Salmon eggs hatch in fresh water. The fish then migrate to the ocean (a hypertonic solution) and, after several years of feeding and growing, return to fresh water to breed. How can these organisms make the transition from fresh water to ocean water and back to fresh water?

Different gill cells are involved in osmoregulation in freshwater than in salt water

Explanation:

Salmon eggs hatch in fresh water and then they move towards ocean (hypertonic condition) but after several years they return back to the fresh water for breeding.

This is possible only because the cells involved during fresh water is different from the cells involved in the marine water. Two different types of gill cells are found in the salmon which helps in the transition of the fishes into two different water.

These different cells allows the regulation of the body internal environment( osmoregulation) by maintaining a proper electrolyte balance.

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom