In order to evaluate the relationship between water quality in ponds and indices of histopathological changes occurring in the vital organs of the common carp (Cyprinus carpio L., 1758), two six-month field experiments were carried out using two different water supplies: from the nearby stream and a tube well. The fish were fed supplemental feed: raw cereals, pelleted and extruded compound feed. Histopathological analysis, alteration frequencies, and semi-quantitative scoring of the changes were used to assess the health status of the fish. Ponds supplied by stream water were characterized by higher water hardness, dissolved oxygen and pH values, while those supplied by the tube well had higher electroconductivity, total ammonium and orthophosphates content. Fish survival rate and habitat suitability index were lower in ponds supplied by stream water, while the weight gain did not differ between the two water supplies. The use of stream water resulted in a higher level of histopathological changes in gills and liver. Among the water quality parameters, pH level had the strongest influence on fish. Differences in water supply produced greater influence on the level of histopathological changes than the type of feed applied. Gills were the most sensitive organ, while the kidney was the least responsive.

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During the period from April to October in the production season of the year 2000, seven chemical parameters of the aquatic environment were analyzed twice a month for the amount of natural food (zooplankton and bottom fauna) on pond No. 4 at the Dubica Carp Farm in Banatska Dubica (Vojvodina, Serbia). Correlation between natural food and carp growth was analyzed as well. The investigations revealed negative correlation between the amount of natural food per kilogram of ichthyomass and pH (p < 0.001, r = -0.875), electroconductivity (p = 0.0294, r = -0.673), COD (p = 0.0052, r = -0.782), total water hardness (p = 0.0186, r = -0.709), and carbonate water hardness (p = 0.0087, r = -0.758). However, statistically significant positive correlation was found between the biomass of natural food (g/kg of ichthyomass) and concentration of nitrates (p < 0.01, r = 0.841) and phosphates (p = 0.0427, r = 0.640). Concerning com­ponents of natural food (zooplankton and bottom fauna), bottom fauna biomass had a very low production and showed no statistically significant correlation (p = 0.137, r = 0.491) with total natural food biomass whereas zooplankton biomass was very significantly correlated (p<0.001, r = 0.830) with the total amount of natural food, indicating that zooplankton was the dominant component of natural food at the investigated fish pond. The daily weight gain (%) of cultured fish was greatest at the beginning of the carp farming season, i.e., in April, when it comprised a value of 2.3%. After negative growth at the end of May (-0.2%), a gradual increase of weight gain was recorded until the beginning of September. By comparing the biomass of natural food with the daily weight gain (%) of carp, it was observed that the weight gain had a trend similar to biomass of natural food with a 15 day interval. This relationship was justified by positive statistical correlation (p = 0.006, R = 0.960). As zooplankton was the major component of natural food, fish relayed on it as the main protein source showed by a significant positive correlation (p = 0.034, r = 0.907)