Looking at the patch history of WoW it states them being introduced in 1.9.
http://wowwiki.wikia.com/wiki/Patch_1.9.0http://wowwiki.wikia.com/wiki/Patch_1.9.0
http://wow.gamepedia.com/Patch_1.9.0
"Schools of fish have appeared around the world that can be fished from a few times before being fished out. These schools tend to contain valuable fish types such as Oily Blackmouth or Firefin Snapper. In some places, wreckage can be fished from for even better rewards."
1.11 added in more schools to Azshara.
