Brian tried for a long time to catch fish. He tried using a spear and a bow at different times too. After being stranded in the wilderness for 47 days, Brian has had plenty of trial and error experiences with surviving. He had luck with small birds and berries, but fish were a tough prey to capture. In chapter 13 Brian recounts his first success at putting an arrow through a fish.
Finally, after hours, he stuck the arrow down in the water, pulled the bow, and waited for a fish to come close and while he was waiting he noticed that the water seemed to make the arrow bend or break in the middle.
Of course—he had forgotten that water refracts, bends light. He had learned that somewhere, in some class, maybe it was biology—he couldn't remember. But it did bend light and that meant the fish were not where they appeared to be. They were lower, just below, which meant he had to aim just under them.
Brian finally was able to spear his first fish with an arrow because he relearned a valuable physics lesson about light. Water refracts (bends) light. Water is a denser substance than air, so water slows light down. Because light is a wave, different parts of the wave exit the water at different times. This causes the light to bend, which makes objects in the water appear to be in locations where they are not. Brian adjusted his aim to below the fish, which is not where the fish appeared to be, but it is where the fish actually was.