Nile tilapia reared under full-strength seawater: Hemato-immunological changes and susceptibility to pathogens

Co-culture of Nile tilapia, Oreochromis niloticus, with marine shrimp has been an increasing farm practice as evidence has suggested that the co-culture is one of a few ways to mitigate shrimp diseases, especially the acute hepatopancreatic necrosis disease that has affected shrimp culture industry worldwide. Since Nile tilapia are basically freshwater species, having them in co-culture with marine shrimp therefore requires a certain degree of salinity adaptation, in which the fish are readily capable to. The present study was aimed at studying certain hemato-immunological changes of the fish reared under elevated salinity, as well as their susceptibility to pathogens. Nile tilapia (180 g initial BW) were reared under gradually increasing salinity within one month from 0 to 30 ppt at 3 ppt/3 d and maintained for another month in 30-ppt salinity. At the end of the 60-day experimental period, 50% of the fish survived with mortality occurred mainly during the second month when the salinity was at 30 ppt. Their growth rate was retarded to 60% of that of the control fish maintained under freshwater (0 ppt). The fish reared under elevated salinity also had significantly lower hematocrit, higher total white blood cell counts, higher absolute numbers of lymphocytes and thrombocytes and higher serum lysozyme activity, compared to those reared under freshwater. Presence of three representative pathogens: infectious spleen and kidney necrosis virus (ISKNV), Francisella noatunensis and Streptococcus agalactiae; were determined in the fish reared under freshwater and in those reared under elevated salinity, using polymerase chain reaction (PCR) method. Of the three pathogens, only ISKNV was detected in 1/10 of the freshwater and 6/10 of the elevated-salinity fish. In both groups, the virus was present in the gills, brain, liver, gonads, skin, kidney and spleen, with moderately and severely positive reaction observed in the kidney and spleen. The detection of ISKNV was confirmed by in situ hybridization of the gonads and kidney, using loop-mediated DNA amplification with digoxigenin, which clearly showed dark-brown stain of the positive signals in the PCR-positive samples. These results suggest that a substantial number of O. niltoicus could survive elevated salinity up to full-strength seawater provided that the salinity had been gradually elevated. And under that situation, changes in hemato-immunological functions of the fish did occur, which probably caused lethal proliferation of pre-existing ISKNV that otherwise would remain dormant.