Fish do not 'feel pain' like higher animals do. Their brains are not set up with that type of nervous system (fish are the most primitive form of vertebrates, and were the first to evolve). Other than appearance and a few minor adaptions, they have changed little since the Cambrian Period (530 million years ago). They are not even conscious, little more than organic robots, really. When they struggle on a line, they are merely reacting instinctively, rather than from any deliberate sensations. A fishes nervous system does not even contain any specialized pain receptor nerve cells, like many higher animals do.

Compared to what happens to fish in the wild, on a daily basis (being eaten, and digested alive, torn to pieces, or used as a host for parasites....), being stuck with a hook is pretty small potatoes.