| Abstract |
The aim of this study was the improvement of the rearing procedure of red porgy Pagrus pagrus, in order to get a final product of higher growth performance and quality. Growth was studied from the stage of juvenile (5 g) till 150 g of body weight, during four successive periods, representing the seasons of the year. Three feeding protocols were applied: a. free feeding 24h/day, b. restricted feeding for six hours in the morning (in the period of highest feeding activity of non restricted fish) and c. restricted feeding for six hours at night (in the period of lowest feeding activity of non restricted fish). Food was administered by the means of self-feeders. The experiments were repeated in two different densities (50 and 100 fish per tank). Growth rate was recorded by individual weighing of fish every 14 days. The physical condition of fish was studied with samplings that involved blood analysis in the beginning and at the end of each seasonal experiment. The following parameters were measured: haematocrite, glucose, lactate and thyroid hormones. After each sampling, 100 g of fish per tank were kept in the freezer for later body composition analysis. Total protein, lipid, humidity and ash content were measured, as well as total energy. Mortality was not affected by rearing conditions (fish density or feeding time). As far as growth is concerned, in the summer specific growth rate (SGR) was maximal for the fish that fed without restriction and minimal for the fish that fed at night, while in the other seasons there was no difference. Feed gain ratio (FGR) did not seem to be affected by any feeding protocol at any season of the year. Voluntary feed intake was maximal at the 50 fish population that fed without restriction during summer, while in the other seasons the differences between feeding conditions were not significant. Blood analysis and body composition analysis did not show any influence of feeding time and fish density to the physical condition and composition of fish, so it is not easy to determine the time interval of the day during which feeding is the most beneficial for fish rearing. By continuous recording of the self-feeders activations of the fish groups that had no restriction in food availability, feeding rhythms of red porgy were determined. Red porgy was a diurnal organism. Its activity started at dawn and kept high till dusk, while during night there was no feeding activity. As fish grew bigger with time and their feeding needs increased, feeding activity continued also during night hours, but in a much lower extent. Fish reared under restricted feeding during night hours, changed their feeding behaviour and adapted to the new situation successfully. Two weeks after the application of the restriction, feeding activity at the hours of food availability increased significantly, and activity in the morning decreased accordingly. The increase of feeding activity at the hours of feed availability in the tanks that fed in the morning was not significant, because it was already high before the restriction. At the end of the experimental period, 82 % of the total self-feeder activations occurred during the hours of feed availability. well as total energy. Mortality was not affected by rearing conditions (fish density or feeding time). As far as growth is concerned, in the summer specific growth rate (SGR) was maximal for the fish that fed without restriction and minimal for the fish that fed at night, while in the other seasons there was no difference. Feed gain ratio (FGR) did not seem to be affected by any feeding protocol at any season of the year. Voluntary feed intake was maximal at the 50 fish population that fed without restriction during summer, while in the other seasons the differences between feeding conditions were not significant. Blood analysis and body composition analysis did not show any influence of feeding time and fish density to the physical condition and composition of fish, so it is not easy to determine the time interval of the day during which feeding is the most beneficial for fish rearing. By continuous recording of the self-feeders activations of the fish groups that had no restriction in food availability, feeding rhythms of red porgy were determined. Red porgy was a diurnal organism. Its activity started at dawn and kept high till dusk, while during night there was no feeding activity. As fish grew bigger with time and their feeding needs increased, feeding activity continued also during night hours, but in a much lower extent. Fish reared under restricted feeding during night hours, changed their feeding behaviour and adapted to the new situation successfully. Two weeks after the application of the restriction, feeding activity at the hours of food availability increased significantly, and activity in the morning decreased accordingly. The increase of feeding activity at the hours of feed availability in the tanks that fed in the morning was not significant, because it was already high before the restriction. At the end of the experimental period, 82 % of the total self-feeder activations occurred during the hours of feed availability. Learning the use of self-feeders was easier for fish in groups than individual fish. Morning or night feeding did not prove to be beneficial for the growth of red-porgy. Growth was independent from feeding time, so one can choose morning or night feeding, according to ones needs, without any negative effect to fish growth.
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