A unique, bluish-colored strain of walleye appears to be increasing in number in Canada, and expanding its range into the northern United States. “We’re finding it in Canadian river systems where it was not present a decade ago, and receiving more reports from the northern U.S.,” says Dr. Wayne Schaefer, a professor of biological sciences at the University of Wisconsin-Washington County.

A lifelong fisherman, Schaefer became interested in blue walleyes more than a decade ago after catching some in a stream near his cabin at Ear Falls, Ontario. Along with grad student Mark Schmitz, UW-Milwaukee, and colleagues at the universities of Iowa and Toledo, Schaefer is spearheading studies on the strange fish. He also maintains a website, bluewalleye.com, where anglers can submit reports of blue walleye catches. To date, the fish have been documented from northern Manitoba and northwestern Ontario to westcentral Quebec, and south into Minnesota, Michigan, and Wisconsin.

Not to be confused with the “blue pike” (Sander vitreus glaucus) of Lake Erie, which the U.S. Fish and Wildlife Service declared extinct in 1983, the blue walleye is a variation of the standard, garden-variety walleye (Sander vitreus vitreus). It is characterized by blue shading on the upper body, most noticeably on the dorsal fins and top part of the tail.

The blue walleye owes its cobalt cast to a lack of yellow skin pigment and the presence of a blue protein in the mucus coating its skin. This protein, which Schaefer and the collaborating scientists who discovered it dubbed “sandercyanin,” does not affect the health or taste of the fish. (Sander is the genus name for walleye and cyanin means blue in Greek.)

Though they can’t say for sure what purpose sandercyanin serves, researchers theorize it may aid in shielding the fish from the sun’s ultraviolet (UV) radiation. “Sandercyanin contains biliverdin, which is a byproduct of the hemoglobin carried by red blood cells,” says Schaefer. “One factor that causes hemoglobin to break down into biliverdin is exposure to UV radiation.”

Because biliverdin is known to act as a sunscreen of sorts in some species of animals, it is possible that, in an ironic twist of science, blue walleyes could be protecting themselves from the sun by excreting excess amounts of the very substance caused by its radiation. “That’s the hypothesis of the biochemists and geneticists who have worked on this project,” says Schaefer. He notes that production of sandercyanin is greatest in the summer months, corresponding to peak periods of sunlight, and the blue coloration is found only on the fish’s dorsal regions—which receive the highest UV doses.

An increase in the number of blue walleyes may be due to decreases in the ozone layer that protects Earth from UV radiation. “They may have always had the ability to produce sandercyanin and are just needing it now,” says Schaefer. He notes that some normally colored yellow walleyes also produce blue mucus, but it is most noticeable on the albino walleyes lacking yellow skin pigment. The albinism is likely the result of a recessive gene.

Schaefer also tracks reports of blue pike, and hopes to one day help discover a surviving population of the near-mystical fish—which was once abundant in Lake Erie but fell victim to overfishing and pollution. Anglers can aid his efforts by reporting blue walleye catches on his website. “Get a photo of the fish, and scrape the skin along the back with a knife to see if the mucus is blue,” he says, adding, “If the mucus is clear, it may be a blue pike. Take a skin sample or fin clip for DNA testing.” Recently, he received a report from an Ottawa River angler who caught a bluish walleye that had clear slime. “That was exciting, except he didn’t get a photo or samples,” says Schaefer.

Along with gathering blue walleye field samples and reports from fishermen, the researchers plan further study on the link between the blue protein and UV radiation.