Fin condition was examined in 100–500 g hatchery-reared rainbow trout sampled in a large range of environmental conditions of an experimental farm. A method that is simple to use, requires little training, and does not need costly sophisticated equipment or reference to a model is proposed to assess fin erosion (defined as a decrease in size and profile alteration of fins). A procedure to reliably measure the length of the longest ray present in two selected fins (dorsal and pectoral) is described. Fin general profile is assessed using five levels of erosion based on the precise rank of intact or eroded rays to limit subjective bias. The suitability of the two descriptors of fin erosion under laboratory, farm or field conditions is discussed