"San Pedro" is Saint Peter from the New Testament. Christ called him from his fishing nets to become a "fisher of men" (i.e. a disciple of Christ who would help Christ seek other followers) in Matthew 4:18-20:
And Jesus, walking by the sea of Galilee, saw two brethren, Simon called Peter, and Andrew his brother, casting a net into the sea: for they were fishers. And he saith unto them, Follow me, and I will make you fishers of men. And they straightway left their nets, and followed him.
(King James Version).
His profession may be the origin of the Early Christian secret symbol, Ichthys.