Detail of a Late Archaic kylix (drinking cup), about 510-500 BC. Period: Archaic Greek: 'An octopus hiding from a fisherman.'
According to Aristotle, an octopus would gather crustaceans, eat their meat, and throw the shells and small fish remains outside its den. For a fisherman, these scraps were telltale signs that gave away the creature's underwater hiding spot. That's why hunting an octopus was different from catching a fish swimming out in the open. It wasn't enough to just spot the prey. You had to know exactly which crevice along the rocky shore was occupied, which hole had shells piling up in front of it, and exactly where to wait.
Also, there isn't just a single catch in the scene. The fish on the line is one catch, and the octopus is another.
The kylix was used in Ancient Greek symposia. A symposium was a gathering where men reclined to drink wine, talk, and have fun. The tondo inside the kylix - its circular central area - would slowly appear as the wine was drunk. The scene would fully emerge as the drinker emptied the cup. There isn't a grand mythological event here. No heroes, battles, gods, or ceremonies. Instead, the painter chose an everyday task: a young figure fishing / hunting for an octopus.