A bristlemouth - the most common fish in the sea

The next frontier for humans to explore, understand and exploit is the mesopelagic zone of the ocean. It is where the most numerous species of vertebrates live. It plays a crucial part in regulating the planet's temperature. And now we have the technology to venture into the twilight zone.

The mesopelagic or twilight zone extends vertically in the ocean from about 200 to 1000m, the depth where sunlight ceases to penetrate.

Scientists estimate it holds as much as 10 billion tonnes of animals - as a comparison, the total biomass of cattle on earth is 500 million tonnes.

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One of the biggest single elements of this vast biomass are the staggering number of bristlemouths living in the zone - there may only 32 species of Gonostomatidae but they are easily the most abundant vertebrate genus on the planet numbering in the hundreds of trillions or even quadrillions.

And the world's fishing industry endless in search of more prey has its eye on them. It has recently developed nets to work at such depths. In the last year, Norway has issued 46 new licences for vessels to fish in the zone. Pakistan is about release rules allowing its fishing fleet to exploit the zone and the North Atlantic  Seafood Forum last month discussed how to regulate commercial fishing in the twilight zone.

Landing just one per cent of the fish in the mesopelagic zone would double the catch of the world's fisheries. However, it is unlikely to end up directly on your plate - most would be used to feed farmed fish.

Most of the research into the twilight zone has relied on sonar. During the Second World War American Navy research into submarine defence first indicated the vast amount of life in the zone - they thought the sheer mass of fish created 'false bottoms' under which submarines would be able to avoid detection. A major study by Spanish researchers in 2010 known as the Malaspina Circumnavigation came up with the 10 billion tonnes estimate using sonar surveys .

In recent years scientists have also realised that the planet's largest migration takes place every night as around one hundred million tonnes of biomass (fish, plankton, jellyfish, comb jellies, cephalopods, gastropods and ctenophores) rise up from the mesopelagic zone to feed near the ocean's surface and descend as day returns to avoid predation. In areas such as Palau and Hawaii this has led to what is called 'Black Water' diving where you can witness this extraordinary spectacle of the beasts from the deep rising to feed and sometimes breed.

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A jellyfish,  Kona, Hawaii

A new generation of research submarines is about to be launch to probe the twilight zone. Engineers, biologists, and chemists at the Woods Hole Oceanographic Institution (WHOI) are developing the Mesobot, a new class of deep-sea robot specifically focused on midwater science. The Mesobot will provide transformative imaging and sampling. At the core of its design is the ability to navigate a three-dimensional environment in a biomimetic fashion inspired by midwater animals like jellyfish and salps. 


The Mesobot (above) will weight 75kg and will hang untethered in the water column and observe mesopelagic life for extended periods. It will be able to track animals up and down during their migration. The WHOI teams have gone to great lengths to ensure that the robot's thrusters do not disturb the creatures it will be observing. It also has a method of capturing samples and preserving them in pressure controlled tanks when it returns to the surface.

The largest on the new submersibles is called Deep See and is a 700kg underwater sledge that will be dragged behind surface research vessels gathering wide angle, high definition video. 

The smallest is called Snowclops which will freely descend down the water column measuring the marine detritus - known as Snow - which is a crucial part of the carbon exchange between the  surface and the mesopelagic zone. 

The mechanics of this complex carbon pump are just starting to be understood - every year it is though as much as 12 billion tonnes of carbon descend into the mesopelagic zone as marine life defecates or dies. But waters welling up from the deep also return vast amounts of carbon back to the surface. Understanding this cycle will be crucial in dealing with global warming.

It is a race for the scientists to understand more about the zone before commercial exploitation has a significant impact on it - not something we have always managed on the surface. And there is the further imperative of understanding the carbon cycle as the planet warms. Combined it makes the twilight zone of crucial importance  - far more than outer space, the mesopelagic zone is the next frontier for humans.




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