Histological alterations under metal exposure in gills of European perch (Perca fluviatilis L.) from Topolnitsa Reservoir (Bulgaria)

Topolnitsa Reservoir is located in a region of Bulgaria rich in copper mines where intensive mining has been ongoing for several decades. General data on the ecological status of the reservoir and the effects of metal on fish is relatively scarce. The first aim of the study was to measure the concentrations of six metals (As, Cd, Cu, Ni, Pb and Zn) in water samples and in the gills of European perch (Perca fluviatilis L.). The second objective was to examine gill structure and determine the severity of histological alteration as a result of metal exposure. Surface water and fish gill samples were collected in spring, summer and autumn in 2012 and metal and histological analyses were performed. Metal concentrations in the water samples varied, but only Cu concentrations were determined in all three seasons and they were higher than the maximum permissible levels. The gill metal concentrations were significantly higher (P<0.05) than in the water. Examination of gill structure revealed the presence of proliferative and degenerative changes, as well as changes in the blood vessels. Histological lesions were similar in their severity in all three seasons. This study provides the first information about metal effects on the morphology of European perch gills from Topolnitsa Reservoir. It can be concluded that the metal contamination of the Topolnitsa Reservoir and fish is chronic and that it can negatively affect the structure and function of fish gills. As metals display a tendency to accumulate in fish gills, their effects are expected to become more severe with time, as they affect gill functions.