EFFECT OF THE C:N RATIO ON Daphnia magna (Straus, 1820) PRODUCTION USING TILAPIA FARMING WASTEWATER

The purpose of this study was to produce Daphnia magna in wastewater from Tilapia farmed in a biofloc system, to evaluate three different carbohydrate:nitrogen ratios. The experiment was conducted for 30 days following a completely randomized experimental design, with the three treatments: C:N 10:1, C:N 15:1 and C:N 20:1, all fed with Chlorella vulgaris. The physical-chemical variables of the water analyzed were: temperature, dissolved oxygen (DO), pH, TAN, NO 2, hardness and alkalinity. The following growth variables of the Daphnia were also evaluated: Maximum average density (MAD), Maximum density day (MDD), Specific growth rate (SGR), Doubling time (DT) and yield (Y). The water quality variables remained in the range of ideal conditions for the species, presenting significant differences (p<0.05) among the treatments for: pH, DO, hardness and alkalinity. Regarding the growth variables, significant differences (p<0.05) were observed, with higher MAD, Y and SGR values for the 10:1 treatment (3,433 +/- 267 ind L-1, 245 +/- 19 ind L-1 day(-1) and 45.3 +/- 0.6% day(-1), respectively) and lower for the 20:1 (1,011 +/- 283 ind L-1, 55 +/- 15 ind L-1 day(-1) and 28.3 +/- 1.6% day(-1), respectively). The MDD occurred on day 12 for the 10:1 treatment and on day 18 for the 15:1 and 20:1 treatments. Alkalinity and hardness had stronger influence on the growth variables, which was also indicated by the simple linear regression. The principal component analysis (PCA), with 80% of explanation, identified high values of SGR, density and Y for the 10:1 and high values of alkalinity, hardness, DT and NO2 for the 20:1. Thus, the use of effluent from Tilapia farming in a biofloc system with a C:N ratio of 10:1 provided better production results for D. magna, demonstrating that it is an option for the production of live feed for aquaculture.