By law, you are required to release all fish that you do not intend to eat. Northern pikeminnow is a native freshwater species that has predated on salmonids since the last glacial period. The predator-prey relationship is a selection force that has kept salmon populations strong, not just quantitatively but more importantly their genetic diversity. That balance has been tampered by overfishing for species that are more commercially desirable to human in the last century. To label certain species as a threat to the survival of salmonids rather than focusing on the true causes of their demise is ignorant. Killing a few northern pikeminnow may give you some warm fuzzy feeling, it does not do your beloved salmon any favour. If anything, it just frees up more niches in the ecosystem for more northern pikeminnow or other predatory fish to occupy.