broodfish and stripping of the sexual products at the moment when the spawners are in optimal physiological stage for coupling. If the hormonal treatment of spawners represents a part of controlled reproduction, the dosage of gonadotrophic hormones and the water temperature at which the fish are kept must be mutually adjusted to avoid the occurrence of amonalies and the poor survival of fertilized eggs and larvae.
These two large groups of factors constitute the environment of the reared fish. The parameters of this category influence the maintenance of homeostasis which is essential for good health. If the value of one or of several parameters exceed the physiologically acceptable limits for the species reared this can reduce the growth, decrease the food utilization, cause stress, predispose fish to infectious or other diseases or even directly cause diseases or fish kills.
The water quality in a rearing facility fluctuates with a narrow or a wider range, as the result of dynamic interactions between several variables. These fluctuations can have a predictable rhythm (such as the decrease of oxygen level after each feeding in an intensively managed trout raceway), a seasonal one, or they can occur at irregular and hardly predictable intervals. They are especially troublesome in ponds.
Securing a good and constant water quality by planning and management is the fundamental zootechnical and zoohygienic prerequisite for success in mass rearing of fish. The status of knowledge, tradition and economic considerations determine whether this prerequisite can be met or not.
The latest quantitative data on fingerling losses caused by poor water quality and rearing conditions were presented by Goltz and Wierowski (1978) for the German Denocratic Republic. During summer periods of 1976, 1977 and 1978 these factors caused 57 to 69 per cent of the total losses in carp, while the corresponding losses in rainbow trout varied between 56 and 85 per cent. It would be unjustified to make wide generalizations from these data, since there are a great number of specificities in aquaculture in the different countries, including variable diagnostic criteria. Nevertheless, it is safe to assume that fish diseases resulting from these factors cause high economic damage to the world aquaculture.
The pH- dependent ammonia intoxication is recently a much discussed topic in warmwater fish farming. This subject was reviewed by Schreckenbach and Spangenberg (1978). The pH in ponds and open waters used for fish rearing often reaches values above 8.5 or 9 due to eutrophication, high stocking densities and intensive feeding with protein-rich diets. Short or prolonged increases of pH or of NH3 and pH cause decreased food utilization, depression of growth, stress, lowered resistance against infectious and parasitic diseases and even macroscopically visible non-specific alterations of organs in ammoniothelic fishes. The autors base the diagnosis of ammonia autointoxication and intoxication on the results of water examination, symptoms in fish and NH3 determination in blood serum. For prevention of intoxications in ponds they suggest the cessation of N- and P- fertilization, systematic calcification with CaCO3, drying of ponds, aeration of water, stocking of phytophagous fishes and some other methods that either are less effective, have a short term effect or need to be improved. It is possible that some of the cases of suspected ammonia intoxication belong actually to the gill necrosis syndrome in carp, that may have a viral etiology (Popkova and Shchelkunov, 1978; Golovnev and Ekelchik, 1978).
The fish health can be impaired by deficiency, excess or imbalance of nutrients, by presence of toxic non-nutrient materials and by spoiled food. Having a high metabolic rate, fry and fingerlings are quite sensitive to malnutrition and food intoxication.
Nutritional diseases and nutritional pathology have been reviewed by Snieszko (1972), Ashley (1972) and by Cowey and Roberts (1978). The symposium on Finfish Nutrition and Feed Technology organized by EIFAC (Hamburg, 1978) provided nutritionists with tools for application of present knowledge and for further research.
Single deficiencies or simple imbalances are rare in the rearing practice. This etiological group of diseases manifests itself mostly by inappetence, growth retardation, poor food utilization or lowered resistance to other diseases. These diseases are best prevented by use of food possessing a standard quality and formulation, that is laboratory controlled, and by adhering to feeding charts, if they exist.
Improper handling, attacks by predators and unsuitable rearing facilities lead to external and sometimes also to internal traumatic injuries. Even light injuries may have severe consequences especially if they become secondarily infected by bacteria or fungi. These injuries can also result in body deformations.
The use of nets with improper mesh size, sharp parts of equipment for sorting, carrying, holding etc, overloading of vessels for carrying of fish out of water and tumbling of fish from too great distances onto hard surfaces are the main causes of injuries during handling. Plant eating fish and some other species are restless in transport vessels at high water temperatures and can abrade their skin by hitting the cover and sides when jumping. Restless behaviour in net cages and enclosures may also result in skin abrasions. The improper overwintering conditions for carp fingerlings lead sometimes to development of decubital lesions in pectoral and pelvic region. Careful and professional fish culturists know how to prevent such damages.
Attacks by predatory insects, fish or birds often result in bites, scrapes or stabs. In ponds, a high percentage of fry and fingerlings can be damaged by such lesions. Injuries and predation by insects can be prevented by chemical treatments of ponds. The prevention of bird damage is difficult, especially when it concerns large ponds that have to be fished out and have an inadequate configuration of bottom.
Microbial and parasitic diseases are the focus of interest of fish pathologists for many reasons. The main ones are the extent of damage caused by these diseases to fisheries and especially to fish culture, their great number and the difficulties in their control. Fry and fingerlings are generally very susceptible to diseases of this group; some of them even occur only in early periods of fish life.
The rate of progress in knowledge on these diseases is fascinating, but there are still many problems which can not be solved in practice. Many disease entities yet unknown to ichthyopathologists will have to be defined and studied in order to enable fish culture to progress at the desired rate.
The appearance of individual diseases of this groups as well as their symptoms and extent of damage can be explained by study of interactions between the pathogenic organism, the fish and the environment. Snieszko (1978) tried to condense and geleralize these relationships in the form of a simple “algebraic equation: H+P+S2 = D, where H means species and strain of the host, its age and inherited susceptibility to any particular disease; P = the agent causing disease with all its variability, S = stress of the environment and D = disease which results if the components of the left side of equation are in proper qualitative and quantitative relationship”. The actual relationships and interactions leading to a disease are often much more complex. The above equation underestimates for instance the factor of environment: it influences the fish, the pathogen and the outcome of the host - pathogen relationship in a very complex way and not just by induction of stress in fish. the variability in survival of pathogens in waters of different quality and the variability of immune response in fish are just two examples of environmental influences on occurrence of diseases. Furthermore, it is well known that communicable fish diseases occur not only as single infections or infestations. Simultaneous presence of several pathogens (mixed, secondary or super-infections and/or invasions) in the reared population are much more common than reported in literature. They are especially frequent in pond fish culture. In such situations the pathogens usually have a combined or cumulative harmful effect, but one should expect that suppressive interaction among some pathogens exists as well. More studies and development of models about interactions between pathogens, hosts and environments are therefore needed to elucidate those relations in order to develop new measures for control of the diseases which is principally described in the next chapter.
Measures for prevention of diseases can be oriented towards one, a combination of two, or to all three components (pathogenic organism, fish, environment) that trigger their outbreaks. The selection of strategy depends upon the biological properties of the causal agents in question, the characteristics of the fish cultural establishment and economic considerations. The best prevention strategy for most diseases is the elimination of the pathogen.
4.1.1 Measures for elimination of the pathogenic organisms
The principal goal of these measures is to prevent the presence of as many pathogens (ideally all of them) as possible in the fish culture establishment or at least in rearing units for fry and fingerlings. To achieve this goal the following measures are used: a) inactivation (or killing) of pathogens or their carriers, vectors and intermediate hosts in rearing facilities and in the whole establishment by disinfection and other measures; b) use of water free of pathogens; c) stocking of eggs, larvae or fry free of pathogens; and d) prevention of introduction of fish pathogens into establishment or rearing unit by carriers, vectors or intermediate hosts (fish, birds, snails, etc.), tools, equipment, vehicles, personnel and food.
Ad a) Disinfection of establishment. Cleaning and disinfection of all facilities for larvae, fry and fingerlings except earthen ponds, is nowadays not a problem. The latter can be disinfected by drying, sunlight, freezing-thawing during winter season and by liming with 1500–2000 kg/ha CaO (CaCo3 is not effective) or by calcium hypochlorite (500–1000 kg/ha).
Sometimes the damage caused by some pathogens is so persistent and has such economic impact that the erradication or stamping out of the fish represents the most rational approach. The stamping out is however not always applicable. The prerequisites for the success of this sanitary measure are: a) perfect knowledge on the pathogen and on epizootiology of the disease to be erradicated, b) availability of eggs, fry and fingerlings free of the pathogen, c) possibility to prevent the entrance of the pathogen into the rearing environment. The examples of successful control by erradication are infectious pancreatic necrosis, viral hemorrhagic septicemia (Vestergaard-Jørgensen, 1977) and mixosomiasis (Ghittino and Vigliani, 1978).
Ad b) Hygienic water supply. Limited availability of spring, well or bore - hole water free of fish pathogens led to development of systems for removal of pathogens from fish-inhabited surface waters by filtration and sterilization by ultraviolet light or ozone. The above types of water supply are now extensively used for rearing of salmonid fry and fingerlings and to a limited extent for other species.
Fry and fingerlings of warmwater fishes are almost exclusively produced in stagnant water facilities fed by surface waters. This is one of the main reasons for relatively low survival rates in general and for health problems due to a wide variety of diseases.
Many pathogens that enter the rearing units by water can be inactivated by the biological measures or chemical treatments. An interval between the end of filling the unit with water and the stocking of about 5–7 days eliminates most of the obligatory pathogens from the environment, if other precautions (a, c, d) are respected. The chlorine treatment (1 ppm of available chlorine) or liming of water for refilling the ponds is not practised, but should be efficient for smaller to medium rearing units.
Ad c) Sanitary quality of stocking material. The transmission of pathogens from broodfish to offspring is nowadays preventable in salmonids and a few other species by checking broodfish for pathogens and elimination of carriers, controlled reproduction, disinfection of eggs or their treatment with antibiotics and separate rearing. In cyprinids, the controlled reproduction is a relatively new development which is now widely accepted and increasingly used. Checking of cyprinid broodfish for pathogens is not gained enough foothold in praxis yet. The disinfection of eggs is not practised per se, but the Woynarowich's method of egg stickyness removal by tannic acid treatment has some disinfecting effect. Further progress in this area is urgently needed. Use of eggs, larvae, and fry from disease-free environment should become a must in rearing of fry and fingerlings.
Ad d) Prevention of entrance of fish pathogens by carriers, vectors, or intermediate hosts, tools, equipment, vehicles and food. This group of measures is increasingly applied. Protection of rearing environment in indoor operations is of course ideal. The wires against gulls, disinfection barriers and disinfection of fishery equipment, tanks and lorries are relatively easily conducted. Feeding of raw fish and fish organs to fry and fingerlings is not seen on most farms any more. In large units for pond rearing of fry and fingerlings the carriers, vectors and intermediate hosts are still an important source of infections and invasions. An unsolved problem is the entrance of the so-called wild fish into ponds during filling and addition of water.
The ideal sanitary situation can be achieved only if all of the above measures are carefully planned and conducted. This does not mean that the realization of even a single one of the above measures will not improve the health status of fish during rearing.
When the presence of some pathogens remains unavoidable, it may help to pursue a reduction of their amount, to prevent an increase of their virulence and to decrease the chance for their transmission. In addition to these listed measures the number of pathogens and their transmission rate can be reduced by chemical treatments of fish and water (see chapter 3.4). The chance for increase of virulence can theoretically be reduced by removal of week, sick, moribund and dead fish. Transmission of some pathogens can be reduced by limiting the close contact of fish out of the water and in nets, tanks, etc.
4.1.2 Increasing resistance of fish to diseases
The resistance of fish to some diseases can be improved by keeping the fish in optimal physiological condition. Other diseases can be prevented by increasing the specific resistance by genetic selection or by vaccination. There are also diseases which can not be prevented by measures of this category. Such an example is the diplostomatosis in fry: when cercariae of Diplostomum sp. meet the tiny fish, they will enter it and kill it by migration through the body regard less of its physiological status. Only at a certain age the fish can better resist the invasion by this parasite.
General resistance to some diseases can be impaired by various stressors - exogenous or endogenous stimulations that are loading the organism and causing the non-specific changes in physiology. Some stress inducing factors, such as netting, sorting and transport are unavoidable parts of production management. The stressors such as inappropriate water quality and traumatization can and have to be avoided by proper rearing methods and technology. One of the oldest remedies in fish medicine - the dip or bath treatment with sodium chloride reduces the response of fish to stressors (Wedemeyer, 1972) and should be used in handling and transport as much as possible.
The vaccination of fingerlings is now starting to become a practically applicable tool in fish culture. Two vaccines (against vibriosis and redmouth disease) are being produced commercially and the research on various aspects of vaccination has provided a considerable flow of information that may eventually lead to development of several other applicable products.
The status of knowledge on immunization of salmonids was reviewed by Evelyn (1977). He also discussed the four vaccine administration methods. The oral vaccination procedure gave mostly disappointing results. The mass inoculation method (intraperitoneal injection of vaccine) worked well, but the number of fish that can be treated was a serious constraint to wider use. Two new methods - infiltration and spray vaccination - which have been developed recently are well suited for mass application of vaccines.
The work on immunization of carp has been making much slower progress. In our experiments (Fijan et al., 1977; and unpublished data) on vaccination against spring viremia of carp, solid protective immunity was obtained by intraperitoneal application of live virus to fingerlings when the water temperatures was aboven 18°C. Somewhat less favourable results were obtained by oral application and by keeping the fish in water with virus for one hour. The infiltration method-immersing fish briefly into hyperosmotic solution and than into antigen solution - has not resulted in a measurable protective immunity in carp. Further work is needed in this area to develop practically applicable and safe vaccination procedures.
4.1.3 Manipulation of environment
Progress in health protection can result also from adjustments of production technology which create a better controllable environment or one faciliting the preventive measures. An interesting approach along this line is the development of a new system for wintering of carp fingerlings described by Kanaev (1976).
In classic carp farming, the fingerlings are kept during winter either in large fingerling ponds with stagnant water or in small wintering ponds to which they are transferred in late fall. In both systems, the losses during winter and in early spring can be high. In Soviet Union, the latter system is practised. Kanaev (1976) reported that wintering in large indoor facilities with tanks (basins), water flow of partially or fully controllable quality and temperature, aeration and possibilities for preventive medication resulted in higher survival rates which economically justified the investments. It seems that such installations may solve some health problems also during carp wintering under conditions of a less prolonged winter than the one in geographic area of Kanaev's work.
4.1.4 Legal regulations and sanitary levels of establishments
Communicable fish diseases constitute such a threat to aquaculture that many countries regulate their control by laws and regulations (Zenny, 1969). To improve the possibility to control the spread of the most important diseases, the Food and Agriculture Organization and the International Office of Epizootics (O.I.E.) have proposed the sanitary norms for certification of live fish, eggs and semen in international trade (Anon. 1977; O.I.E. 1977). These norms aim at the control of the most important virus diseases - viral haemorrhagic septicaemia (rhabdoviral infections) of salmonids, infectious pancreatic necrosis and spring viremia of carp. The recommendations of O.I.E. include myxosomiasis as well.
The above proposals and recommendations of FAO and C.I.E. introduced the concept of sanitary levels of piscicultural establishments and of their products (Ghittino and de Kinkelin, 1975; Anon, 1977; O.I.E., 1977). This concept takes into account the presence or absence of above mentioned fish pathogens in the piscicultural establishment, the microgeographic aspects of the sanitary situation and the policy of fish introductions into the establishment and watershed. It requires a regular, official fish health control accompanied by appropriate laboratory examinations of fish samples. The FAO/O.I.E. proposal (Anon, 1977) classifies the fish culture establishments into four categories: coded pathogen free (CPF), coded disease free (CDF), specified diseases free (SDF) and establishments with minimum disease (MD). The norms recommended by O.I.E. (O.I.E., 1977) have foreseen CPF and SDF sanitary levels while all other pisciculture establishments fall into the category that does not comply with the norms for the above two levels (actually MD category). We are quoting below the text from the Zoosanitary code of O.I.E.
“The definitions specified hereafter apply to establishments as well as to fish originating from these establishments following a period of surveillance of at least two years. They take into account the conditions of water supply of the fish farms.
Three sanotary levels are provided for:
Establishments recognised as being free from pathogens of the fish diseases included in the Code (CPF).
This definition applies to the pisciculture establishments subjected to official sanitary control since at least two years, in which none of the pathogens of the fish diseases included in the Code could be demonstrated during the controls and the water supply of which makes it impossible a contamination through wild fish or restocking fish carrying these pathogens occurs.
Establishments free from specified fish diseases included in the Code (SDF).
This definition applies to the establishments subjected to control since at least two years, in which the causal agent of a disease or agents of several diseases included in the Code have not been demonstrated during the controls, irrespective of the water supply system of these establishments.
Establishments which do not comply with the above norms but are subjected to control, and of which animals do not show clinical signs of the fish diseases included in the Code.
The Code recommends that such fish without clinical signs should be reserved for human consumption”.
The recommendations (Anon., 1977; O.I.E., 1977) contain the instructions for sanitary inspections of establishments, laboratory examinations of fish, methods of disinfection and certification procedures. With adequate modifications these recommendations are applicable for prevention of some other diseases. Work performed by the FAO and C.I.E. can be a useful guide for improving national legislation on fish diseases.
Most facilities for rearing of salmonid fry and fingerlings can comply relatively easy with norms for the CDF or SDF level. The situation with existing types of pond establishments for carp is less satisfactory. The small ones, having a controllable water supply can comply with norms for CDF level, but they are rare. Large farms can at present reach only the SDF level. Fish culturists and ichthyopathologists will have to work together to establish procedures and methods to improve the generally unfavourable sanitary situation on large pond farms.
Most bacterial and parasitic diseases can nowadays be prevented and treated by application of drugs to fish or to the water. For some of them and for viral diseases there is still no medication.
Treatments can have a very beneficial effect if used prophylactically, i.e. at the anticipation of a possible disease outbreak, or at the onset of a disease in a few fish. Application of medication to a population of predominately sick fish, i.e. therapy in proper sense, is much less effective.
The common methods of drug administration to fish were reviewed by Ponpard (1978). Snieszko (1978) listed most chemicals now in use in fish medicine. The treatments for individual diseases are given in textbooks and manuals.
Some diseases with possibly serious consequences for fry and fingerlings occur at a predictable time or at regular intervals. In many establishments they can not be avoided except by prophylactic treatments. Preventive treatments, adjusted to a particular rearing period, system or establishment should therefore be a regular part of technology planning.
The unrestricted and indiscriminate use of chemicals that are seemingly useful for fish culture can cause hazards for the environment, fish and human health. It is quite natural that national regulations provide guidelines for avoidance of drug misuse.
Health protection requires a system of health monitoring. Such a system should consist of the following four components: a) daily observation of fish in each rearing unit, b) sampling and examination of fish at regular intervals, c) sampling and examination of fish at onset of health distress or disease and d) sanitary inspection. In addition to this, regular monitoring of water quality should provide information on influence of environment on fish and enable timely interventions. Such a system enables early detection of pathogens, diseases and unfavourable water quality.
In order to be rational and efficient, the components b) and d) of monitoring have to be adjusted to specific health problems of various species and rearing technologies. The sanitary inspection is carried out by authorized official inspectors, as part of a national and international sanitary system for prevention and spread of diseases. A model of sanitary inspection for monitoring of some diseases has been suggested in annexes of the proposed convention on fish diseases (Anon., 1977) and by Zoosanitary Code (O.I.E., 1977).
The health monitoring can not be carried out without specialists for fish diseases, supported by laboratory facilities for diagnostic work and research. Standardization of fish sampling methods and methods for detection and identification of pathogens allows high dependability, efficiency and speed of laboratory diagnosis.
From such a system, the fish culturist should expect a quick diagnosis accompanied by recommendations for prophylactic and therapeutic measures, as well as advices about possible improvements in rearing technology.
The health protection in mass rearing of fry and fingerlings should secure high survival rates, good growth as well as good health status and sanitary quality of the final product. The basic prerequisite for materialization of these goals is a rearing environment at a high sanitary and hygienic level.
The fish health has to be defended by a complex of preventive measures against all groups of factors that can adversely affect the sanitary outcome of rearing. The measures have to be selected and adjusted to specific rearing technologies and incorporated into every step and procedure of fish cultural operations. Gaps in knowledge and economic constraints limiting the application of known facts are still often leading to partially or completely unsatisfactory health status of reared young fish. The treatments of sick fish populations are the last resort for reduction of losses.
The planning and conducting of the above prophylactic measures is a complex task requiring up to date knowledge on all groups of fish diseases and efficient cooperation between various specialistic prophyles of fish culture technologists and ichthyopathologists.
The system of fish health protection must be backed up by health monitoring and inspection, by legal regulations and by research on fish diseases.
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