Some quick notes/tips for Freeplay:-
1. The fewer pieces you cut the fish into the more you get for the fish and the less room it takes up in your storage but it will take longer to bleed out and die.
2. Try not to cut the dimmer/thinner lines, just cut the brighter/thicker ones in the center.
3. Be sure to keep other fish from eating your catch while you wait for it to bleed out.
4. The biggest fish down very very deep usually require their core to be cut completely apart in three sections. It's best if you can avoid cutting the dimmer lines when doing this but often it's very hard to do so.
5. The green and blue dots of the fish are multipliers which are super important for getting a really high score.