A few minutes ago, I just fished up the Dark Herring, so I now have my "One That Didn't Get Away" achievement! I was fishing in Valgarde (mostly in open water; fishing in schools only when they spawned next to me), and he was my 266th catch/250th cast (not including the less than 5 "Your fish got away!"s). I fished him from a Fangtooth Herring school. Fishing skill is maxed at 450 (and +20 from my pole), and I was not using lures.
The catch breakdown is as follows:
Bonescale Snapper - 108 (40%)
Fangtooth Herring - 95 (35%)
Glacial Salmon - 39 (14%)
Pygmy Suckerfish - 16 (5%)
Crystallized Water - 5 (1%)
Reinforced Crate - 2 (<1%)
Dark Herring - 1 (<1%)
It technically took 250 casts, as the Pygmy Suckerfish is looted alongside other fish.
Best of luck to everyone!
Now, all I need to do is manage to finally win the Extravaganza, and I shall be Salty Pwnica! :)