To catch a fish, it has to bite your bait.

When something bites, you can then catch it, or lose it. If you catch it you get X amount of SP, and if it gets away you get half that.

So at a grade 10 spot, you'll get 300 SP for catching, or 150 SP for getting a bite and losing it.

If nothing bites, or if you lose the fish before you get a chance to jig, you get no SP.