After fish eggs spend a few months living in a cool gravel stream, tiny fishlings called alevin (or sac-fry) emerge from the eggs. These inch-long fish still carry a yolk sac twice their size. They remain in the gravel for several more weeks, absorbing their nutritious yolk and beginning to feed on bits of food that float through the gravel. One night, the alevins swim out of the gravel and begin feeding on microscopic water animals called zooplankton. They are now salmon fry.
Salmon fry develop some spiffy skin marks — vertical black bars, called parr marks. For tiny fish living among underwater vegetation, parr marks are great camouflage. They help the tiny fish blend in with stream-bottom vegetation instead of catching the eye of predators.
About the time that salmon fry become as long as your hand, they begin to lose their stripes and take on a sleeker appearance. They are becoming smolts, the teenagers of the salmon world. Teenage salmon have an irresistible urge to leave home and begin traveling. The signal is in their genes. The smolts head into the current and begin their long journey to the ocean. Smolts are definitely going through some major changes, called smoltification, on this journey: their bodies are changing so that they can live in salt water. The changes that occur allow the young salmon to begin maintaining their body chemistry in saltier water.
When the rivers in the Columbia River Basin were running free, smolts from high in the Sawtooths could reach the sea in about eight days. Today, they must travel through water slowed by eight huge dams, and they must get through or around those dams somehow. Their fast-water float downstream now requires several months.