I understand that, once you hit your ESA listed fish limit, you have to stop fishing for the non-listed target fish.

My question is whether the target fish population is practically unlimited.

If it's not, then there is a chance that a method that greatly reduces the rate of non-target fish take would result in exhaustion of the target fish resource *before* you hit the ESA-listed fish limit.

I'm ignorant regarding the actual numbers involved. But, for a probably-not-realistic hypothetical example, if seine netting reduced the rate of bycatch to one ESA-listed fish for every 20,000 target fish taken, and the ESA-listed bycatch limit is 20,000 fish, then you would exhaust the target fish limit before exhausting the bycatch limit, unless there are actually 400,000,000 target fish available for harvest.

So, my point is, you can't just assume that the ESA bycatch will be the limiting factor as a theoretical matter. Maybe the actual numbers work out so that it will be the limiting factor in most cases, but that's not obvious without more analysis of the numbers.