WordNet records no synonym and no antonym for whitefish. 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.
any market fish--edible saltwater fish or shellfish--except herring
flesh of salmon-like or trout-like cold-water fish of cold lakes of the northern hemisphere
silvery herring-like freshwater food fish of cold lakes of the northern hemisphere
Synonyms, antonyms and the broader and narrower relations for whitefish come from Princeton University’s WordNet. Full sources and licences.