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Scientists Catch Live Fish At Record Depth In The Atlantic

July 31, 2008

Scientists say they have captured a live deep-sea fish and three shrimp species at a record depth of 2,300m on the hot vents of the Mid-Atlantic Ridge.

A new trapping device has been developed that allows recovery of live animals under their natural pressure at greater depths than previously achieved.

The researchers hope to be able to transfer the animals into an experimental lab to study their normal biology.

“Pressurized recovery has been around for the past 30 years, but this is the deepest fish-capture under pressure – the previous record was 1,400m. This is also the first time pressurized capture has occurred at a hydrothermal vent,” said Dr. Bruce Shillito, marine biologist at the Universite Pierre et Marie Curie, Paris, France.

The shrimp species were caught at 1,700m (5,600ft; Mirocaris fortunata and Chorocaris chacei) and 2,300m (7,500ft; Rimicaris exoculata) at two vent fields, Lucky Strike and Rainbow, on the Mid-Atlantic Ridge.

“At depths of over 1,000m, it is difficult to recover animals alive. Catching with no pressure is as good as catching dead. Fish are the most fragile – even a fisherman with a 100m line will probably reel in a catch whose gas bladder is in its mouth,” said Shillito.

He said that although the fish caught by the team was a zoarcid (Pachycara saldanhai) and had no gas bladder, it was sensitive to full decompression.

The fish was active and remained upright under the pressure of the surface, however upon release of pressure its movement became uncoordinated and within a few minutes it was totally motionless.

The same effects of decompression were also observed in the shrimp species. At the surface, under pressure, most shrimps were in an upright position and swimming actively and continuously.

A separate shrimp sample was caught and pulled to the surface without pressure, the animals jerked violently, and after a few hours were dead.

The samples were examined onboard the ship “Pourquoi Pas?” during the Momareto cruise, which was organized by Ifremer, the French Research Institute for Exploitation of the Sea.

Dr. Shillito’s team hopes to be able to transfer its catch from the sampling device into a better-equipped experimental tank, without decompression, allowing the scientists to observe the animals’ normal behavior and responses to different environments.

“We are particularly interested in the Pompeii worm (Alvinella pompejana), a vent worm which is thought to be the most thermo-tolerant marine organism, yet remains to be recovered in good enough shape. It is intriguing to find out how heat-resistant this animal is,” said Shillito.

Despite covering about 60% of the Earth’s surface, the deep-sea floor ecosystem is poorly understood.

“We urgently need to find out more about the place we are destroying,” Shillito added.

“At a time when we are over-fishing the depths of the ocean, we know more about cooking recipes than the biological features of deep-sea fauna.”

Shillito in conjunction with Gerard Hamel, a mechanics engineer at the Universite Pierre et Marie Curie, developed the new sampling system for pressured recovery, which has been named Periscop.

Exocet/d, a large European research program has provided funding for the project.

The system is made up of three compartments that perform different tasks – capture at depth, recovery of the deep-sea species under natural pressure, and transfer to the lab with no decompression.

“In most previous attempts involving pressurized recovery, a single container fulfilled these three tasks – this may lead to contradictory technical requirements,” explains Shillito.

The plastic capture box is attached to a submersible arm that allows movement and suction for sampling. The animal is then transferred into a pressurized box. This is kept at the same pressure as the sampling depth during ascent by a pressure compensator.

He said they used pressurized water to maintain pressure, which is a safer and a simpler alternative to gas and he hopes this method of pressurized recovery will become standard.

As well as pressure shock, when animals are pulled to the surface they suffer from changes in temperature.

He said that the temperature at depths below 2,000m is pretty constant all over the world – around 2-4C, yet the surface waters where they were sampling were 22-25C.

“Heating is difficult to prevent without getting out the heavy gear – using active cooling systems, requiring energy and computer controls – but at least we know that every sample has had the same temperature history; they have the same background story,” said Shillito.

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