It is quite true that sea lions and especially seals will go for an easy meal. Even when fishing from a pier or dock, those darn things will grab your salmon. I've had it happen such that the second after I "hooked" a salmon at the surface, a seal or sea lion gobbled it right up.
I used to go smelt netting with my uncle and I always will remember the time when a monster sea lion made a big hole in our net.
I will be a devil's advocate and argue that marine mammal predation on recreationally valuable species is a perfectly natural occurence that doesn't warrant the shooting of seals or sea lions. Mind you, I'm no seal/sea lion lover, I've merely come to accept the fact that we must co-exist with them.
If we started killing seals and sea lions, the food chain would be disrupted. Orcas would be present in fewer numbers, so too would sharks due to decreased prey abundance. The theoretical result would be that salmon will rebound greatly. However, the oceans and streams can only support a certain amount of life before populations start to crash. When you eliminate a salmon's predators, then the salmon becomes the king of the sea and what starts happening is that the baitfish stocks start dwindling. Baitfish also play an important role in the food chain. They consume both phytoplankton(plants) and zooplankton(animals). If there are fewer baitfish, then there will be more plankton around. When phytoplankton dies, the process of decomposition sucks oxygen out of the water. So basically something that affects the top of the food chain will affect the bottom and vice versa. Nature has always found a way to adapt. Present salmonid predator populations are either balanced or in the process of balancing out. Taking the life of any creature has its consequences whether "good" or "bad". So take your pick, kill the marine mammals or leave them be. Either option will leave a profound impact.
Don't get me wrong, I feel sympathetic for anyone who has had his/her day broken all because of "that darn sea lion".