The method of ploidy level determination depends on the equipment, expertise of the lab or farm staff, and on the material studied (fish species, specimens' age and size, etc.). The combination of some simple techniques can be very helpful for fast, reliable and inexpensive assessment of effects of genome manipulations. Among indirect methods the nucleoli quantification seems to be the most convenient one, except that it is limited to species possessing one locus of nucleolar organizer regions (NORs) (rainbow trout as an example). Species with multilocus NORs (like brook trout) need another method, for instance sizing of erythrocytes nuclei. These techniques can be useful for identification of products resulting from new approaches in biotechnology applied to fish culture, as polyploidisation or hybridisation, for characteristics of transformation levels of organisms found in the wild or those produced due to experimental treatments.
