New research has finally shown what many marine biologists suspected but had never seen before: fish migrating through the depths of the sea.
The study, published this month in the Journal of Animal Ecology, used analysis of deep-sea photographs to show a regular increase in fish numbers in particular months, suggesting seasonal migrations.
Tracking fish in the deep sea is a challenge. They are sparsely distributed, the water is almost devoid of sunlight, and the monitoring equipment has to withstand enormous pressure.
The study used photographs taken by the Deep Ocean Long-Term Environmental Observatory System (Delos), two observatories on the seabed 1,400 m below the surface, off the coast of Angola. The researchers analyzed 12,703 photographs, of which only 502 had managed to catch a fish, taken over seven and a half years, and found that each year, at the end of November and June, there was an increase in the number of fish.
"It's certainly unprecedented, but it's never really been proven," says Rosanna Milligan, assistant professor at Nova Southeastern University in Florida and lead author of the paper. "That's what we were able to do with this study."
"Even after all these years, one of my favorite parts of being a scientist is when you do those first graphs of your results and you start to see something emerging from the data," says David Bailey, senior lecturer at the University of Glasgow and co-author of the Article. “That is one of the greatest thrills of the entire scientific endeavor. 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 is to figure out where the fish are coming from and going when they move," says Bailey. "What's going on? What does it mean and what are the fish doing?
Very little is known about the behavior of any of the fish photographed. Grenadiers, a family of fish seen in more than 100 of the Delos photos, have long tails that can allow them to move great distances at low speeds, but despite being a relatively common deep-sea fish, there is little information on how far they can swim.
A 1992 article, for example, put acoustic tracking devices on bait and fed them to grenadiers, but the devices only tracked fish up to 1 km away.
Milligan believes the fish could be migrating to follow dying organisms on the surface. Plankton flourish off the West African coast every year four months before deep-sea fish migrate to the area. Since the deep waters depend on life on the surface that dies and sinks to the bottom, other animals may be gathering to take advantage of the dying plankton, and fish from the deep migrate to eat them.
"We just have no idea how these things work," says Tim O'Hara, 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 miles deep in the ocean and we get these little bits of information from one or two places and we're trying to put together a big picture. "
Milligan and Bailey hope that this discovery will encourage 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 looked at this level of detail during this time. This could be happening all over the place. "