I hate to disagree with you, Mike, but, I disagree with you. Actually, freshwater fish are more prone to mercury contamination. Mercury in water, whether natural or industrial takes the form of methylmercury.
You are right to an extent, methymercury latches onto chloride in saltwater and doesn't degrade as easily as it does in freshwater where it tends to unionize with decaying carbon.
The age and size of the fish is the reason for saltwater fish to eventually have higher mercury levels than smaller, younger freshwater fish. However, freshwater contamination is trending down
