Evaluating the use of the sea weed Padina pavonica (L.) as a diet for the mass production of the rotifer Brachionus plicatilis

Abstract

In this study, the suitability of using Padina pavonica (L.) cake as a diet for the rotifer Brachionus plicatilis was investigated on a laboratory and commercial scale. The study was carried out using L-strain rotifers. The laboratory trials, carried out in 90L cylindroconical fibreglass tanks, varied depending on the diet used to feed the rotifers. Hence, the trials were differentiated on the grounds of using the Padina cake (PC), Culture Selco® (CS), and microalgae (ALG) combined with Baker's yeast (BY), in different proportions. From all trials performed at laboratory scale, those of most commercial interest were selected and repeated in full-scale 900L cylindroconical tanks. Each trial required two experimental tanks to obtain duplicate results. The rotifers were fed 4 times per day using the standard feeding regimes for the various formulated diets. Data collection was carried out from inoculation all the way until the death phase of the rotifers was reached. Each day the rotifer populations, their fertility, the percentage increases in total rotifer population from Day 1 of culture, and various growth parameters were determined. Quality of the culture medium was also monitored daily. The total proximate nutritional analyses and fatty acid profiles of the diets used in the trials and the rotifers cultured on these diets were carried out. The operating costs for Live Food culture required for the production of 1 million Sea Bream fingerlings was performed to evaluate the economic advantages, if any, of using Padina pavonica cake as a substitution diet. The results were analysed statistically using the Paired t-test and Wilcoxon test. The best substitution ratios to use for both the CS and Alg+BY trials in the 90L chemostats, was found to be 50:50. The total amount of rotifers cultured using the 50:50 ratio in 1 tank for the CS trial is 1.48 times greater than when using the commercial Culture Selco diet alone, and 1.43 times greater for the Alg+BY trial. At the 900L chemostat level, the best substitution ratio was 60% CS and 40% PC. Rotifer growth was 1.45 times better using this diet then when using CS. The 50:50 diets do not have a significant effect on the fertility of the rotifers, but maintain a growing population for longer periods than both the CS and Alg+BY diets because it prolongs the ageing process of the rotifers, and maintains a clean culture medium. The same applies for the 60:40 trial at the 900L chemostat level. Although the growth parameters for these diet mixtures are relatively lower than for the rotifers fed the standard diets, in the long term, rotifer growth performance is better. This is because rotifers can be maintained for longer periods in the late exponential phase of growth. Rotifers fed the 50:50 diet, for both the CS and Alg+BY trials, show an increase in percentage crude protein and crude lipid content. Fatty acid profiles also indicated that rotifers feeding on the 50:50 diets showed adequate levels of EPA and DHA. PC therefore seems to induce lipid and fatty acid metabolism in rotifers. As a result of using the 60:40 diet as a replacement for the standard Culture Selco® on a commercial scale, Live Food Culture operating costs may be reduced by at least 25%.