Ports rank among the most disturbed marine environments. Pollutants like trash, oil and toxins from antifouling paints tend to accumulate in enclosed harbors. Invasive species arriving in ballast water may displace native wildlife, while ship propellers, sonars and dredging cause noise pollution that can interfere with animal communication. There is thus a great need for effective ways to clean up harbors. One promising method is renaturalization, in which hardy species that can survive under these conditions are deliberately introduced to act as pioneers.