52 March 2009 corn and wheat meal, rice bran and soybean oil. Except for fishmeal, all ingredients were available in the current pacu production region. Fish were fed daily, six days per week. From December 1999 to March 2000, food was given three times a day. Between April and May it was supplied twice a day and in winter, early June to late August, once a week in the afternoon. Twice daily feedings began again early in September. From November until the final culture stage, only one feeding was administered at 1600. Food ration rate varied according to body weight, 10 percent during the initial stage, decreased to four percent in March 2000 and to two percent in October 2000. Rations were offered at a maximum of 45 kg/ha/day and it was not offered if water temperature remained below 20ºC during the winter season. The Fish were not fed if the dissolved oxygen level was ≤2 mg/L and water temperature was above 35ºC. Fish were supplied by a local commercial fish farm. Their initial average weight was 0.25 g. Water variables recorded twice daily during the early morning and late in the afternoon prior to feeding were temperature, dissolved oxygen and pH. Ten percent of the fish population, were sampled on a monthly basis to determine fish growth, fish health and to calculate required feeding rate. All the fish were weighed at the end of experiment. Survival and overall production were determined, as well as feed conversion ratio (FCR) and specific growth rate (SGR). Statistical data analyses were performed and treatment groups compared by ANOVA and Duncan test, according to Hintze (1998). Study Results Water quality results Mean water temperature during the culture cycle ranged from 14 to 31ºC, with an optimum for growth appearing to be 26-28ºC (Figure 3). The lowest temperature was 7ºC and had little effect on fish mortality. Dissolved oxygen levels (Figure 4) were always >3 ppm, except for the second summer when extreme values (< 1 ppm) were recorded very early in the morning because of high temperatures and high biomass. In one case, 80 percent mortality was recorded because of a sudden DO depletion during the night. Water pH values exhibited slight daily and seasonal variations and ranged from 6.6 to 8.0. Growth response Growth response is shown in Figure 5 for each pond density employed and the outcome for each treatment is detailed in Table 1. The D density was the result of high mortality in six ponds s. Density A (1.0 fish/m2). Results from 10 out of 12, 300 and 500 m2 were assessed. Final average weight was 560.3 g (ranging from 443.3 to 627.7 g). Average FCR was 1.45 (ranging from 1.30 to 1.66) Density B (0.5 fish/m2). Results from 11 out of 12, 300 and 500 m2 ponds were employed in the analysis. Final average Fig. 2. Pacu ponds (CECADAC) Fig. 3. Mean water temperature registered during the culture cycle. Fig. 4. Mean dissolved oxygen levels registered during culture cycle
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