I'm not sure about the effects on trout fishing, but I seem to remember that in the past few years there have been concerns in Long Island Sound about the decline in lobster numbers. The main culprit was suspected to be runoff that included the toxic residue from mosquito spraying, and the spraying was in response to the WNV cases that were popping up in CT (avain, not human).
For a while I didn't buy into this, but then I read an article somewhere showing data that the toxins were VERY likely to blame with the lobster issues. Being a research scientist, I'm very skeptical of data, but I found this piece to be compelling. I just wish that I could remember where I read it.
Whether or not it's true, I'd say that it's a safe bet that this spraying is affecting the environment on many more levels than we realize. I could go on and on about viruses and their mutation rates and eventual resistance to toxins, but that's a whole other animal. Suffice it to say that nature has a way of taking care of things.