When you say “the killing of fish”, you're referring to the definition of serious harm? I don't think you can possibly say beforehand in most instances what will or will not significantly harm a fishery. You can do things to assist an ecosystem that supports a fishery that absolutely nobody would predict would have deleterious effects. It doesn't happen very often, but it does sometimes happen that only with the wisdom of hindsight do you realize what happened.
Nonetheless, you can take a risk-based approach to these sorts of things. Uncertainty is king in fisheries management. If you don't have a system that's built around an admission of that, and tools for handling it in an open and transparent fashion, with some realistic limits put on it, you don't have a system that does the job, or can possibly be expected to.