Tracker (2001)

Cole spots a face on a milk carton of a missing boy, whom he recognizes from the video recording he took from the train station. He and Mel investigate and determine that an alien Odulian is inhabiting the body of the 15-year-old boy, and is luring unsuspecting teen street kids for his experimentation. The boy and another alien in a doctor's body are trying to extract chemicals for Zin. The Odulian, an aquatic creature, dodges their first attempt to capture him by taking refuge in Lake Michigan. Cole determines the alien needs certain elements to survive, and normal lake water won't suffice because of its high zinc content. Cole tracks down the source for the proper chemicals the alien needs, and finds the base where the alien is performing his experiment. The doctor escapes but Cole captures the Nodulian in a net to prevent him getting back to the water, then extracts the alien life essence.