Mackerel, tuna and herring are fish that live their entire lives in mid-water or just below the surface of the ocean. They are called "pelagic fish". The fish often follow the ocean currents, which are like big rivers in the ocean. The fish habitat in these areas can change rapidly due to the actions of the wind, rain, sun and air temperature. The fish follow these "rivers in the ocean" to areas that are suitable for them to reproduce and feed. They lay their eggs in the open ocean or shallow coastal water. When the eggs hatch, the young drift with the ocean currents. Out of millions that hatch only a few survive to become adults.