Cast your line, don’t be fooled by the little “dwoop” sounds, you have a fish when you get a bigger sound with an exclamation point on your head. Click to hook the fish then, and this is the tricky part….you have to keep the little fish picture inside the green box.
You do this by moving the green box up and down using the left-click on your mouse with the fish until you fill the vertical bar just to the right to the top (any time the green bar is off the fish you lose progress on this bar and a depleted bar will cause the fish to escape). If you keep the fish icon fully in the green bar the entire time you will get a “perfect” notice, this gives you more points. As you level up and get better equipment your green bar gets much bigger.
Here is a tip: you will mostly need to TAP the mouse button to control the green box. If you PRESS the mouse button it will zoom your bar to the top, letting it go will drop the bar to the bottom. When the bar hits the bottom it will bounce, which can cause you to lose a fish, so be sure to use the mouse button to soften the landing. It will take lots of practice to get the feel of the tap/hold system so you can follow the movement of the fish, but it comes.
Most of all, don’t give up! It’s a hella frustrating mini game and some of the fish are a PITA, but it CAN be done for ALL fish. (Source: me, having caught all fish)
Once last point. You can get “treasure” icons, you have to catch those at the same time as the fish, but not lose the fish. You do this by having the green bar sit under the treasure icon until the bar over the treasure icon fills, then finish catching the fish. It’s best to try and get the fish bar ALMOST full, then snag the treasure, then go back to the fish before the fish “escapes” with an empty bar. This also takes practice.