Dusky spinefoot (Αγριόσαλπα μαύρη)

The dusky spinefoot (Siganus luridus) is a species of rabbitfish (Siganidae). It is native it the western Indian Ocean, as well as the Red Sea and Persian Gulf. It can now be found in the Mediterranean Sea because of transport through the Suez Canal. It is often found in small schools feeding on benthic algae in shallow water with compacted sand substrates. Its dorsal and anal fin spines contain venom that is not life-threatening to humans. (Wiki)
I spotted these fish washed ashore, in Agios Nikolaos marina, Crete island, Greece. The fishermen do not use the fish, and they throw them back in the water, usually dead.