A Fisher of Fish
A Fisher of Fish is one of the optional Stranger missions in Red Dead 2. Start it by speaking to Jeremy Gill on his dock along the north-east shoreline of Flat Iron Lake. (South East of Flatneck Station)
Gill will first explain that he is incredibly famous and can make a fair amount of money off sending people stuffed fish. He’ll say he could be making even more money if he had more fish and enlist your help.
Gill will then provide you with a map showing the approximate location of 13 Legendary Fish found throughout the world. Catch them and bring them to a post office to mail them to Gill for a reward.