I think most people are mistaken when it comes to how fish affect an in-game economy. From my understanding fish ruin the economy when they vendor for a decent chunk of change. This affects the economy by bringing more gil in to circulation, devaluing what's already there (Hyper-Inflation, this happened in FFXI at one point as a result of RMT, not necessarily from fishing, but from a large influx of gil). RMTs capitalize on this because they use bots, and it's very simple to just vendor things. How do you reduce the affect on the economy? Simple, reduce the price that you can vendor the fish for.

I think most of you are taking "It won't affect the economy" too literally. Making fish desirable on the AH does not affect the economy in the way you are thinking. At most, it'll lower the cost of things by creating alternatives.