The first tentative identification was that it could be a very juvenile deep water Oarfish – which can grow to 8m in length.
Since then, Andrew Green has been sleuthing and sent the following comments: “I sent my pics of your “oarfish” to Mark McGrouther who is head of fish at the Australian Museum in Sydney. He kindly forwarded the pics to his colleague and eel expert Mike in Tokyo.
Turns out it is not an oarfish, but an eel leptocephalus, as I wondered initially; although not our common local freshwater eel, but a conger (hence the large size of the leptocephalus).
See this: http://australianmuseum.net.au/blogpost/Fish-Bits/Large-Muraenid-Leptocephali to see some of Mike’s work.”
Do have a look at these videos, they are superb and show exactly how this tiny fish moves and hunts.