Officials Emptying Reservoir To Eliminate 'Voracious Predator' Before It Wipes Out Several Endangered Species The complete draining of a western slope reservoir is underway in an effort to rid the small lake of northern pike before the illegally introduced predators devour four species of endangered fish that have been recovering there for more than three decades, CBS Denver reports. The northern pike were first spotted at the Mack Mesa Reservoir by anglers last fall, according to the Colorado Parks and Wildlife. CPW officials conducted an immediate sample count and removed several adult northern pike. Additional fish were later discovered during a time when northern pike are known to spawn.