Some immediate challenges in this regard are studies
on the possible effects of xenobiotics on population heterogeneity through selection pressure. Because of the potential long-term impacts on both individuals and populations, better definition of normal variability in reproductive parameters and more comprehensive tem- poral data are needed, so that potential trends can be identified more readily and reliably, and hypotheses tested regarding their causation Campbell and Hutchinson, . A comprehensive method of data collection should involve extensive studies of highly exposed fish populations, in addition to more information on normal population variation, as well as regional and seasonal variations. The persistent and bioaccumulative properties of the lower potency endocrine modulators, coupled with their widespread use, certainly warrants continued investigation. Furthermore, more attention should be directed towards the examination of potent natural and synthetic estrogens in the aquatic systems, especially those originating from human female contraceptives such as ethinylestradiol