I suspect the biggest (closest) concentration is in the Frozen Sea, just south of Borean Tundra. Sholazar Basin also has a lot of pools, fairly close together. Both are higher skill areas, which will tend to reduce the number of people fishing.
Of course there are locations such as Lake Kum'uya (near Warsong Hold in Borean Tundra) where pools generally respawn fast enough to allow one angler to go round and round the same lake. So you might not need a huge concentration. Likewise, it is reasonably effective to fly round the coast of Northrend, or along the rivers of Grizzly Hills.
There are other things to consider, such as who else is fishing, the value of the normal fish you are (mostly) catching, and whether you can (mentally) survive fishing the same handful of pools for days. Personally (over 6,000 pool catches so far - and no turtle), I have to keep on moving to new areas, just to break up the boredom...