Turtle

[Skyshot]

I managed to fish mine up from a Dragonfin Angelfish school at Lake Indu'le in Dragonblight. I was there for cooking mats and ended up getting a turtle for my efforts. You should see about trying to fish it up there. The schools seem to respawn decently fast. By the time you go full circle fishing from schools around the lake, the first one should be very close to respawn. If all else fails, the angelfish normally sell very nicely on the AH. On my server they can go for 50-95 gold per stack of 20. =)

[Slickrock]

The other option is to fish in Howling Fiord where the Herring come from, as it gives you a chance to catch the rare Herring for the fishing achievement. If you are NOT going for that achievement, or already have it, then probably fish in pools that net the most money (Dragonfin or Salmon, probably).

[Saturo]

A good, and not so known about place is the Frozen Sea! There will always be at least two chools on your minimap tracking, and it's surprisingly easy to get between the spots, even without mounting up.
http://www.elsanglin.com/fish/the_frozen_sea.html

[FuzzyDolly]

Well another 200 salmon today and still no mount. Tomorrow I'll try the Herring place since I don't have that other achievement.