There is a "insectide" chemical called Endosulphen.
It is very effective for knocking off caterpillars in leaf crops (eg lettuce)etc, and has a short withholding period.
I am led to believe it has a brilliant hit rate in knocking off aquatic life too.
I am also led to believe you do not need very much of it, for example, rinsing out an empty Endolsulphen tin works wonders in cleaning out a dam.
But as I said, that is just what I have heard and I could be completely wrong.
Cheers
Phillip