G. Zhelyazkov*

Department of Biology and Aquaculture, Faculty of Agriculture, Trakia University, 6000 Stara Zagora, Bulgaria

Abstract. The aim of study is to determine the effect of two stocking densities on some hydrochemical parameters and growth traits in cultivation of perch /Perca fluviatilis L./ in a recirculation system. The fish stocking material consisted of 555 perch (Perca fluviatilis L.) with live weigh of 21.05–28.85 g, that were divided in two groups, each of them with three replications, at a stocking density of SD1 – 137 fish/m3 and SD2 – 94 fish/m3. The fishes are cultivated in concrete tanks with a volume of 0.8 m3 which are part of the recirculation system. They were fed ad libitum three times a day with extruded pellets for trout “Aqua UNI“, a product of “Aqua garant”, with size of the pellets 2 mm. The trial lasted 60 days. The hydrochemical parameters in the recirculation system during the trial were determined through methods adapted for fish breeding. To study the influence of the stocking density on the weight gain and the feed conversion in perch (Perca fluviatilis L.), cultivated in a recirculation system, the average live weight (g) was determined as the fish were individually weighed at the beginning and the end of the trial. During the experimental period the mortality of the perch was also studied as the dead fish were daily controlled. At the end of the trial period the weight gain (g), survival rate (%) and feed conversion ratio (K) of the fish were determined. In the conditions of the experiment we found that the higher stocking density does not evoke changes in most of the hydrochemical parameters of the recirculation system. The only exception is the decreased content of oxygen dissolved in water in SD1 – 7.51±0.12 mg.l-1 compared to SD2 – 8.31±0.13 mg.l-1, as the differences are significant (P<0.05), although its values are within the optimal range of the species cultivated. At the end of the trial a tendency toward higher live weight in the fish from SD2 – 41.82±15.01 g was observed being 4.89% higher than the SD1 fish. The analysis of the data concerning the survival rate shows a trend toward higher value in SD1 – 92.12±4.58%, while in SD2 it is considerably lower – 72.00±20.78%; however no significant difference was found (P>0.05). The weight gain of the perch from the three replicates of SD1 shows that it is 16.18±4.18 g , this being 14.71% lower than that of the specimens of SD2, but the difference was not significant (P>0.05) . The feed conversion ratio of the specimens of SD1 is 1.06±0.04 and it is 17.92% lower than that in the SD2 fish and the difference is significant P<0.05.