I can introduce you to a number of fish eating body builders and lunberjacks proving conclusively that protien plus exertion leads to muscle mass ;)
Also consider that poor Africans have a metabolism adapted to hot climates. Fat is genetically less likely. North dwellers have a genetic imperative to store fat for insulation.
I never met many waiflike northerners. Sure there are some. But overall they are bigger people.
Body builders eat protein shakes and all kinds of ridiculous supplements and weight train, let muscles rest, then resume weight training.
Point me towards lumberjacks that eat primarily a fish based diet and have significantly increased muscle mass. I call bovine feces.
Working everyday fishing and farming(and raiding in the Summer) from sun up to sun down while eating a fish based diet does not lead to significantly increasing muscle mass.