Black walnuts contain chemicals called "juglins" that, as said above, have been used to stun fish. Although I have plenty of husk dust, I've never tried using it to poison fish since that is illegal where I live and I could explain the difference between an experiment and a crime to a judge. Juglins are found mostly in the husk of the nuts but also in the leaves and roots. There may well be lower amounts in the bark and wood that would probably leach out over time. I suspect that might take a while. You may not get much action the first couple of years but it is probably worth trying.