FIRST RECORD ON CHILLODONELID PARASITE IN TROUT FISHERY OF SERBIA

Up to now, investigations of ectobionts, including parasitic organisms, were not accomplished on the rainbow trout Oncorhynchus mykiss reared in Serbia. Reports on that topics from European trout-rearing farms (M 0 r a v e c and S c hoi z 1994; R i n tam a k iKin nun e n and Val t on e n 1996, 1997; B u c h man nand U I d a I 1997) and those out of Europe (U raw a and Jam a

Up to now, investigations of ectobionts, including parasitic organisms, were not accomplished on the rainbow trout Oncorhynchus mykiss reared in Serbia.Reports on that topics from European trout-rearing farms (M 0 r a v e c and S c hoi z 1994; R i n tam a k i-Kin nun e n and Val t on e n 1996, 1997; B u c h man nand U I d a I 1997) and those out of Europe (U raw a and Jam a 0 1992) provided a solid basis for introducing into that field of fish parasitology.The present study was undertaken in order to find if there are any ectoparasites on the farm-reared rainbow trout in Serbia and to monitor their occurrence through the year.In considering the results of this survey, all kinds of ectobionts found' on the rainbow trout were considered parasites, since such kind of their relationship to the carrier, i.e. the host organism, was reported previously (U raw a and Jam a o 1992).
During the nine month (March -November) survey of three closely distanced fish farms, situated in the eastern Serbia, in the area of Homolje through the year i'1998, a total of 5 ectobionts was recorded.Three of them were ciliates (Chilodon ella hexasticha, Ichthyophthirius multifiliis and Apiosoma piscicola var.minor), one was a trematode (Gyrodaetylus sp.) and the last one was a mould (Saprolegnia sp.).
Samples from the live yearling of rainbow trout (25 individuals on each month sampling, a total of 675 individuals in a year) were taken from the skin, and examined under the microscope (magnification 400x).From the data obtained, the average intensity (from a number of individuals in 10 visual fields) and extensity (i.e., the prevalence) of infestation were calculated and further analysed.
On the analysis of prevalence of rainbow trout infestation with particular parasites (Fig. 1) by Friedman ANOVA test, it appeared that Chilodonella hexasticha occurrence was significantly variable through the year (F = 3.237, df = 8, P < 0.01), mainly due to a significant difference in prevalence of that ciliate between May and July -September period (Scheffe test, p < 0.05).On comparing the prevalence of the sole Chilodonella hexasticha with the prevalence of that parasite found in mutual infections with the other parasites (either Apiosoma piscicola var.minor, Gyrodactylus or both) occurring on the same fish, the independence in infestation pattern was found, meaning that there was no difference in prevalence (X 2 = 4.651, df = 40, P > 0.1), i.e., that prevalence of the Chilodonella hexasticha was neither favoured, nor suppressed in the mutual infestation with the other parasites occurring on the rainbow trout.

Sole infestations by Chilodonella
During the period of investigation, no prominent pathogenic effect of ectoparasites was noticed on the farmed rainbow trout, either in general, or on the skin and gills.Similar to data of R i n tam a k i Kin nun e n and Val ton e n (1997), we found Chilodonella hexasticha in mutual occurrence with the other parasitic species (e.g., with Apiosoma piscicola var.minor, with Gyrodactylus sp., and both of them).However, the type of infestation (sole vs. mutual) revealed insignificance in occurrence for rainbow trout we examined, thus implying that occurrence of any of them should not favour the additional or subsequent infestation, and that mutual occurrence of several parasites does not increase their harmness, considering the lack of any noticed pathogenic effect on rainbow trout.