That's a squid, but I could see how it could be hard to tell the difference, especially since the fins run the length of its mantle. The fisherman could tell by seeing if there was a gladius or cuttlebone. Also, the pupil would be different on a cuttlefish.
I know we have an Atlantic squid that is often mistaken for a cuttlefish because of a similar fin arrangement and
I know the pen vs cuttlebone distinction but how do you tell squid from cuttle without cutting it open (ie from a photo)? In this case one could make an argument for what looks like an oval support OR a thin pen, depending on how you interpret the rigid part of the mantle.
I think cuttlegirl gave a good suggestion: look at the shape of the pupil. I know many cuttlefishes have a W-shaped pupil (as opposed to a squid's round pupil), but is that universal among cuttlefish? Could that be a defining characteristic to examine?
So this one must be australis where Sepioteuthis sepiadea is squid here that is often confused with a cuttlefish. Here (US East Coast) it is not difficult to decide since we don't have a native cuttle but Queensland (the source of the photo) would have both and the full skirt made me wonder how to tell from the picture (pupil not displayed well enough to make a call on shape). What about head to mantle connectivity? I note, though, that this one has a small external connection and I thought part of what separated squid from octopuses (apart from tentacles) is the internal vs external connection.