Alevin

An alevin is a recently hatched salmon or trout larva that retains a large yolk sac as its sole nutritional source. Alevins remain hidden within the gravel of the spawning redd, surviving entirely on yolk reserves until they are large enough to emerge and begin feeding exogenously. Alevins have poorly developed eyes and swim bladders; they phototaxically avoid light and remain in the dark interstitial spaces of gravel. The alevin stage lasts 6–14 weeks depending on water temperature (longer in colder water). Emergence from the gravel coincides with yolk absorption and the onset of exogenous feeding (the "swim-up fry" stage).