SURVEY OF THE HEALTH STATUS OF FISH IN WATER RESERVOIRS OF THE MORA VA RIVER BASIN

Lucky Z., K. Kd.l: Survey of the Health Status of Fish in Water Reservoirs of the Morava River Basin. Acta vet. Bmo, 52, 1983: 83 93. In the years 1979-1980 the health status offish in water reservoirs having a controlled stock was investigated. The study was carried out in 9 water reservoirs situated in the basin of the river Morava, each supplied by different natural water sources, and supervised by the Podnik Povodi Moravy enterprise. Necropsy and a parasitological examination were used to assess health status of 281 fish specimens representing 19 species. A total of 58 diseases were diagnosed: 3 infectious diseases caused by 2 bacterial and one fungus species and 55 invasive diseases produced by many different species (19 protozoan, 19 monogenean trematode, 1 plerocercoid, 4 metacercariae, 6 tapeworm, 2 nematode, 1 spiny-headed helminth, one mollusc larvae, and 2 arthropod). All infectious diseases deteriorated markedly the health status of the affected fish. Parasites producing the most serious pathological changes were Hexamita salmonis, Trichophrya piscium, Chilodonella cyprini, Tetraonchus monenteron, Gyrodactylus salaris, G. luciopercae, Diplostomum spathacewn, Tylodelphis clavata, Triaenophorus nodulosus and Ergasilus sieboldii. A detailed evaluation of the results is provided along with suggestions for measures aimed at fish disease control, successful management of the water reservoirs, and protection of the environment. Infectious fish disease, parasitic disease, water reservoirs. In the years 1979 -1980 a survey of fish diseases and their causative agents was conducted in fish from the basin of the river Morava (belonging to the Danube river basin). Monitoring the health status of fish in these water reservoirs along with regulating their stocking rate and species distribution are of enormous economic and biological significance. This complex problem has to date been studied by few workers in this country. Attention was concentrated primarily upon causative agents of fish parasitism in several dam lakes along with faunistic investigations offish parasites (Ergens 1956; Pribyslavsky et al. 1966; Jilek and Lucky 1967; Pribyslavsky and Lucky 1967). Materials and Methods The health status of the fish was investigated in 9 water reservoirs. Characteristic features of these reservoirs are given in Table 1. From these reservoirs a total of 281 fish specimens representing 19 species were examined during the years 1979 and 1980. All species are listed in Table 2 and further data are provided. The fish were caught in nets and by pole fishing. They were transported to the laboratory packed in ice and examined within 24 hours after catching.


Materials and Methods
The health status of the fish was investigated in 9 water reservoirs.Characteristic features of these reservoirs are given in Table 1.
From these reservoirs a total of 281 fish specimens representing 19 species were examined during the years 1979 and 1980.All species are listed in Table 2 and further data are provided.The fish were caught in nets and by pole fishing.They were transported to the laboratory packed in ice and examined within 24 hours after catching.

Water reservoir Locahty
____ ~ _______ i _ (district~  Opatovice, IX.Slusovice. The health status assessment of the fish was based on results of necropsy and a parasitological examination according to Lucky (1976) of the skin, gills, body cavity, stomach and intestine, liver, spleen, musculature, eyes and kidneys.
The extensity of invasions (i.e. the total number of affected fish) or diseases (or their causative agents) are expressed in per cent even when only one fish was examined.The intensity of invasion is expressed either by the relative number of parasites in the microscopic field at a given magnification (see Table 3) or, with helminths, molluscs and anhropods, the actual numbers are presented.
Mild invasions by only moderately pathogenic parasites were rated as O. Local, single pathological changes of the invaded organs resulting in minor damage (e.g.mild hyperplasia or encapsulation) were rated as l.Pathological changes resulting in serious lesions of the organ caused by a ratther massive invasion of parasites were rated as 2. Marked changes with damage and necrosis in the organs were rated as 3, and lesions impairing the overall health condition of the fish were rated as 4.

Results and Discussion
The present results are summarized in Tables 2 and 3. Table 2 provides information about occurrence of diseases and their causative agents in the individual water reservoirs.Table 3 presents a list of fish diseases and their causative agents according to the zoological system.
All investigated water reservoirs are situated in the basin of the river Morava and they were built on well stocked streams.Later they were further stocked with various fish species without veterinary supervision.
In the two years of the study (1979,1980), a total of 58 diseases and causative agents were diagnosed.They impaired the health status of the fish to various degrees.
The infectious diseases erythrodermatitis and saprolegniosis were diagnosed in several reservoirs; myxobacteriosis was found in one reservoir.All infectious diseases deteriorated the health status of the fish, but did not cause any increase in their death rates.
The parasitic fauna diagnosed in the fish was extensive.These different species were identified: 19 protozoan, 19 monogenean trematodes, 4 metacercariae, 1 spiny--headed helminth, 1 mollusc, and 2 arthopods.These findings are not surprising from the faunistic point of view as in fish taken from both running and still waters we earlier found l34 parasitic species in 2067 fish representing 36 species (Lucky and Dyk 1964).We conclude that nowadays the collection of parasites in the above-mentioned river basin has greatly increased, e.g. according to the data reported by Ergens and Lom (1970) a total of 300-400 fish parasites can be expected to occur in the basin of the river Morava.
Several of the fish parasites found in the investigated reservoirs had impaired the health status of the fish markedly.Among the protozoan species, the most severe pathological lesions were caused by Hexamita salmonis in the brown trout, and Chilodonella cyprini in the pike.Other protozoan species occurred rather sporadically and caused no serious pathological changes in the affected fish.Not even the highly pathogenic Ichthyophthirius multifiliis produced disease in the water environment of reservoirs with lesser concentrations of fish.
The majority of monogenean trematodes caused only local changes on the gills and skin of the affected fish.Heavier infestations resulting in pathological lesions and severe consequences to the fish with high oxygen demands were observed with the species Tetraonchus monenteron in the pike, Gyrodactylus salaris in the brown trout, and Gyrodactylus luciopercae in the pike perch.
Serious damage was produced by metacercariae of Diplostomum spathaceum and Tylodelphis clavata invading fish eyes, causing "pop-eye" or lens opacity and leading to blindness and starvation when present in larger numbers.Metacercariae of both trematode species are common parasites infesting many fishes in both running and still waters.In the locality under study, they were found in         -------~------------    -----but two reservoirs (Bojkovice and Landstejn), both situated far from nesting sites of water birds.In the remaining reservoirs the eye metacercariae occurred frequentlyand caused serious lesions in the brook trout, perch, pike perch, and minnow.All tapeworm species affected the health status and growth performance of the fish stock.The most sever effects were caused by Triaenophorus nodulosus, especiall its plerocersoids in the perch.Both the extensity and intensity of infestations were usually high and lesions in the liver parenchyma were extensive.Other parasitic helminths were found only occasionally and caused no significant pathological changes.
Among parasitic arthropods, the species Ergasilus sieboldii was most often diagnosed.Mild lesions caused by this parasite were found in the tench and silver bream.
Results of the present field study show that the health status of stock fish in these reservoirs, as defined by the Veterinary Act No. 66/61 Sb., is fairly good.No dangerous epizootics requiring veterinary public health measures were diagnosed.The etiology of fish erythrodermatitis is not fully understood as yet so that it cannot be included among the dangerous epizootic diseases.Infectious diseases diagnosed in this study did not reach a level that could threaten the fish stock in the reservoirs.
The numerous collection of parasites may be explained by fish aquaculture management methods and a rather wide geographical distribution of the reservoirs even though they belong to one river basin.Several parasitic species, especially the monogenean trematodes, are highly species-specific.
Direct chemotherapeutic and preventive measures aiming at fish disease control in these reservoirs are impracticable from the public health point of view.Occurrence of this wide variety of causative agents can best be controlled by management means.Fish in good condition are more resistant to disease and combat it better.It is necessary therefore to maintain proper stock density and a balance of individual fish species; the overstocking of one species may result in outbreaks of species-specific infectious diseases.Prior to planting, the fish fry should be carefully inspected by a veterinarian to ensure that only healthy fingerlings are used.These measures can substantially reduce the occurrence of protozoan, trematode and arthropod parasitic species.
Similar measures are implemented in the control of helminths requiring intermediate or definitive hosts for their complete life cycle.The dynamics of several helminthoses is often directly related to density and species density of the inter-~o     mediate host organisms.In some reservoirs, as suggested by our results, the density of water birds should be controlled.They often carry trematodes whose metacercariae can infest the fish.Indeed, this study has shown a high incidence of eye metacercariasis in the fish.The incidence could be reduced by controlling the nesting sites of water birds (black-headed gulls) at or near the reservoir, as eradication of the first intermediate hosts -molluscs -is impracticable.Such measures should be adopted for specialized fish-rearing ponds.They could also be adapted to water reservoirs.However, one must here consider more complex matters of ecology, especially the protection of wild birds.On the other hand, water birds do catch and remove injured and diseased fish floating near the water surface.
Heavy tapeworm infestations were found in the pike caused by Triaenophorus nodulosus and by its plerocercoids in the perch.The later fish is the most common intermediate host of T. nodulus.The perch stock should be more strictly monitored in coarse-fish holding reservoirs with pike planted regularly to eliminate those small cyprinid fish that feed on small water algae.If under such aquaculture schemes, the perch proved to be more efficient, the pike should be replaced by a different predatory species, e. g. the pike perch.
agents of fish diseases in the individual water reservoirs in the years 1979 and 1980

Table 2 continued Causatlve agent of the Fish species
I No. offish I