In the Seattle area we've had those warnings for years now, in regards to eating salmon, halibut,and rockfish caught in Puget Sound.
My take is that most salmon are migratory, and return for spawning up the rivers in late Fall. Some species of salmon do put on a pound per week upon return.. but just catch them early for good eats. LOL Halibut on the other hand are a long-lived fish, so I'd keep portion size very small for any Halibut that weighs over 50 lbs. From what I've read, the mercury content greatly increases as many large breed fish get older and obviously larger.