Manatee Response to Interruption of a Thermal Effluent

Manatees (Trichechus manatus latirostris) aggregate at warm industrial effluents during winter in Florida, raising concern that cessation could expose individuals to cold stress. We determined the abundance and distribution of manatees in the Caloosahatchee River system in the winters prior to and during a temporary cessation of the heated effluent at a power plant near Fort Myers, Florida. Manatee abundance did not change significantly between winters (1984-85), as measured by 2 indices derived from aerial surveys. Distribution changed significantly among 3 types of survey strata. When the heated effluent was off, manatees gathered in an area of deep water that cooled more slowly than river and bay water. Most, but not all, manatees returned to the discharge area when the heated effluent was restored. Because the manatees apparently had no other short-term alternative, the decision of the power company to resume discharge of heated effluent was warranted and averted probable mortality that otherwise would have occurred. Changes in operation of power plants may influence indices of manatee abundance based on winter counts at power plants.