It takes two to five years for a manatee to give birth to a calf. manatees are sometimes caught in fishing nets or hit by boats. what effect is human activity having on the manatee? a) manatees are moving to safer waters to breed. b) manatees are changing their habits to avoid boats. c) their population is unaffected because manatees reproduce. d) their population is decreasing because the rate of reproduction is low.