Ask the trout of Bonnie Brook about their politics and they might tell you they are of the conservative persuasion. They’ll proudly describe growing up from eggs spread naturally by their parents. No hatchery trucks pulling up by the side of the road. No official opening day or closed season. No need for state interference.

What they don’t tell you is, by the nineteen-sixties the stream’s native brook trout had all but been obliterated by over-fishing and environmental degradation. To compens...