Nineteen pages into the hunt the shark finally takes the bait. Brody reels in the line to discover that the wire attached to the squid bait has been "neatly severed [and] chewed clean through". When a second line goes the same way as the first, Quint breaks out "a four-foot length of three-eights-inch chain." The scene made it into the movie, but, like many others, went through significant changes. In the movie Quint first hears the tick of the reel as the line plays out and straps himself into the fighting chair before the fish runs. Hooper thinks they have hooked nothing more than a marlin or a stingray, but is chastised when their catch bites through Quint's #12 piano wire and both he and Brody are thrown across the deck.
Die-hard Jaws fans know the number of the wire because they have seen the deleted scene on the DVD extras where the fisherman goes to Amity's local music store to buy a supply of it. Piano strings have to be tough because they take a regular beating - a piano is, after all, a percussive instrument. Even so, #12 has a breaking strain of only three hundred pounds, which is several thousand short of the weight of the three ton Great White.
In fact, Quint has no intention of hooking the shark and reeling it in. "All I'm trying to do is goose him a little and bring him to the surface." In the movie Quint says more or less the same line when he instructs Hooper to secure the floating barrels with a boat hook just before the shark lunges at the two of them.