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Deep-Sea Fish Trap Sheds Light on Ocean's Slowest Denizens

John Roach
for National Geographic News
May 31, 2007
 
A new, high-tech fish trap is bringing to light why life in the cold abyss of the deep dark sea moves really, really slow.

The trap has thick, pressure-retaining steel walls that allow scientists to capture fish in the deep sea and bring them alive to the surface to study their exceedingly slow metabolism.

Sea creatures caught several thousand yards deep with nets or cages like those used to snare lobster and crab almost always die on their way to the surface.

"It's very frustrating working on dead things," said Jeffrey Drazen, a marine biologist at the University of Hawaii at Manoa who designed the new device.

The trap is a 6-foot-long (1.8-meter-long) cylinder with two- to three-inch (five- to eight-centimeter) thick stainless steel walls. It weighs about 1,500 pounds (680 kilograms).

One of the end caps is fitted with a door that opens inward. A baited hook dangles on the end of a spring-loaded steel cable. When a fish takes the bait, the hook sweeps the fish into the trap and the door snaps shut.

The trap is attached under an array of spherical glass floats that provide enough buoyancy to lift the trap to the surface once the researchers jettison iron ballast that weighs the contraption down.

Instruments and cameras inside the trap allow scientists to monitor a captured fish's behavior and biology.

Preliminary tests with the trap allowed Drazen and colleagues to snare fish at depths of 11,500 feet (3,500 meters) and study them at the surface for several days.

Slow Zone

The animals' metabolic rate—how quickly they use energy—is of key interest.

"The pace of life down there is incredibly slow," Drazen said. The researchers measured metabolic rates about ten times lower than at the surface.

The question is why.

Intuition suggests the lack of food at ocean depth slows everything down to a state of quasi-hibernation, but that assumption appears wrong, Drazen said.

The fish captured in the trap have watery muscles, suggesting they don't swim much. And since they don't swim much, their metabolism is low.

He explained that in the deep, dark sea, sight distance for predators and prey is limited. A little shrimp, for example, only has to swim a few feet to escape the gape and eyesight of a rattail, a type of deep-sea fish. (Related pictures: "Weird New Animals From Antarctica's Deep Seas" [May 16, 2007].)

A surface-dwelling tuna, by contrast, may have to chase an anchovy for several hundred yards. Thus tuna and anchovies have bigger swimming muscles and higher metabolism, Drazen said.

"It really seems to be light that controls the metabolism of these animals and how they sense predators and prey," Drazen said.

James Childress is a marine biologist at the University of California at Santa Barbara who studies fish metabolism. About 25 years ago, he first connected metabolic rates to vision.

At the sea surface there is strong selective pressure for organisms to use visual information, he explained.

"That is, if they see a predator, to move quickly to get out of there. If they see prey, to move quickly to capture it," he said.

The deep sea, where there is little light, lacks the selective pressures for strong swimming muscles, he added.

Drazen's trap, Childress noted, is allowing scientists to measure for the first time the metabolism of animals that until now scientists were unable to recover.

"It's a terrific engineering thing," he said of the trap.

Aquarium Display

Ultimately, Drazen said, he'd like to use the trap to acclimate deep-sea fish to atmospheric pressure at sea level, like a diver in a decompression tank.

But acclimation is a huge challenge, Childress said. While the properties of certain body elements such as tissues and fats can be acclimated, proteins that trigger cell activity are genetically fixed and sometimes cease functioning completely.

Nevertheless, he said, the researchers should try, as it would allow scientists to build up a stock of deep-sea organisms to study at the surface.

Drazen suspects he'll be successful with certain fish that live up to 6,560 feet (2,000 meters) below the surface.

"If this is true," he said, "now we can have animals living in aquariums for the public to see or in aquariums for scientists to observe and study."

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