SURVIVAL AND HEAVY-METAL ACCUMULATION OF 2 OREOCHROMIS-NILOTICUS (L) STRAINS EXPOSED TO MIXTURES OF ZINC, CADMIUM AND MERCURY

Two Nile tilapia strains of Oreochromis niloticus (L.) (Cichlidae, Tel eostei) fingerlings were exposed to mixtures of zinc, cadmium and merc ury. The two strains used were Chitralada or NIFI (originally from the National Inland Fisheries Institute, Thailand) and CLSU (from the Fre shwater Aquaculture Center of the Central Luzon State University, The Philippines). Short-term (10 days) exposure to a metal mixture of 5 mg l-1 zinc (Zn), 0.5 mg l-1 cadmium (Cd) and 0.02 mg l-1 mercury (Hg) g ave significantly higher survival percentage in the NIFI strain compar ed with the CLSU strain. Similar exposure conditions using larger and older fingerlings of the two strains also showed a slightly higher sur vival percentage in the NIFI strain but the difference was not signifi cant. Prolonged exposure of the fingerlings to a lower concentration o f the metal mixture (1.0 mg l-1 Zn, 0.1 mg l-1 Cd, 0.01 mg l-1 Hg) als o resulted in similar survival percentages between the two strains at the end of the 60 days run. Whole body accumulation of Zn was signific antly higher in CLSU than in NIFI after 14-day exposure to the low con centration metal mixture. There was no significant difference in the a ccumulation of Cd and Hg between the two strains. Of the three metals, Hg had the highest bioaccumulation factor (BF) which was approximatel y 900-1000, followed by Cd with 255-280 and Zn with 180-195 times the nominal concentration in the water. Concentration of Cd and Hg in fish tissues increased with exposure period while the concentration of Zn was maintained in NIFI and decreased in CLSU between the 6th and 14th day of exposure, suggesting that Zn (an essential element) accumulatio n maybe regulated by both strains.