Indeed, and if you really want to make money fishing you'd probably have to either just fish for fish feast fish in WG, or various locations in northrend for just the high level fish, or travel around all areas to not saturate any part of the market too much.
Or simply fish in multiple locations at once, which would be handy and cool, though would make travel more tedious and less convenient, but would at least keep each char's bobbers and splashes from interfering with each other, having only one bobber on each window makes it easier to see which ones got the bites.