Mr Duncan said the fish was a bit decayed which didn't help with identifying it.
"It looked foreign, it didn't look like something which was either out of New Zealand or from the very deep sea."
Maybe someone caught it and threw it out of the boat - I can't imagine that but you never know," he added.
After tweeting about his find, Mr Duncan had received a few suggestions, with one person saying it could be a barracuda and another suggesting it could be an oarfish.
Japanese legend has it that oarfish wash up on beaches before or after earthquakes.
Mr Duncan said it was possible the fish was an oarfish based on pictures he'd seen, but if it was it was a very small one.