He finds a survival guide and learns how to fish, quickly realizing that he does not have the luxury of being a vegetarian anymore. This just goes to show how the mind is able to preserve itself even under terrible circumstances.
The theme of survival is not really stated implicitly, but is a very strong and noticeable theme in the book. While interpretation of this story is left open to the reader, one theory interprets the animals as a defense mechanism used by Pi in order to survive the conditions of being lost at sea while preserving his sense of morality.
Pi told his journey in two versions, an animal version, and a human version. This acknowledgement suggests that somewhere deep inside he knows the story with people is true, but that it does not matter.
After all he has been through, Pi does not doubt. Richard Parker is the antagonist in the story, although this depends on which of the two stories the reader choses to believe.