Stunning Display

The quest giver is throwing fire balls to the lake, when they hit, they will stun the fish, collect them. Make sure you collect the stunned ones, if they're moving it won't count.

You can pick up the fish while mounted. So you can fly around, if you want to try and get close to the water. Or, you can use a sea turtle if you have one. A the very least you can outrun the other players.

I preferred the use of the turtle as with flying mounts I had a tendency to overshoot where the fish were. Not good when there's a dozen people questing.

Edit: I also recommend using a water strider if you have one, this is by far the most optimal way of picking up the fish.

"As an added bonus, my method ensures they're already cooked! If that's not a marvel of modern efficiency, I don't know what is."
-Ren Firetongue, leveling up fishing AND cooking at the same time!