Map of Great Lakes showing the difference in their treatment cycles of sea lamprey among streams. Category 1 streams are treated on a 3, 4, or 5 year cycle. Category 2 streams are somewhat variable in their sea lamprey treatment. Category 3 streams are highly variable in their sea lamprey treatment, meaning the years between treatments are not consistent. Category 4 streams are streams where sea lamprey have been found, but no longer inhabit the area and the streams are no longer treated.