It is customary to tip your crew 15-20%. It should be based on the professionalism and efforts of the crew, not how many fish are caught. On our charters, all crew members take fishing very serious. The crews work very hard to catch you that trophy, and/or as many fish as possible; but rarely, due to numerous external factors (moon, tide, weather, etc), fishing can be slow regardless of crew efforts.
The tips are split evenly between the crew, captain and mates, and it's an important part of their income.