Ontogenic changes in the osmoregulatory capacity of the Nile tilapia Oreochromis niloticus and implications for aquaculture

The Nile tilapia, one of the most extensively cultured tropical freshwater species, offers considerable potential for culture in low-salinity water. The ontogeny of osmoregulation in the Nile tilapia was studied from spawning to yolk-sac absorption after exposure to different experimental conditions ranging from freshwater to 25 ppt. Eggs were able to withstand elevated rearing salinities up to 20 ppt, but transfer to 25 ppt induced 100% mortality by 48 h post-fertilization. Across all stages, embryos and larvae hyper-regulated at lower salinities and hypo-regulated at higher salinities. Osmoregulatory capacity increased during development and from 2 days post-hatch onwards remained constant until yolk-sac absorption. Adjustments of larval osmolality following abrupt transfer from freshwater to experimental salinities (12.5 and 20 ppt), followed a pattern of crisis and regulation, with values for larvae stabilising at c. 48 h post-transfer for all treatments, regardless of age at time of transfer. Age at transfer to experimental salinities (7.5-20 ppt) had a significant positive effect on larval ability to osmoregulate, with larvae transferred at 8 days post-hatch maintaining more constant whole-body osmolality over the experimental salinities tested than larvae at hatch. Concomitantly, survival following transfer to experimental salinities increased with age. There was a significant effect (GLM; p < 0.05) of the salinity of incubation and rearing on the incidence of gross larval malformation that was seen to decline over the developmental period studied