On the trail of a massive, mysterious fish
The fight is over for the 300-pound bluefin tuna. It's been reeled in by an expert fisherman and hauled aboard the Calcutta, a 55-foot sport-fishing boat. As the fish lies on a plastic pad, a seawater hose is placed inside its mouth. Its staring eyes are covered with a wet cloth. The fish doesn't struggle, but stretches out its fins as if feeling for the water that suddenly vanished.Skip to next paragraph
Subscribe Today to the Monitor
Most people think of tuna as sandwich filling. But this silver-blue fish is a perfect swimming machine, one of the most remarkable animals in the ocean. Unfortunately for the bluefin, it is also one of the tastiest animals in the ocean. Bluefin are a great delicacy in Japan, served as sushi (raw fish with rice) or sashimi (simply raw). Japanese diners will pay very high prices for a prime fish. In 2001, a 444-pound bluefin sold for $175,000 at a fish market in Tokyo. That's $394 per pound!
The Calcutta's bluefin, however, won't be put on ice and flown to Tokyo. It's about to become a living "spy probe."
The Calcutta's science team kneels around the bluefin. "Two hundred centimeters," says Dr. Barbara Block of Stanford University, as she measures the fish. Chuck Farwell, an aquarist at the Monterey (Calif.) Bay Aquarium, holds the hose in the fish's mouth. The hose forces seawater through its gills and helps it breathe while out of water.
Dr. Block pushes a pointed metal barb the size of a large paperclip into the muscle on the bluefin's back. A pop-up satellite tag the size and shape of a small microphone is attached to the barb by a plastic cord. Dr. Block pushes in a smaller barb to anchor the other end of the tag. When she is done, she stitches up the small cuts.
In just a few minutes, the bluefin's discomfort is over. Block, Mr. Farwell, and the rest of the team pick up one end of the pad and carefully slide the fish back into the water. This time next year, the high-tech tag on the bluefin's back will report back to Block, giving her a fish's-eye-view of life in the open ocean.
"Bluefin are incredibly tough fish," says Block as the tuna disappears into waters off North Carolina. "We know from tag data they are soon back to normal." Block has put high-tech tags on 680 Atlantic bluefin tuna since 1997. Through the tags, she is learning more about bluefin and their ocean habitat - information which could help her protect the bluefin from being overfished.
Atlantic bluefin tuna aren't the only creatures carrying "spy probes." Block and other scientists have already tagged great white sharks (see "Shark trackers," the Dec. 3, 2002 "Kidspace"), salmon sharks, Pacific bluefin, albatrosses, elephant seals, sea turtles, blue whales, and large squid.
But studying marine life isn't easy. A scientist doing chimpanzee research can follow a troop through the forest, watching every move and nibble. That is impossible with fast-moving, deep-diving, wide-ranging ocean animals.
For many years, scientists have used metal or plastic tags to find out where fish go. If a tagged fish was caught, the fisherman would (hopefully) send the tag back and tell the scientist where and when the fish was caught. However, these simple tags can only show that a fish traveled from point A to point B, not what happened in between.
Back in Block's lab, Farwell shows me a pop-up satellite tag like the one now carried by the Calcutta's bluefin. "These three dots are the sensors. This one senses swimming depth, and this one water temperature. This one records light levels, which can be used to calculate the fish's location," explains Farwell. "The tag takes and records measurements every two minutes."
The pop-up tag looks like a microphone and weighs no more than a candy bar, but it is really a minicomputer. After a set time - usually one year - the tag pops off the fish and floats to the surface. It sends its data to an orbiting satellite, and the satellite sends the data to Block's computer.
Adult bluefin are among the farthest-ranging creatures on the planet, built to cover long distances quickly and efficiently. Their "superfish" swimming design is the envy of submarine engineers.
You might expect a perfect swimming shape to be long and thin. Yet bluefin are stout. Scientists have discovered that the bluefin's body thickness compared to its length perfectly minimizes drag in the water.
Every bit of the bluefin's body is designed to cut smoothly through the sea. Its fins fit into notches on its body. Its eyeballs are recessed into its head and its scales are slick. The bluefin's design, millions of years in the making, is far more sophisticated than a racecar's.