Epizootic observations in relation to growth performance of rainbow trout Salmo Gairdneri (Richardson) at commercial fish farms on the rivers Itchen and Test, Hampshire, U.K.

The Itchen Abbas and Romsey fish farms on the rivers Itchen and Test, Hampshire, U.K. were selected for the investigation. Both farms are large intensive units specializing in producing rainbow trout Salmo gairdneri (Richardson). 1,702 fish of age 0&#43; were examined during six intensive surveys between September 1986 and August 1988. The prevalence and intensity of the protozoan parasites Trichodina, Hexamita salmonis, Ichthyophthirius multifiliis, Costia necatrix, Chilodonella, Sessile peritrichs and one monogenean Gyrodactylus were recorded. Proliferative Kidney Disease caused by PKX was also noted. The majority of the parasites were noted during the spring and summer periods. Parasitic abundance was high at higher temperatures. During the concurrent surveys (4th and 5th surveys) at two fish farms, Trichodina, Gyrodactylus and sessile peritrichs showed higher abundance at Romsey fish farm, whereas high prvalence of I. multifiliis gill infection was noted at Itchen Abbas fish farm. PKD infection prevalence was highest in the 6th survey at the Romsey fish farm. By using the analysis of covariance, the majority of the parasites, incuding PKD, showed significant correlation of prevalence with fish weight, length and condition factor (P&#60; 0.05). Both one-way and two-way analysis of variance revealed significant interactions between certain pairs of parasites in their effects on fish weight, length and condition factor (P&#60; 0.05). A log-logistic model also revealed significant relationships of the infection rate of certain parasites with both time and temperature. Intensity of I. multifiliis on the gills and density of Gyrodactylus infections in mucus showed significant relationships with time and temperature in some surveys by a simple regression analysis. Using a simple regression analysis it was evident that the fish weight and length of both uninfected and infected groups have a significant relationship with age. However, there was very little difference between infected and uninfected fish, age for age, especially where the sample was big enough to be useful. The effects of chemicals used by the management were investigated during the surveys namely Chloramine-T- for external parasites, Oxolinic acid for bacterial diseases, Furazolidone for H. salmonis and Malachite Green for PKD. Other management parameters such as food consumption and feeds per day were recorded and related with other data.</p