I think the branch was put there by the photographer, for scale. A can of soda would have been better --

I don't think the branch dented the fish --

I think the photographer probably pushed down against the fish's decay-bloated belly, so the branch would not wash away in the surf. Why? Becasue he could NOT move the fish --

If the branch had dented the fish with its own weight, that fish would weigh a ton.