Dictionary of Ichthyology  

yolk-sac larva = a fish larva which has already hatched from the egg but has not started feeding yet and still absorbs the yolk in the ventrally attached yolk sac. At the end of this stage in development, the yolk and oil globule(s) have been used up, the major organ and sensory systems required to capture and digest prey are functional, and the primordial pectoral fins are present. 

Salmo salar from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Salmonlarvakils.jpg

Canadian Museum of Nature