Periodic Effects of Salinity on Compensatory Expression of Phenotypic Traits in Nile Tilapia (Oreochromis niloticus)

Salinity fluctuation is a great threat for fish as most species either do not grow well or eventually die in coastal regions where salinity changes happen regularly because of different natural calamities. This study was conducted to explore the effects of periodic changes of salinity on different phenotypic traits in Nile tilapia (Oreochromis niloticus) in order to find out any traits that can be expressed through compensatory growth. Equal sized juvenile Nile tilapias were randomly assigned into two treatments including control treatment (CT: constantly 0 ppt) and salinity treatment (ST: 25 ppt in 1(st) and 3(rd) month and 0 ppt in 2(nd) month). Each treatment contained 45 fish having three replications and the entire study was carried out up to three months. After each experimental month, all the selected phenotypic traits (i. e. standard length, tail length, body area and color pattern) were measured. The results revealed that ST fish had significantly smaller standard length and body area compared to CT fish especially after the 1st month of the experiment, while no significant variations were found in other months. The study also showed no significant variations in tail length and color patterns between CT and ST fish throughout the experimental months. The findings revealed that ST fish showed a trend of compensatory growth (i.e. recovery of standard length and body area) after 2(nd) month of rearing at control condition (0 ppt). The overall results conclude that periodic salinity fluctuation can affect adversely the growth of fish which is an alarming issue for tilapia farmers and other stakeholders. However, the compensatory development of certain traits through phenotypic plasticity can minimize at least these losses.