Fingerling is a broad term applied to a juvenile fish that is about the size of a finger.  The definition of what length classifies a fish as a fingerling varies between species, but the Food and Agriculture Organization (FAO) generally defines a fingerling as ranging in length from 10 to 15 centimeters.  

Fingerling size fish (photo credit: Food and Agriculture Organization).

The term fingerling is most often used in aquaculture and fish stocking programs because fish at this life stage are more physically developed and better equipped to survive the harsh outdoor environment than earlier life stages.  Most fish stocking programs stock fish at this life stage in order to increase the survival rate of the newly released fish.(fishionary.fisheries.org/fingerling)