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A tagging method for very small fish

The ability to reliably identify individuals over time and across contexts is essential in numerous areas of science. There are a variety of well-established methods for uniquely marking individuals, such as using paint or dye, visible implant elastomer tags, numbers or barcodes glued to the animal, passive integrated transponders, and more. For some species, life history stages, and/or experiments, however, these existing tagging methods are not sufficient. Here, we describe the method we developed for tagging juveniles of the African cichlid fish, Astatotilapia burtoni, which are too small for the methods used to tag adults. We used fishing line threaded through the needle of an insulin syringe to tie a loop of line through the dorsal muscle of juveniles as small as 10 mm standard length. Unique color patterns on the line can be used to distinguish among individuals. The tag is compatible with normal locomotion and social behavior, discernible to the eye and on camera, durable enough to last at least months, and the juvenile can grow with the tag. For A. burtoni, which is a model system in social neuroscience, the lack of an appropriate tagging method for very small juveniles likely contributes to the relative lack of early-life studies, and the same may be true for other small species. We expect this method to be useful in a variety of species and will facilitate the integration of organismal and behavioral development into more research programs.