Santiago suspects that the fish on his line is a marlin. How does he know this?

Santiago notices a bite on his hundred-fathom-deep line. The first bite is hard, and the stick to which the line is connected drops sharply. The next tug is more tentative, but Santiago knows exactly what it is. "One hundred fathoms down a marlin was eating the sardines that covered the point and the shank of the hook where the hand-forged hook projected from the head of the small tuna" (41). Encouraged by a bite at so deep a depth so far out in the Gulf, Santiago reasons that the fish much be very large.