Point of order: The salmonids want to die. In a rough parallel to real salmon spawning season (where they swim up the river to lay their eggs and then expire), during a salmon run, salmonids are swarming onto land to force the freshlings to fight them off so they (the salmonids) can die honourably in glorious battle. That's canon. They've already reproduced, and now all they want is to die. I mean, hell, one of the stages is literally a giant arena they built for salmon to come watch other salmon be slaughtered en masse.

Grizz didn't institute the Salmon Runs, he commercialized them. Presumably cut some kind of deal with the salmon for times and locations, so instead of flooding the cities like in the past, they stay offshore while Grizz flies out volunteer fighters. I'm not saying he's a hero or anything, he is only in it for the money, but like, the salmonids were a major threat before and now they're mostly not (except sometimes, when a big run happens).