Question

Assuming that the concentration of oxygen in the water is the same, why does freshwater fish die if it is released into the ocean?

Answer 1

Most freshwater fish are not adapted to saltwater conditions and cannot effectively osmoregulate. In freshwater a fish is hypertonic to its environment, meaning the fish is "more salty" and water continually flows into the fishes body via osmosis. In saltwater a fish is hypotonic or "less salty" compared to its environment. This meas water continually flows out of the fish's body. Fish gills are adapted to handle either situation, but often times not both. A freshwater fish would quickly become dehydrated after being placed in saltwater.