The most popular rainbow trout (Oncorhynchus mykiss, Walbaum 1792) production technologies include both an extensive method with the flow through system (FTS) and an intensive method with the recirculating aquaculture system (RAS). Their impact on the fish was evaluated with a morphological assessment of the gills, as these are organs susceptible to environmental changes. Trout of 350 - 500 g body mass were caught for trial in spring and autumn, with 36 fish originating from 3 fish farms with the FTS system and an equal number from 3 RAS fish farms (n = 6). The fish were macroscopically examined and the gills were investigated microscopically (haematoxylin and eosin staining). Hypertrophy and hyperplasia were most commonly detected, amounting to 89% of all structural abnormalities. These lesions were slightly more common in the FTS, especially in autumn, whereas the changes to the blood vessels in the gills were observed more frequently in the rainbow trout from the RAS system than in the fish obtained from the FTS technology (the difference was statistically significant). The morphological lesions in the mucous cells of the gills were detected at a comparable severity regardless of the technology or production season. The predominantly low values of the histopathologic indices, which described the microscopic lesions in the gills of fish from the FTS and RAS systems, showed that the examined organ was most often free of lesions or demonstrated only minor morphological damage regardless of the production technology.
Keywords: fish production technology; gills; morphological lesions; rainbow trout.