At least 50 percent of chinook salmon found in Lake Michigan over the last five years have been naturally reproduced, according to the Milwaukee Journal Sentinel. Chinooks have shown that they are very good at producing large year classes of wild fish, thanks to Michigan streams they spawn in.
Of course, the ratio of stocked fish to wild ones varies depending on where sampling is being carried out. Scientists at the U.S. Fish and Wildlife Service and the Wisconsin Department of Natural Resources are looking to reduce the confusion, however, by using adipose clips and wire tags to track stocked populations of the fish.
These are neat because they allow fisheries scientists to more easily determine if a fish was stocked in their investigations. But the tags also allow anglers to know if a fish they’ve caught was stocked or reproduced naturally, as well as contribute to fisheries research in Lake Michigan.
U.S. Fish and Wildlife experts say the best way for a fisherman to tell if a fish was stocked or not is to look for the adipose fin that sits between the dorsal fin and tail. If it’s not there, the fish was likely raised in a hatchery.