Silt can plug up the alevin's gills. Alevins do not like the light. If light penetrates into the gravel the alevins wriggle down further into the streambed or the incubator. Sometimes too many alevins end up in one area. They become too crowded and many of them die.
Many animals hatch from eggs.
Contrast and compare the hatching of the salmon to the hatching of a bird. Make a chart. When salmon eggs hatch they are known as alevins. They are still under the gravel. Alevins have a yolk sac on their underside.