I'm fairly certain (although I've been known to be wrong before) that successful casts are the only thing that matter as far as skill ups are concerned unless this has changed in the last couple patches.
Spending all day fishing in Durotar/Elwynn Forest will be just as good as fishing in Dalaran/Northrend waters provided of course that the casts are successful.
The great thing about Duro/Elwynn are that you have literally no chance of missing on a cast as long as your skill is >20.
Looks like
El says "I'm right".
reprinted from El's Extreme Anglin' (great fishing website):
To improve your fishing skill, you simply need to catch something. And wherever you fish, you will always catch something.
1. You will need to be at least level 10, feeling the Wrath of the Lich King, with about 50 gold.
2. Buy a basic Fishing Pole. These are sold by many trade goods vendors and fishing suppliers.
3. Travel to Howling Fjord: Horde to Vengeance Landing, Alliance to Valgarde. At lower level, Horde travel by zeppelin (from Undercity), Alliance travel by boat (from Wetlands).
4. Find the fishing trainer. For Horde, Angelina Soren, on the docks next to the sea, with an umbrella. For Alliance, Byron Welwick (illustrated right), by the water, with an umbrella, behind the flight master. Learn Apprentice fishing.
5. Fish open water next to the fishing trainer. To learn about basic fishing techniques, please read Your First Catch. Between the fishing skills below, learn a new rank from the trainer:
* 50-75: Journeyman.
* 125-150: Expert.
* 200-225: Artisan.
* 275-300: Master.
* 350-375: Grand Master.
6. And that's it! Overall, half your catches will be junk items. But that does not matter, because every catch contributes to improving your skill. Both trainers in the Howling Fjord are next to water, and train all 6 ranks. So you won't waste time moving.