They get oxygen from the water. They cannot swim around because they have a large yolk sac attached to the underside of their body. This is the alevin's food source. Alevins live under the gravel in redds. Sometimes the water becomes silty.
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.