THE EFFECT OF SALINITY AND FEED TYPE ON THE GROWTH AND NUTRITIONAL QUALITY OF THE S-TYPE ROTIFER Brachionus rotundiformis

Abstract

The growth rate and nutritional quality of the S-type rotifers Brachionus rotundiformis under four different salinity levels fed three different food types were tested and these resulted in differences in maximum density (production No. ml-1), increment (%) and specific growth rate (SGR % day-1) after 84 hours trial culture period. The maximum production rates were higher in the cultures fed on baker’s yeast in all experimental groups. The production rates of rotifers (mean No±sd ml-1) fed baker’s yeast at 20, 30, 40, 50 ppt were (±SD) ix 320.7±3.80, 219.3±19.5, 211.3±3.2 and 204.0±5.3 individuals ml-1, respectively. The lowest production rates were observed in the cultures fed instant algae® Isochrysis sp. under all salinity levels and the mean production rates recorded at 20, 30, 40, and 50 ppt were 218.3±15.6, 205.0±7.0, 198.0±10.0 and 177.3±27.9 individuals ml-1 respectively. The increments (%) and specific growth rate (SGR % day-1) were significantly higher in the cultures fed on baker’s yeast in all experimental groups and were lower in all treatments fed instant algae® Isochrysis sp. . Resting eggs production during the culture period was positively related to the production rate of rotifers. The highest number of resting eggs produced (no. ml-1) was recorded for the rotifers cultures fed baker’s yeast at all salinity levels at different intervals. On the other hand, the rotifers fed instant algae® Isochrysis sp. showed variations in the numbers of resting eggs produced during all intervals. The numbers were observed to decline with time and increasing salinity until it reached the minimum in the 50 ppt cultures. The study suggested that there is a strong relationship between the food type and culture conditions. Accordingly, protein content and ether extract were found to be higher in all treatments fed baker’s yeast under all salinity levels. The lowest protein and ether extract contents were recorded in the cultures fed instant algae® Isochrysis sp. Saturated fatty acids (myristic acid C14:0; palmitic acid C16:0; margaric acid C17:0 and stearic acid C18:0), a monounsaturated fatty acid (oleic acid C18:1 ω9) and one polyunsaturated fatty acid (linoleic acid C18:2 ω6) were detected. x The results clearly showed that the concentration of excretory products (nitrogenous compounds) was inversely correlated with the density of rotifers per ml under the affection of salinity and food type. The cultures of rotifers fed backer’s yeast at 20 ppt salinity level recorded the maximum ammonia concentration and the minimum concentration of ammonia were measured in the cultures of rotifers fed on chaetoceros sp. at 50 ppt salinity level. However, nitrite concentrations were maximum in the cultures of rotifers fed on chaetoceros sp. at 40 ppt salinity level and the minimum concentrations were measured in the cultures of rotifers fed on baker’s yeast at salinity level of 50 ppt. Rotifers cultures fed Chaetoceros sp., instant algae® Isochrysis sp. and baker’s yeast at 20 ppt salinity level showed relatively higher levels of dissolved oxygen concentrations which were 2.67±0.15, 2.63±0.06, and 2.53±0.12 mg.l-1, respectively. The minimum dissolved oxygen concentrations were recorded in the cultures of rotifers fed on instant algae® Isochrysis sp. at 30 ppt salinity level followed by the cultures fed Chaetoceros sp. at 50 ppt salinity level and the cultures of rotifers fed instant algae® Isochrysis sp. at the same salinity level. The range levels of pH recorded during the experimental period was 7.22-7.78±0.01. This was considered to be within the optimum range for rotifers multiplication