Salt water is rich in minerals. Salt water fish are built to dump minerals quickly from their body systems to prevent overload. If these fish are put in fresh water, they continue to dump minerals and will rapidly die from mineral deficiency.
Fresh water is normally poor in minerals. Fresh water fish are built to absorb minerals to prevent deficiency. If these fish are placed in salt water, they continue to absorb minerals and will quickly die from a mineral overload.
Salmon and similar fish that move between salt and fresh waters undergo metabolism changes in their body systems to take account of the change in mineral density.