WordNet records no synonym and no antonym for conservancy. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a commission with jurisdiction over fisheries and navigation in a port or river
the official conservation of trees and soil and rivers etc.
Synonyms, antonyms and the broader and narrower relations for conservancy come from Princeton University’s WordNet. Full sources and licences.