Fishing skill is a bit different than some of the other professions. You could fish in the pond in your starting area, assuming it has one, from 1 to 300 skill. You don’t need to move to higher level zones to increase your fishing skill. However, all you’ll be fishing up are the same low level fish. Higher level fishing areas allow you to fish up higher level fish. So if you need oily blackmouth you aren’t going to fish any up in your starting area, even if you are gaining fishing skill.
So you have a couple of choices. You can fish in low level areas until your fishing skill is high enough to fish in the areas that have the fish you want. Or you can use fishing poles and fishing lures that give you a bonus to your fishing skill and thus fish successfully in those higher level zones much earlier.
Also be aware that when you go to fish in the highest level areas you’ll be required to use the best pole and the best lures even if you have 300 fishing skill. My rogue had 300 skill and a fishing pole with +20 fishing. He still was not allowed to cast his line in the Sea of Storms. With a +50 lure he could fish but 3 out of 4 fish would get away. With a +100 lure, a total of 420 fishing skill, he can fish reasonably well there.