Officials say the invasive sea lamprey is gaining ground in Lake Superior and Lake Erie, while its numbers remain at near-historic lows in the other Great Lakes.
Sea lampreys are eel-like parasites that attack fish such as trout, salmon and whitefish. They invaded the lakes in the last century and decimated native fish until a poison was developed that brought them under control.
The Great Lakes Fishery Commission says treatment of key tributaries is keeping a lid on lampreys in Lakes Huron, Michigan and Ontario.
But their populations are rising in Superior and Erie for unknown reasons.
The commission says lampricides have been applied in several rivers flowing into Lake Superior. The effects won't be known until next year.
Work will continue to determine what's behind the Lake Erie upsurge.