A UK-Japan team found the 17-strong shoal at depths of 7.7km (4.8 miles) in the Japan Trench in the Pacific – and captured the deep sea animals on film.
The scientists have been using remote-operated landers designed to withstand immense pressures to comb the world’s deepest depths for marine life.
Monty Priede from the University of Aberdeen said the 30cm-long (12in), deep-sea fish were surprisingly “cute”.
The fish, known as Pseudoliparis amblystomopsis, can be seen darting about in the darkness of the depths, scooping up shrimps.
Alan Jamieson, from the University of Aberdeen, said: “It was an honour to see these fish.
“No-one has ever seen fish alive at these depths before – you just never know what you are going to see when you get down there.”
The deepest record for any fish is Abyssobrotula galatheae, which was dredged from the bottom of the Puerto Rico Trench at a depth of more than 8km (5 miles) in 1970. However, it was dead by the time it reached the surface.
The previous record for any fish to have been spotted alive was thought to have stood at about 7km (4 miles).
The Hadeep project, which began in 2007, is a collaboration between the University of Aberdeen’s Oceanlab and the University of Tokyo’s Ocean Research Institute (Ori) and aims to expand our knowledge of biology in the deepest depths of the ocean.
It is funded by the Nippon Foundation and the Natural Environmental Research Council (Nerc).
The researchers have been looking at the Hadal zone – the area of ocean that sits between 6,000 and 11,000m (20,000-36,000ft). It consists of very narrow trench systems, most of which are found around the Pacific Rim.
The researchers are able to explore them using specially designed remote operated vehicles that are fitted out with cameras.
Professor Priede, director of Oceanlab, said: “There is the question of how do animals live at all at these kinds of depths.
“There are three problems: the first is food supply, which is very remote and has to come from 8km (5 miles) above.
“There is very high pressure – they have to have all sorts of physiological modifications, mainly at the molecular level.
“And the third problem is that these deep trenches are in effect small islands in the wide abyss and there is a question of whether these trenches are big enough to support thriving endemic populations.”
But this species appears to have overcome these issues, added Professor Priede.
“We have spotted these creatures at depths of 7,703m (25,272ft) – and we have actually found a massive group of them.
“And this video is pretty tantalising – the fact that there are 17 of them implies that they could well be a family group, begging the question of whether some form of parental care exists for these fish.”
The researchers said they were surprised by the fish’s behaviour.
“We certainly thought, deep down, fish would be relatively inactive, saving energy as much as possible, and so on,” Professor Priede told BBC News.
“But when you see the video, the fish are rushing around, feeding accurately, snapping at prey coming past.”
Because the fish live in complete darkness, they use vibration receptors on their snouts to navigate the ocean depths and to locate food.
Professor Priede added: “Nobody has seen fish alive before at these depths – only pickled in museums – and by the time they come up from the depths they look in a pretty sorry state.
“But these fish are actually very cute.”
Alan Jamieson added that he believed the team would find more fish during their next expedition in March 2009, which would probe the ocean between depths of 6,000m and 9,000m.
He told BBC News:: “Nobody has really been able to look at these depths before – I think we will see some fish living much deeper.”