by
O. N. Bauer
State Scientific Research Institute of Lake and River Fisheries
Leningrad, U.S.S.R.
Selection of fish for resistance to diseases may prove, and in some cases has already proved, to be an important means of control of fish diseases. Selection work often necessitates transportation of fish from one area to another and this often leads to introduction of diseases and parasites into pond establishments and commercial waters where previously they were absent.
Selective breeding of animals and plants for higher resistance to diseases assumes ever greater importance. Often resistance to a specific disease is dependent on a relatively simple mutation involving one or a few genes. Any gene mutation is accompanied by change in the synthesis of proteins and may cause protein incompatibility of parasite and host (Kirpichnikov et al., 1967).
Selective breeding for higher resistance to disease has been applied in fish culture only in recent years and the data available are therefore limited. Insufficient study of fish diseases, especially the infectious ones, and contradictory observations on immunological reactions in fish have also contributed to this. Until recently some authors considered that fish were not capable of producing antibodies. The studies of recent years show that fish do produce antibodies but not so intensively as the higher vertebrates. Immunobiological reactions of fish are undoubtedly affected to a great extent by water temperature; the higher the water temperature, the more pronounced the reaction. At very low water temperature the processes of antibody formation are almost imperceptible (Krantz et al., 1964; Vladimirov, 1966).
Bactericidal activity of organs and tissues has been discovered in fish as well. It is probably dependent on the presence of such antimicrobial proteins as complement, lysozyme and properdian (Lukyanenko and Mieserova, 1962; Lukyanenko 1965; Bauer et al., 1965). At temperatures lower than 10°C the mechanism of excretion and bactercidal action of organs and tissues predominate over other immune mechanisms (Avetikyan, 1959). Phagocytosis, a mechanism of antibody formation begins to function with increase in temperature (Goncharov, 1963, 1967). Recent data on fish immunity provide selection breeders with a theoretical background for the breeding of disease-resistant fish strains.
When a disease occurs in a stock, it does not affect all fish to the same extent; some fish, sometimes large numbers, are not affected. Schäperclaus (1953) cites this example: In the spring of 1950 a small pond in Peitz fish farm was stocked with 665 two-year-old carp which were kept there until early June. In June, an outbreak of infectious dropsy occurred. When the pond was fished out, it was found that 4 percent of the fish were either dead or doomed to die; 54 percent of the fish had external ulcers typical of the disease; and 42 percent showed no symptoms of the disease. Even progeny of a single pair of spawners reared under the same environmental conditions did not all possess the same resistance. Individual specimens may serve as starting points for selection work.
Fish belonging to different strains and forms of any species differ in their resistance to disease. The wild carp (Cyprinus carpio) and especially its Amur sub-species (C. carpio haematopterus) are known for their higher resistance to infectious dropsy than the cultured carp (Karpenko and Sventycki, 1961). Amur wild carp × cultured carp hybrids also possess high resistance. The replacement of cultured carp with hybrid forms in recent years in a number of Ukrainian fish farms have resulted in considerable decrease in the incidence of dropsy in this region.
The same was observed in relation to another widespread infectious disease of carp, the air-bladder disease; it is widespread, not only in the USSR but in the other European countries. Arshaniza (1966) carried out experiments on joint rearing of the cultured carp and the hybrid of the fourth generation of cultured carp x Amur wild carp. The results showed that the incidence of airbladder disease amounted to 78 percent in the former, while it occurred in only about 30 percent of the latter. Symptons of illness were more pronounced in the cultured carp than in the hybrid.
Considering these examples, we may assume that work on development of disease resistant strains of fish may be started not only with the selection of specimens possessing higher individual resistance (fish that remained healthy within a stock affected by the disease), but also with the development of breed groups of hybrid origin, distinguishable by higher resistance to a particular disease. Naturally, such selection work should be carried out at those fish farms which are always under the effect of the disease. Otherwise the increased disease resistance of the selected stock would easily be lost.
As stated earlier, data are scant on the selection of disease resistant stocks of fish, and many of the studies undertaken on the problem have either not been carried through or are still in progress. Only three examples of such studies have been carried to conclusion, two of which concern infectious dropsy.
For a long time dropsy was considered a bacterial disease, Aeromonas punctata being its pathogenic agent (Schäperclaus, 1930, 1954). Subsequently, many scientists have questioned the bacterial nature of the disease and have come to the conclusion that dropsy is caused by virus.
Direct evidence of the viral nature of dropsy was obtained several years ago by Yugoslavian scientists (Tomasec et al., 1964). They succeeded in growing the causative agent of dropsy on artificially-cultured tissue of carp kidney, and infecting healthy fish with it. The second International Symposium on Fish Diseases held in Munich in 1965, recognized a virus as the primary pathogenic agent of dropsy, while A. punctata and other saprophytic bacteria complicate the course of the disease.
Attempts to create a stock of dropsy-resistant carp were made for the first time by Schäperclaus (1953). Work on selection of dropsy-resistant carp was started by him in 1935. Carp not affected by the disease, in ponds where dropsy outbreak had occurred, were chosen for selective breeding. For two successive years the carp were exposed to large amounts of A. punctata with a view to artificially strengthening their natural immunity. Fishes having even the slightest symptoms of disease were eliminated. The offspring of both selected and non-selected fish were compared for dropsy resistance; the results showed that of the former only 2 percent contracted dropsy, but of the latter 30 percent did. No dropsy was recorded among the progeny of the selected spawners. From 1939 to 1943 the mortality rate for the ponds (65 ponds with a total area of 180 hectares) where the progeny from selected spawners was reared, averaged 11.5 percent, its range varying between 0 and 37 percent. The average mortality rate for the control ponds (76 ponds with a total area of 474 hectares) was 57 percent, its range being 4 to 96 percent. Decrease in mortality was observed in subsequent years as well. From these observations Schäperclaus concluded that resistance to dropsy is inheritable and it is possible by means of correct selection to obtain stocks possessing relative immunity to this disease.
However, it is possible to obtain this effect only where a selected stock is not mixed with any other and where no infection is brought in from outside, either with the water or with the fish delivered to the station. Experience over many years has shown that infection brought into a specially selected stock manifests, as a rule, in an acute form.
But such complete isolation of the selected stock cannot be maintained in the carp farms situated within the region of the natural habitat of Cyprinus carpio. Infection is continually brought in either by the wild fish penetrating into the fish farm, or by the water supply or perhaps by some other unknown agent. It was suggested by Kirpichnikov et al., (1967) that such fish farms should select spawners from breed groups that are most resistant to dropsy and increase their resistance further by means of controlled selection. Such work has been started by Kirpichnikov at the fish farms of the Northern Caucasus. The aim of the first phase of this work was to compare the resistance to dropsy of different breed groups of carp. The following groups of carp have been tested, singly as well as in mixed groups.
Ropsha carp (hybrid Amur wild carp × mirror carp, F4 and F5),
Ukrainian carp selected by A. Kuzema,
local scaly carp and low-bred mirror carp of unknown origin,
wild carp of the Don river.
The trials for resistance to dropsy were carried out at one of the fish farms. The larvae were reared separately. Fish of different breed groups were combined and reared for three years (1963–1965). The fish were marked by fin clipping; however, under the conditions of a hot summer, the fins grew, causing errors in differentiation of breed groups. The results of the test showed that the various groups showed marked differences in their resistance to dropsy. The Ropsha carp, local scaly carp and Don River wild carp proved to be the most resistant to this disease. The enhanced resistance of the above groups is apparently connected to their wild carp origin. Besides high resistance to dropsy, the Ropsha carp was characterized by generally higher viability, and this carp was therefore selected for breeding at the fish farms of the Northern Caucasus, where outbreaks of dropsy often occur. In the spring of 1966, three-year-olds of all groups had attained sexual maturity and served as brood stock of the second generation, which also was subjected to intensive selection for resistance to dropsy. Though this work is far from completion it promises great economic possibilities, especially in the Northern Caucasus, where dropsy is a major hazard in fish culture.
In the early fifties, work was started on the creation of trout stocks (the brown trout, Salmo trutta m. fario and the brook trout, Salvelinus fontinalis) resistant to two widespread bacterial diseases, furunculosis and ulcer disease (Wolf, 1954). The initial phase of the work was to test the susceptibility of different fish populations to these diseases. For this, ponds at one trout farm were stocked with fish brought from different bodies of water. Fish affected by the diseases were released into these ponds and the dead fish were fed to the experimental ones. The diseases broke out in all experimental ponds, causing mortality. In the first year, the mortality rate of the brown trout in the experimental ponds varied from 35.7 to 65.8 percent; that of the brook trout fluctuating between 5.3 and 99.6 percent. Thus the two species were seen to differ in their resistance to the diseases. Populations with high resistance were chosen for further selection work. Results obtained in subsequent experiments are not available in the literature accessible to us.
Further work on selection of trout for resistance to furunculosis has been reported by Elinger (1964). Diverse geographical strains of the brown trout and the brook trout were intensively exposed to Aeromonas salmonicida, and the less resistant individuals were removed by mass selection. Survivors were selected as brood fish for the next generation, which was similarly exposed to the disease. Hybridization through crossing more promising strains in some instances improved resistance. Resistant strains were compared with regular hatchery stock under actual hatchery conditions. Results obtained thus far indicate that resistance to furunculosis is being achieved. The work is in progress.
For carrying out selective breeding of fish it is necessary to obtain fish belonging to different populations, forms, subspecies, or species from different regions for rearing in experimental fish farms. It is apparent that pathogenic agents of different diseases might be brought in with these fish. The danger of bringing in infection is negligible when eggs are chosen as the initial material for selection. There is only one instance on record where infection was introduced into a hatchery with eggs; the pathogenic agent of whirling disease, one of the most troublesome trout diseases, Myxosoma cerebralis, was introduced with trout eggs. It was supposed that the spores of the organism could stick to the trout eggs and be introduced in this way into new fish farms and bodies of water. However, there is no corroboration of the possibility of such introduction. Efforts in obtaining carp eggs under hatchery conditions, with a view to propagate them artificially, have shown that healthy progeny may be obtained from the eggs of diseased fish. It has been demonstrated that such widespread disease of carp as infectious dropsy and air-bladder disease are not transmitted germinatively. However, there exists the possibility of introduction of parasites infesting the eggs into new bodies of water and fish farms. Fortunately, such parasites are few. Only one species of Coelenterata, infesting acipenserid eggs (Polypodium hydriforme) has been recorded in the U.S.S.R. The danger of bringing it along with eggs is not excluded, and every precaution should be taken while transporting eggs for fish-cultural purposes.
The danger of possible introduction of pathogens while transferring fry or older fish is more real. Soviet ichthyo-pathologists have collected a good deal of data on this. The following is one example. From 1958 to 1962, large numbers of fry of the grass carp Ctenopharyngodon idella, caught in the rivers of China, were transferred to the U.S.S.R. In spite of all the precautions taken, more than 20 species of parasites were introduced with these fish. Many of these parasites cause severe epidemics, not only among phytophagous fish, but among other cultured and commercial fish as well. Thus the cestode Bothriocephalus gowkongensis came to be introduced into the fish-farms of the European part of the U.S.S.R. and Central Asia; then it infected the carp and many other fishes, causing high mortality. It has since penetrated into many bodies of water (Musselius, 1967), and has been introduced into Ceylon (Fernando and Furtado, 1962) and into Romania and other countries of eastern Europe (Radulescu and Georgescu, 1964).
There are many examples of accidental introduction of pathogenic agents of fish diseases along with fish brought in for selection work. The selection of the cold-resistant strain of carp is based on the crossing of Amur wild carp and the cultured carp (Cyprinus carpio × C. carpio hematopterus). In the course of this work a large number of parasites of Far-Eastern origin came to be introduced into the various fish farms.
Back in 1936 the first lot of Amur wild carp was brought in, resulting in the introduction of two monogenetic trematodes which parasitize the gills, and a cestode. Dactylogyrus extensus and Khawia sinensis cause severe diseases, and at present it is difficult to find a fish farm where these do not occur.
In 1949 some Amur wild carp were introduced into fish farms in the northwestern regions of the U.S.S.R. and Byelorussia. Some of these fish were infected with the parasitic protozoan, Ichthyophthirius multifiliis, which was unknown in the carp ponds of the U.S.S.R. Long-distance transportation which lasted about 20 days promoted the development of further infection and when transferred to fish farms, the wild carp passed on the infection to the local spawners. In the spring of 1950 a heavy infection caused high mortality among the spawners. At the Velikolusky fish farm 100 percent of the spawners were lost, and the mortality in the Byelorussian farms was not less than 50 percent. The disease was introduced with the stocking material into the other farms of Byelorussian and the northwestern regions of the U.S.S.R., and by the mid-fifties it had spread to the fish farms of the Ukraine, Central Asia and Kazakhstan.
It was proved experimentally that fish exposed to infection for the first time turned out to be carriers of the largest numbers of Ichthyophthirius. The intensity of disease is 20 times less in fish exposed to the infection repeatedly and further exposures helped to develop immunity to the disease (Bauer 1955, 1958).
Toward the end of the fifties a new lot of the Amur wild carp was introduced into three fish farms of Latvia for selection work. In a year a severe outbreak of dropsy occurred. It was also found that a large nematode, previously unknown in the U.S.S.R., was brought in with that lot of fish. Vismanis (1962 and 1967) described the nematode as a new species, Philometra lusiana. Large females of this nematode, parasitizing under carp scales, cause the formation of ulcers and consequent deterioration of the market quality of the fish.
In 1966 a new lot of Amur wild carp was transferred to Ropsha, an experimental fish farm of the State Research Institute of Lake and River Fisheries; every precaution had been taken. In the autumn of 1967, myxosporidians of the genus Sphaerospora were found on the gills of carp in a number of ponds; this parasite had never been found in Ropsha ponds before.
It is on record that fish in a number of fish farms were infected by air-bladder disease, introduced there with the carp broodstock brought in for selection purposes.
Thus any transfer of fish for the purpose of selection is always accompanied by the danger of introduction of pathogens, even though every precaution is taken.
Only introduction of fish at the egg or at the larval stage (during the first few days after hatching) eliminates this danger. This is possible with adequately worked-out methods of artificial spawning. For salmonids and whitefish such methods have been developed since the close of the last century and, as a rule, these species are transplanted at the egg stage. Only this may explain the fact that not a single pathogen has been introduced with rainbow trout exported from America to Europe and other continents. At present there are well-developed methods for obtaining fertilized eggs of Acipenseridae and phytophagous Cyprinidae. The eggs of the latter are pelagic. Until recently no effective method was known for taking and incubating adhesive eggs as those of carp. Removal of the adhesive material of carp eggs by any of a number of recently developed methods would enable artificial reproduction of this species, as well as long-distance transportation of the eggs and larvae. In recent years the possibility of obtaining healthy progeny from carp spawners affected by infectious diseases and parasites has been examined. Thus in 1965 (in Byelorussia) and 1967 (in Yazhelbyzy) eggs were obtained from carp spawners affected by air-bladder disease. These eggs were incubated in sterile water, and the larvae were released into ponds where the disease had not been recorded, or to carefully disinfected ponds supplied with water from a non-infected body of water. When mass examination of fingerlings was carried out, not a single individual was found affected by the disease.
Artificial reproduction of wild carp has been carried out at the Tsymlansk hatchery-nursery for a number of years and in 1967 about 70 million larvae were obtained. Observations have shown that if larvae are released into carefully disinfected ponds, they will be practically free of parasites when fished out in autumn. Transporting of fish at the egg and larval stage delays the process of obtaining progeny from the transferred fish by several years. But the delay is worthwhile for it excludes the danger of introduction of infectious diseases with fish brought in for selection purposes.
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Bauer, O.N., 1955 Ichthyophthirius and its control in pond fish farms. Izv.gosud.nauchno.-issled.Inst.ozer.rech.ryb.Khoz., 36:184–223 (In Russian)
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