So the way that it is supposed to work, at least if you look at it like the fishing tournament, is that you will still be able to fish just like you can today, just flipping out into the briny deep to catch whatever comes your way. The drop rates for more valuable fish may go down quite a bit, but Darkclaw Lobster and other commons are going to be just as plentiful in the open sea as they ever were.
There will also be fishing spawns, like in the tournament, that will be much like mining. The spawn will likely be labelled, as it is in the tournament, "School of X". Your drop rate for that type of fish/eel/whatever will likely be in the 85% range, just as it is in the tournament, with the possibility of getting other valuable catches such as stranglekelp, lockboxes, etc.
So if you just want to feed your kitty, keep on keepin' on. But if you make a living fishing stonescale eel at night (like I do, I don't like to farm Tyr's Hand or any of that when I can make 30g+ per hour fishing and drinking cold beer) then you may have to make some adjustments to your gameplay. There will likely be eel spawns up and down the coast in the zones where that type of fish is currently caught, and there will be some level of competition for those spawns. Raiding guilds may have to go out and get a guild fisherman to round up stonescale for MC/BWL/Ony runs, as the supply may dwindle a bit with lower drop rates and low spawn rates. On the bright side, pricing may actually go up a bit due to demand.
Fishing is currently nerfed. It has been since 1.1. For those of us who enjoy fishing as a significant part of our gameplay and income, this is high on our list of things for the 1.9 patch, and I for one am glad to see it coming. Sure I won't be able to drink as many beers as I have been up to now while fishing, but I could spare to lose the beergut anyway.
Anyway, just my two pesetas.