Effect of dietary sodium chloride acclimation on growth and plasma thyroid hormones in tilapia Oreochromis niloticus (L.) in relation to sex

This trial was conducted in order to test the effect of supplemental dietary sodium chloride on growth and plasma thyroid hormone levels of male and female monosex groups of tilapia Oreochromis niloticus (L.) reared in fresh water and 10 parts per thousand brackish water. At the end of the experiment, the gonadal development of females was observed. A high-salt diet (8% NaCl) was fed to female and male tilapia for a period of 3 weeks. After this time. both groups were transferred directly to fresh water and 10 parts per thousand brackish water, and a growth trial was conducted over 90 days. The results indicate that dietary supplementation with NaCl improved the growth of females and males at 30 days, except for females maintained in fresh water. Females reared in 10 parts per thousand brackish water showed an increase in growth rates that was accompanied by the highest plasma T3 levels. In contrast, males did not exhibit the same relationship in 10 parts per thousand brackish water, showing lower plasma T3 levels than in fresh water. Histological sections of O. niloticus ovaries revealed that female groups contained oocytes at different stages of development. At 90 days, females reared in 10 parts per thousand brackish water showed oocytes in maturation, while in fresh water, they showed the highest proportion of vitellogenic oocytes and no mature oocytes. The highest proportion of mature oocytes observed in brackish water was associated with the better growth rate in relation to fresh water. These results confirm that tilapia breed freely in low-salinity brackish water as well as in fresh water.