I live near a legit fish monger. his main clientele are people that like fish, but dont "like-like" fish. he is 90% neat tidy fillets of fish all in baggies sitting on pristine ice. very little whole fish options.
a few years ago: I asked about the carcasses. salmon carcasses in general. he replied, "how many do you want?"
what? two! he bagged up two whole carcasses and gave them to me. FREE. he was thrilled I was not wasting the stuff. I took it home, scrapped the bones, made sushi wraps, grilled the heads and collars, etc.
near him is an Asian grocery store. people saw me with bag on occasion...."me..hey go get a free carcass!". word spread.
now the dude charges a few dollars. hahahha...and he sells the collars. a Japanese grilled delicacy. the owner grins at me now when I go in to buy them. last night. miso glazed grilled collars.
I should have snuck out the back door at closing hours...