Moby-Dick never has just one of something when it could collect the whole set. It doesn’t have just one epigraph – it has 80. Literally 80. We counted. We won’t reproduce them all here, but they’re in the preface titled "Extracts," which appears in most editions of the novel. If your edition doesn’t have it, you can read it in Project Gutenberg's free e-text.
Oh, and before that "Extracts" section, there’s another preface titled "Etymology," which discusses the origin of the word "whale" and the different words for "whale" in a variety of languages. You might want to take a look at that one, too.
Melville introduces his collection of 80 epigraphs with two paragraphs describing how they were compiled by a sub-sub-librarian who "appears to have gone through the long Vaticans and street-stalls of the earth, picking up whatever random allusions to whales he could anyways find in any book whatsoever, sacred or profane" (Extracts.1).
Of course, this is totally made up; Melville himself found all the quotations and put them together. All this means is that, when the author sympathizes with his imaginary sub-librarian, he’s actually saying, "oh, poor me, I had to work really hard to find all this stuff, which you’re probably not even going to care about."
In the opening paragraphs about the sub-sub, Melville also makes a claim about the purpose of the quotations: "these extracts are solely valuable or entertaining, as affording a glancing bird’s eye view of what has been promiscuously said, thought, fancied, and sung of Leviathan, by many nations and generations, including our own" (Extracts.1). Like most claims by the author or narrator of Moby-Dick, this one is both helpful and misleading. On the helpful side, Melville is certainly trying for "a glancing bird’s eye view": the quotations begin with the Bible and include everything from accounts of sea voyages and naturalistic facts to uses of the whale as a literary metaphor and an example of "W" from a child’s primer.
On the unhelpful side, usually the point of an epigraph is to make a reference to give a piece of literature a new context, metaphoric resonance, or association, Here, though, the point is to show how many possible associations there are with the word "whale," and, by invoking all of them, to expand, rather than limit, the whale’s symbolic potential. Melville’s quotations show us that the whale means everything and nothing.
Melville is really upfront about this: if we, as readers, expect him to give us a little guidance and direction, we’d better get used to disappointment. Given the choice, Melville will always rather complicate than streamline. Is Moby-Dick best interpreted through religious symbolism? Well, there are Biblical epigraphs here. Is it best read as an adventure story? There are quotations from travel narratives here. Is it best read as a sort of novel version of a nature documentary? There are quotations from field guides here. You get the idea.
By the way, Melville didn’t just transcribe quotations as he found them. In some cases, he edited and revised his selections to make the references to whales more obvious, so you can’t accept this list of extracts as authoritative without checking up on them.
Both the "Extracts" and "Etymology" sections present us with little fake stories of the researchers who supposedly put them together. At the beginning of "Etymology," Melville makes up a character, an usher in a grammar school, who compiles the list of different words for "whale" and what they mean; the usher "loved to dust his old grammars; it somehow mildly reminded him of his own mortality" (Etymology.1). At the beginning of the "Extracts" section, of course, there’s the character of the sub-sub-librarian.
As we hinted at the beginning, these characters become figures for Melville himself as the author-compiler of Moby-Dick. All the information he sews together only works for Melville when he can attribute the research to one of his fictional characters. Every definition in Moby-Dick has someone who wrote it or is reading it, while every quotation belongs to someone who finds it relevant. In Moby-Dick, facts can only be interpreted through character.
We imagine Melville reading something with a traditional epigraph – one quotation alone on an otherwise blank page – and demanding, "Who put that there? Why? Why that one? How did they find it? How did they feel about it? What kind of person decided to use it? If I wrote a novel, I wouldn’t leave these questions unanswered!" He’s nosy that way.