I gather that the "other identifying characteristics" referred to in the news report were probably concrete-scraped pectorals. While larger trout do suffer the effects of antimycin or rotenone poisoning more slowly, I find it really hard to believe that a fourteen incher would have gone unnoticed by previous shocking crews (unless this was the first time this stream was shocked)?
It's doubly distressing in that it is now June, and that sexually mature rainbow may already have spawned. I am sure Steve and his crew are just sick about this.
My solution to horse traffic is more pragmatic: (1) make them wear bags like the horse taxis have to in every major city (that will cut down on the besmeared trails) and (2) limit them to a handful of routes. They may already have regs on that; I have never tried horsebacking a trail.
As far as stupidity goes, I saw a study the other day. On one wall of a building, researchers posted a sign saying "Please do not write on this wall." On the other they wrote "Writing on this wall is FORBIDDEN!" No one wrote on the first wall; the second got graffitied heavily. Some people will do a thing just because they've been told not to.