Department of Natural Resources
Anglers often notice distinct yellow marks that are circular in shape and are often found on the belly near the pelvic fins. Because fisheries professionals use artificial dyes or ink to mark fish, these circular-yellow marks found on Great Lakes Chinook salmon are often misidentified as an artificial mark. However, these marks are not artificial marks used by fisheries professional to track salmon in the Great Lakes, but instead they are naturally occurring in Great Lakes Chinook salmon. These circular-yellow marks are usually noticed in late July through early September as a salmon undergoes the process of maturating and preparing to spawn. In general, there is a good deal of variation in Chinook salmon coloration especially as they start this metamorphosis and are staging prior to running the rivers. What is really unique about these marks as the fish starts to turn from a silvery-white to a yellowish-brown in color is that the areas beginning the change can appear as a perfect yellow spot or circle about the size of a quarter. Again, these spots are naturally occurring pigmentation and not an artificial dye or mark used by fisheries professionals.
See below for photographic examples of yellow spot.