New research has finally demonstrated what many marine biologists suspected but had never before seen: fish migrating through the deep sea.

The study, published this month in the Journal of Animal Ecology, used analysis of deep-sea photographs to show a regular increase in the number of fish in particular months, suggesting seasonal migrations.

Tracking fish in the deep sea is challenging. They are sparsely distributed, the water is nearly devoid of sunlight, and the monitoring equipment has to withstand enormous pressure.

The study used photographs taken by the Deep-ocean Environmental Long-term Observatory System (Delos), two observatories on the sea bed 1,400m below the surface, off the coast of Angola. The researchers analysed 12,703 photographs – only 502 of which had actually managed to capture a fish – taken over seven-and-a-half years, and found that each year, in late November and June, there was a spike in the number of fish.

“It is certainly not unprecedented but it has never really been demonstrated,” says Rosanna Milligan, an assistant professor at Nova Southeastern University in Florida and the lead author on the paper. “That is what we were able to do with this study.”

“Even after all these years, one of my favourite parts of being a scientist is when you do those first graphs of your results and start to see something emerging from the data,” says David Bailey, a senior lecturer at the University of Glasgow and a co-author on the paper. “That’s one of the greatest thrills of the whole scientific endeavour. It was really, really amazing.”

Even with this new discovery, Milligan and Bailey still say there is a lot they don’t know.

“The natural thing to do is to find out where the fish are coming from and going to when they move around,” says Bailey. “What is going on? What does it mean and what are the fish doing?”

Very little is known about the behaviour of any of the fish photographed. Grenadiers, a family of fish seen in more than 100 of the Delos photos, have long tails that may allow them to move great distances at low speed – but despite being a relatively common deep-sea fish, little information exists on how far they can swim. A 1992 paper, for instance, put acoustic tracking devices in bait and fed them to grenadiers, but the devices only tracked the fish up to 1km away.

A remotely operated vehicle approaches one of the two observatory platforms used to monitor the fish, in water 1,400m deep off the coast of Angola.

Milligan thinks the fish might be migrating to follow dying organisms on the surface. Plankton blooms off the coast of west Africa every year four months before the deep-sea fish migrate into the area. Given that the deep sea is dependent on life at the surface that dies and sinks to the bottom, it is possible that other animals could be gathering to take advantage of the dying plankton, and the deep-sea fish migrate to eat those.

“We just have no idea how these things act,” says Tim O’Hara, a researcher and senior curator of marine zoology at Museums Victoria, Australia, who was not involved in the research. “We are literally groping in the dark. It’s kilometres down in the ocean and we get these tiny bits of information from one or two locations and we’re trying to put together a big picture.”

Milligan and Bailey hope this discovery encourages other researchers to look for similar patterns in the deep oceans.

“Maybe if we had more of this level of surveillance in other places, we would find fish migrations in all kinds of places,” says Bailey. “It’s just that [Angola] is where we happened to be observing in this level of detail for this amount of time … This could be happening all over the place.”


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