The prospects of mass tagging of whitefish using otolith recording structures (review)

Sequential formation in the otolith of registration structures consisting of calcium-protein layers growing on the surface is the basis for using otoliths as a carrier. The rate of formation of these layers depends on the physical and chemical properties of the environment in which the organism develops. This makes it possible to actively influence the otolith structure during the formation of the tag. The existing methods of tagging otoliths can be divided into two groups: chemical and physical. Chemical methods are based on the deposition of fluorescent compounds — fluorochromes, which include forms of tetracycline and alizarin compounds — in the surface layer of the otolith. Fluorochrome introduction into the organism, with further transformation in the form of layered otolith deposits, is ensured by their ingestion with food, temporary immersion of eggs, larvae, or juvenile fish in the appropriate solution, or by injection. Chemical tags are recorded by the fluorescence of tagged layers of otolith at a specific wavelength. Thermal tagging belongs to the physical group of methods. The mark is formed due to the directed periodic change of temperature regime in this case. The unique alternation of light and dark concentric formations on a polished slice of otolith, appropriate to the periods of exposure of the organism to physical environmental factors during tagging, is analyzed in the identification of the mark. The listed methods of tagging have been tested on whitefish. The relatively small size of otoliths during embryogenesis and larval development, which technically complicates the registration of tags obtained by physical methods, is a specific feature of whitefish species. Tagging with fluorochromes, in particular alizarin compounds, is the most promising method for mass-marking otoliths of this group of fishes.