What is the ruling on fishing with the intention of sustenance, providing for one's family, and giving charity, knowing that it is the primary profession on the island, even though it was learned from polytheists in the past?

A Muslim is permitted to benefit from the useful professions and sciences he has learned, even if his teacher was a polytheist, because this does not cause any harm. Muslims learn from polytheists what benefits them in their worldly affairs. Therefore, you are permitted to hunt professionally and benefit from the skill you acquired from the person you mentioned.