What is the theme in The Yellow Admiral by Patrick O'Brian?

Slavery and the abolitionist movement are key concepts in the Commodore. Jack Aubrey, in his first mission as a Commodore of the British Navy, is given the assignment of ferreting out slave trading ships departing from the west coast of Africa. The transition from the condoning of slavery to the rise of abolition in England was less bloody than the same transition in the United States, but there were still a great many difficulties.