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Deep fish and distant jets

Q: How deep in the ocean do fish live? Why doesn’t pressure crush deep-sea fish? (Ron, Sun City, California)

Fangtooth (Anoplogaster cornnuta) captures food with large sharp teeth. He opens his big mouth, sucks the stray fish or shrimp in, and snaps the trap shut. Found in deep waters 2,000 - 16,100 feet (610 - 4900 m). [©David Wrobel, 2004 Monterey Bay Aquarium Foundation]

A: Your questions were on my mind when I attended a lecture during a week’s fellowship at Woods Hole Oceanographic Institution (WHOI) this September. Senior Scientist Laurence P. Madin was lecturing.

I waited for my chance as he talked about "Life in the Sea". The sea is divided into three parts — the sunny shallow waters, the twilight region of deepening darkness, and the deep zone of eternal night. When he spoke of the various creatures that dwell in deepest waters, I thought, "Aha!"

My hand shot up. "How deep in the ocean do fish live?" I asked. "As deep as the ocean gets," Madin said.

The deepest place in the ocean — the Mariana Trench — is about 100 miles (160 km) southwest of Guam. The trench bottom lies 6.8 miles (11,000 m) below the sea’s surface — farther below sea level than Mount Everest is above sea level. Water pressure there is over a thousand times greater than at sea level.

Until 1960, no man had ever dived that deep. On Jan. 23, U.S. Navy Lt. Don Walsh and Swiss oceanic engineer Jacques Piccard took the U.S. Navy bathyscaphe Trieste down. Gravity pulled Trieste, loaded with iron shot, to the bottom of the Mariana Trench. She dropped into growing darkness. A soft thump, four hours later, and she was down.

"The bottom appeared light and clear, a waste of ...firm diatomaceous [algae-skeleton] ooze," said Piccard.

Peering out the 2.5-inch (6.35-cm) porthole and probing total darkness with his spotlight, Piccard saw flat fish (about a foot long) on the bottom just before they landed. Perhaps a type of flounder or perch. That’s the deepest fish ever sighted — as deep as the ocean gets.

Further Surfing

Monterey Bay Aquarium Foundation: Fangtooth

Whitman College: Deep sea fish

Q: Why doesn’t pressure crush deep-sea fish? (Ron, Sun City, California)

WHOI biologist Larry Madin smiled when I asked the question.

The trick is to keep the pressure inside the fish the same as that outside. Only pressure difference can crush. So, if the difference is essentially zero, the fish is OK. Fish tissue is at about the same pressure as the outside pressure and thus experiences little if any pressure.

Swim bladders that expand and shrink as fish go up and down in the water column would collapse in such deep waters. However, "...most deep-sea fish don’t have gas-filled bladders," Madin said.

"Those with gas bladders manufacture the gas in special glands that have the same pressure as the surrounding water." So, the gas is at the same pressure as the water and, therefore, doesn’t compress. The gas presses back on the water with the same force that the water presses it.

Q: I noticed at sunset that I can see jet trails at a very far distance and, if the sun is just right, I can see the jet sparkling in the setting sun. How far away is the horizon from the jet if it is flying at 36,000 feet? Is it 50 miles, 100 miles, 200 miles? (Pete, East Lansing, Michigan)

A: The horizon is 230 miles from a jet flying that high, neglecting atmospheric refractive effects.

So, the last rays of the setting Sun graze Earth 230 miles west of the jet, glance off its shiny surface, and reflect as a sparkle in your eyes.

Here’s the math: We know that the line from the jet to Earth (see figure) touches Earth tangentially because that’s what "the horizon" means. We want to know its distance, d.

"d" is the distance from the jet to Earth's horizon.  "R" is the radius of the Earth.  The jet is flying 36,000 feet (6.818 mi) high.  [April Holladay]

We also know that the radius (in the figure, R) of the Earth is about 3963 miles (6378 km). The jet is 36,000 feet high, which is 6.818 miles high. So, using the Pythagorean Theorem to relate d to R, we get

(R + 6.818)² = d ² + R²

Substituting R = 3963 miles in the equation, we solve for d and get

d = 232 miles.

By the way, flying at 36,000 feet, we can see "quite a bit more than the usual 180 degrees of sky," says Robert Massey, astronomer at the Royal Observatory Greenwich. Our horizon up that high extends an "extra 3.5 degrees over the ‘flat’ horizon" we see on Earth’s surface.

(Answered Oct. 22, 2004)