Influence of salinity on embryogenesis, survival, growth and oxygen consumption in embryos and yolk-sac larvae of the Nile tilapia

Abstract

The Nile tilapia is a euryhaline species that offers considerable potential for culture in low-salinity waters. In order to investigate the ability of this species to be reared during the hatchery stages in brackish water, the effects of varying low salinities (0, 7.5, 15, 20 and 25 ppt) on hatchability, survival, growth and energetic parameters until yolk-sac absorption were assessed. Salinity up to 20 ppt was tolerable, although reduced hatching rates at 15 and 20 ppt suggest that these salinities may be less than optimal. Optimum timing of transfer of eggs from freshwater to elevated salinities was 3-4 h post-fertilisation, following manual stripping and fertilisation of eggs, however increasing incubation salinity lengthened the time taken to hatch. Dry body weight was related to salinity, with larvae in salinities greater than 15 ppt displaying, at hatch, a significantly (GLM: ρ < 0.05) lower body weight but containing greater yolk reserves than those in freshwater or lower salinities. Survival at yolk-sac absorption displayed a significant (GLM; ρ < 0.05) inverse relationship with increasing salinity and mortalities were particularly heavy in the higher salinities of 15, 20 and 25 ppt. Mortalities occurred primarily during early yolk-sac development yet stabilised from day 5 post-hatch onwards. Salinity had a negative effect on yolk absorption efficiency (YAE). Salinity-related differences in oxygen consumption rates were not detectable until day 3 post-hatch. Oxygen consumption rates of larvae in freshwater between 3 and 6 days post-hatch were always significantly higher (GLM: ρ < 0.05) than those in 7.5, 15, 20 and 25 ppt, however, on day 9 post-hatch this pattern was reversed and freshwater larvae had a significantly lower QO2 than those in elevated salinities. Salinity had a significant inverse effect on larval standard length, with elevated salinities producing shorter larvae from hatch until day 6 post-hatch, after which time there was no significant differences between treatments. Salinity had a significant effect on whole larval dry weight, with heavier larvae in elevated salinities throughout the yolk-sac period (GLM; ρ < 0.05).