I had a fisherman in M, definitely fit looser than a stark from what I remember. Really regret selling it
Weird - I have two Fisherman sweaters (naval and bubble) and I take XL for those, while L fits me perfectly for Stark. Fairly logical to me since Stark stretches and has a very loose knit. IME, the Fisherman doesn't stretch at all and has a tighter knit. I would think most people would size up from their Stark size for a Fisherman
Stark is knit loosely - it will stretch/spring significantly when worn , which is why people have such different views on sizing. I could actually wear a M and wouldn't have any issues buttoning it - it would feel tight around my chest but the loose weave stretches out.
jet - I would size up for the Fisherman so if you're typically a 40, go L. If you're typically a 42, go XL