What does the big fish do after it takes the bait?

From the text:

Nothing happened. The fish just moved away slowly and the old man could not raise him an inch. His line was strong and made for heavy fish and he held it against his hack until it was so taut that beads of water were jumping from it. Then it began to make a slow hissing sound in the water and he still held it, bracing [44] himself against the thwart and leaning back against the pull. The boat began to move slowly off toward the north-west.