It has to do with freshwater habitat needs. Chum and pinks migrate to sea, or estuaries, as fry, only months after the eggs were laid in the gravel. They also tend to spawn lower in river systems. Steelhead and Chinook migrate as smolts, much larger and older, up to a couple of years. Since they spend much more time in freshwater, steelhead and chinook are more dependent on a healthy freshwater habitat.