The Effects of Various Salinities on the Growth, Reproduction, and Pathology of Blue Tilapia (Oreochromis aureus)

Blue tilapia, Oreochromis aureus were cultured from fingerling through maturation, in outdoor tanks, at varying salinities, for a period of nine months, from September 19, 1997 to June 19, 1998. Twelve tanks were used for testing four different salinities, with three replicates each. Each tank had a surface area of 0.8 m2, a water depth of 48cm, and held a volume of 350 liters. Each tank was stocked with ten O. aureus, with a total mean weight of 100 grams, and fed 5% to 10% body weight per day depending on size throughout the study. Salinity was the only induced variable throughout the experiment. Three tanks were the freshwater control group and salinity was held at 0 ppt. Salinities in three tanks were maintained at 10 ± 2ppt. Salinities in three tanks were maintained at 20 ± 2ppt and the salinities for the remaining three tanks were maintained at 30 ± 2ppt. Complete water changes were done each month, and the fish were individually weighed and an estimate of uneaten food was done. Any morphological or behavioral changes were noted for each fish, and each was regularly checked for eggs. During the ninth month, reproduction resulted in viable eggs occurring in the freshwater control group and two tanks with salinities of 10 ppt. Light orange colored eggs were found in the mouths of females in tanks with salinities of 20 ppt, but the eggs were not viable. The fish in one tank suffered a mass mortality in month seven, so it was removed from reproduction computations. Reproduction also occurred during the ninth month in the freshwater control groups. Light orange colored eggs were found in the mouths of females in tanks with salinities of 20 ppt, but the eggs were not viable. Eggs with a grayish tint were found in the mouths of females in tanks with salinities of 30 ppt, but the eggs were also not viable. One high salinity tank was removed after nine months due to disease, but females were holding eggs with a gray color and eggs were examined and found not to be viable. No reproduction occurred in tanks with salinities of 20 ppt or higher during the experiment. Weight gain was shown to be positive and linear with salinity. The control group had the least weight gain with a final mean weight gain of 565 grams per tank. In tanks with salinities of 10 ppt, final mean weight gain of the fish was 697 grams; and in tanks with water salinities of 20 ppt, a final mean weight gain of the fish was 815 grams at the close of the experiment. Fish within tanks with salinities of 30 ppt, showed the most gain, with a mean weight gain of 927 grams these differences were shown to be statistically significant (P\u3c .004). Morphological changes and the occurrence of disease were evident between the fish in the control tanks and the fish in all other tanks. Tank 3, with 20 ppt salinity developed bacterial septicemia, caused by Streptococcus sp .. This disease produced some lesions, enlarged heads and sunken bellies as well as a dark green cast to the dorsal area. During the eighth month, the fish in a tank with salinity of 30 ppt developed a toxic algae bloom and the fish contracted a disease, which manifested itself by producing epithelial erosion, highly suggestive of the dinoflagellate pathogen Pfiesteria piscicida. Within a week, nine of the ten fish suffered severe epithelial loss and were close to death. Other changes accompanying increased salinity were brighter red flashing on the dorsal and pectoral fins and tail. Thus the results are consistent with a hypothesis that higher salinity severely restricts reproduction and increases growth rates, but it was also appears that increased disease accompanies increased salinity. Thus the use of saltwater does not appear to be advantageous for the culture of this species