For the sake of completion and humility I am going to keep this post up, but I've decided I don't hold the opinion below all that tightly. I'm also willing to accept that I might be wrong and that Jim (read the comment below) is more likely to be more informed.
Flakey? Moi? Yup.
Go to the Wikipedia entry on DDT. When you get there read the section on the environmental effects of the pesticide. In particular, read the piece on the effects on humans. Surprising, huh? Then go and read the section on the effect that DDT has on malaria.
So DDT might kill a few fish, but think about the number of human lives it would save.