Why does Ishmael discredit the work of more than half of his listed experts on Cetology?

Ishmael is more interested in the symbolism and mythic idea of the whale rather than scientific jargon by people who have never lived among the creatures. The effect of this description of cetology, however, has an opposite effect; instead of giving a clear indication of the Sperm Whale, the chapter on cetology contributes to the idea that the sperm whale is something difficult to study and define. This lends credence to more symbolic definitions of the white whale that is the novel's title character; since it is more difficult to define the whale in scientific terms, the whale thus lends itself to more vague and creative symbolism.