The fecundity of Gambusia affinis affinis, the common mosquito fish, that has inhabited a lake contaminated with radioactive waste for over one hundred generations, was compared with a control population. The calculated dose rate from the bottom sediments to the Gambusia population in this contaminated environment was 10.9 rads/day. A significantly larger brood size occurred in the irradiated than in the nonirradiated population, although significantly more dead embryos and abnormalities were observed in the irradiated broods. These results suggest that an increased fecundity is a means by which a natural population of organisms having a relatively short life cycle and producing a large number of progeny can adjust rapidly to an increased mortality caused by radiation.

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