Relationship of RNA/DNA ratio to somatic growth of Nile tilapia juveniles (Oreochromis niloticus) under joint effects of temperature and salinity

Tilapia juveniles are a very important life stage, and reliably assessing their growth performance is of prime importance in aquaculture production. The suitability of the RNA/DNA as a bioindicator for growth evaluation in tilapia has not yet been reported. In this study, we examined suitability of RNA/DNA ratio for assessing growth of tilapia juveniles and variation in the ratio under the concurrent influences of temperature and salinity using the central composite design and response surface methodology. Results showed that under our experimental conditions, the synergistic effects of temperature and salinity on the RNA/DNA ratio were highly significant (P < 0.01), and the ratio varied with the two factors in a nonlinear fashion; the temperature 9 salinity interaction was detected at 1% significance level, and had a negative effect on the RNA/DNA ratio particularly at higher temperatures. Temperature was found to have a greater influence on RNA/DNA ratios than salinity. A model of the relation between RNA/DNA, temperature and salinity was developed, with an R-2 greater than 98%. The linear relationships of absolute growth, cumulative growth and specific growth of tilapia juveniles to RNA/DNA ratio all showed an R-2 of greater than 95%. Our results indicate that the RNA/DNA ratio was a responsive and accurate bioindicator for evaluating the growth performance of juvenile tilapia. These ratio-based models are neither temperaturenor salinity-dependent, and could be readily applicable to freshwater or brackish water aquaculture within temperature-salinity combinations ranging from 16 to 36 degrees C and 0-16 ppt.