RESISTANCE TO A HEAVY-METAL MIXTURE IN OREOCHROMIS-NILOTICUS PROGENIES FROM PARENTS CHRONICALLY EXPOSED TO THE SAME METALS

Adult Oreochromis niloticus were mass spawned in conrete tanks. The one-month old progenies (F-1) were exposed for two months to a mixture of 0.01 mg L(-1) Hg, 0.1 mg L(-1) Cd and 1.0 mg L(-1) Zn. The survivors were grown to sexual maturity in a natural environment (lake). The fish were spawned and the progenies (F-2) of the exposed F-1 (EF(1)) were exposed to another mixture of the three metals: 3.0 mg L(-1) Zn, 0.30 mg L(-1) Cd and 0.01 mg L(-1) Hg, both in a static and static-renewal system. Another group of F-2 from unexposed F-1 (UF1) received the same treatment. Results showed that in both exposure systems, survival:of the F-2 of EF(1) was significantly higher (P<0.05) than those from UF1. The median lethal time (LT(50)) of the F-2's were estimated from the time-response curve following regression analysis: 5.16 days (F-2 of UF1) and 9.03 days (F-2 of EF(1)) in the static exposure experiment; 3.34 days (F-2 of UF1) and 5.52 days (F-2 of EF(1)) in the static-renewal run. Exposure of the parental stock resulted in the culling out of individuals which were more susceptible to the heavy metals. The more resistant members of the population (survivors) which have the ability to adapt to the toxicants were able to pass on the resistance to their offspring. The results are supported by other studies in the field which demonstrate high resistance in populations of organisms living in contaminated sites.