MINNEAPOLIS (AP) — The walleye population in Mille Lacs Lake has fallen to its lowest levels in 40 years, while the lake’s larger walleyes may be having trouble finding food.

Officials aren’t sure what’s happening to Minnesota’s most popular walleye fishery. But Department of Natural Resources fisheries research manager Don Pereira tells the Star Tribune they’re concerned.

Causes could be include the harvest regulations governing sport anglers and Chippewa netters, the explosion of zebra mussels in the lake, improved water clarity or a potential lack of prey fish such as perch and tullibees.

Pereira says it’s too early to say if the DNR might tighten regulations on the lake in east-central Minnesota.

Business owners say further restrictions could undercut tourism.

DNR and tribal fisheries researchers will discuss the lake next week.