Final answer:
In the sentence provided, the comma after 'An experienced fisherman' sets off an introductory phrase, providing context about Jake Webb before the main clause.
Step-by-step explanation:
In the sentence "An experienced fisherman, Jake Webb had mastered the art of patience," the comma is used to set off an introductory phrase. An introductory phrase provides context or background information for the main part of the sentence and typically appears at the beginning of a sentence. In this case, "An experienced fisherman" is an introductory element that gives us information about Jake Webb before the main clause of the sentence. According to the provided reference material, a comma should be used after an introductory element at the start of a sentence, except when the introductory phrase is less than four words long, although using a comma in such instances is never incorrect.