is easily observed in mass infestations of the young of cultured carp by 

 Dactylogyrus vastator Nybelin (Fig. 111). Pathological changes under the 

 parasitical action of monogenetic trematodes can bring with them a very 

 dangerous character for the host. We know that with a strong infection 

 many species of Monogenoidea can cause 'death of the host, and under very 

 unfavorable conditions (e.g. overcrowding of fishes, etc.) can also even 

 cause mass epizootics which have great significance in pond culture of fishes. 

 Thus, in Silesia numerous epizootics of carp caused by Dactylogyrus vastator 

 Nybelin caused millions of losses to pond farms. A very well-known serious 

 incident of the mass destruction of the small Aralsk sturgeon in a natural 

 reservoir (occurred, nobis) as a result of insufficiently planned attempts 

 at acclimatization of the stellated sturgeon in the Aralsk Sea which carried 

 Nitzschia sturionis (Abildgaard), which was never encountered there before, 

 with it into this reservoir (or body of water, nobis) . (For greater detail 

 see Lutta, 1940). There is reason to believe that during the attempts at 

 acclinnatization of Gwiniads (Gang Fish, nobis) the Discocotyle sagittata 

 (F. Leuckart) which parasitize them can cause undesirable epizootics. 



The incidence of occurrence of parasites on their hosts is still 

 very insufficiently known for a majority of monogenetic trematodes. It is 

 not something constant for each species, but depends on a number of various 

 factors: geographic situation of the region of research, the location of the 

 latter in the range of the host, or the time of the year, or the biolocy of the 

 host, etc. Very often the parasite which is encountered on 100 per cent of 

 the hosts m the main portion of the range of the latter is encountered very 

 seldom near the edge of the range. For instance, Dactylogyrus simplici- 

 malleata Bychowsky in the Delta of the Volga is encountered in 100 per cent 

 of the hosts, while in the Bay of Finland it is a great rarity. Ancylodiscoides 

 siluri (Zandt) in the lower part of the Volga is encountered on 100 per cent 

 of the Sheetfish (Siluris glanis , nobis), whereas above Seratov it is encountered 

 only in rare cases. For the most part the majority of representatives of 

 Dactylogyridae are encountered on 100 per cent of the fishes in the middle 

 of the range of the host in the sunnmer, and in the winter, as a rule, not p. 79 



more than 20 to 25 per cent. On the other hand, among the same family 



a number of species are very rarely encountered throughout the year, as for 

 instance Dactylogyrus similis wagener on the Roach of the Leningrad region. 

 For many Monogenoidea the incidence of occurrence changes to a great 

 degree depending upon the age of the host. Thus, nunnerous Gyrodactylus 

 which parasitize 100 per cent of the young fishes are discovered in the adult 

 only in exceptional cases. Protancyrocephalus strelkowi Bychowsky is a 

 typical juvenile parasite of the flounder, among the young of which it occurs 

 in 100 per cent of the cases, whereas it is practically absent among the 

 adults. On the other hand, many monogenetic trematodes are completely 

 absent in the young individuals of the host and infest the adult fishes in large 

 percentages. Thus, Diclybothrium armatum Leuckart is not encountered on 

 young sturgeon and very often occurs on from forty to eighty per cent of the 

 adults. According to our studies, this parasite is not encountered on Sterlets 



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