In The Old Man and the Sea, how does the marlin's behavior change on the third day?

Santiago has spent two days holding the line while the
marlin swims in a straight line, tiring itself out. The marlin is so big that Santiago
must be pulled behind it, and cannot pull it in until it is dead. On the third day, the
marlin begins to circle, showing that it can't keep swimming against the
current:

He could not see by the slant of the line
that the fish was circling. It was too early for that. He just felt a faint slackening
of the pressure of the line and he commenced to pull on it gently with his right
hand.
(Hemingway, The Old Man and the Sea, Google
Books)

This behavior proves to Santiago that the
marlin is on its last reserves of strength, and that it will soon come to the surface
where he can harpoon it. The marlin is strong enough to pull the boat, but it can't pull
forever without resting; the circling behavior shows that it is losing strength because
Santiago never lets the tension of the line go.