Fish biologists mark fish with a variety of techniques, but always with the same goal of being able to identify previously captured fish, usually for the purpose of estimating the size of a population or efficiency of a capture technique.
This Chinook salmon fry was just marked by injecting a photonic dye into its caudal fin. Although it looks extreme, very little dye is actually injected into the fin tissue, leaving only a small mark that can be seen with careful examination of the fish. The dye is temporary and experiments have shown that it does not substantially affect fish health or survival.
Photo source: FISHBIO