"Thermal pollution" is a relative term because, roughly speaking, one person's trash (undesired "pollution") may be another person's treasure.
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For example, a power plant may use water as a coolant and return it to a bay at higher temperature. This "thermal pollution" may kill fish that have adapted to particular temperature range. On the other hand, the warm water may benefit some creatures in the ecosystem, and "thermal pollution" may be considered "thermal enrichment". Manatees have been observed to cluster near power plant discharge sites during winter months.