Collect any Large Fish that is stowable on the back of your horse (see Images for Musky/Pike spawns below)
Meet Up with Friend at any butcher
Have friend and Horse with Fish (Fish Stays on Horse) stand next to the butcher for this entire process
Stand just infront of butcher with option to interact or speak to butcher
Open Interaction menu (left d-pad) Go to Online Options and Respawn
Once your character respawns make your way back to the butchers table (do not call horse)
Once back at the table, Remove fish from back of horse and place it back on to the horse
interact with butcher, open sell menu and sell the fish
back out of sell menu and the fish should still be on the horse. (if not join new free roam session and setup again) If so Repeat step 7-9 as much as you would like
Remove fish from horse, place fish back on horse, interact with butcher and sell