I got my hands on the turtle after a number of long fishing sessions, counted over 6500 casts to get it on my level 80 main. Also took me upwards of 9 hours to get the Dark Herring for my Salty title and 730 odd casts for Old Ironjaw.
Two weeks ago decided for a laugh to try and get fishing up on my level 34 Priest, started in Ironforge, got IJ in under skill 200, hit 200 grabbed some +100 fish hooks and set off to Howling on the boat, did an hour or so in the afternoon then went to raid, decided to go back to Howling and tried the lake near the UP meeting stone, got The one that didnt get away Dark Herring skill level 255, cast again got Herring + Suckerfish, cast again Sea Turtle, yep within 3 casts at level 34 obtained 2 very rare achievements, no clue how to upload pictures as yet but happy to upload it if I can figure it out, as you can see the difference between the 2 characters is a big one in terms of level and skill and once again shows that nasty word "Random" at its best.
Additional other info I got Sewer Rat cast 12 on main, cast 6 on another 80 Druid I have yet I know people on cast 2500+ no Rat, sometimes it just works sometimes it doesn't. My advice after the original 6500 turtle, don't fish for hours and hours, an hour here and there is far less stressful and enjoyable and one day it will pop, good luck and happy fishing ;-)
Things like the Rat and Turtle are what keeps us hooked(see what I did there....;p)
Silvasurfa 80 Human Paladin,
Silvashadow 34 Night Elf Priest,
Azjol Nerub EU.