Feb. 3, 2006 — A technique for sensing huge populations of fish swimming together in murky depths is providing unprecedented real-time observations of the shape, movement and behavior of shoals.

The remote sensor system, reported by associate professor Nicholas Makris of the Massachusetts Institute of Technology and his team in the Feb. 3 issue of Science, could provide ecologists and government agencies with more accurate information about fish in danger of rapid decline.

"In terms of insight into the large-scale movements of fish populations and dynamics, this is really quite revolutionary," said fish ecologist Tony J. Pitcher, currently with the University of Concepcion in Chile, the Max Planck Institute for Evolutionary Ecology and the University of Hong Kong.

Conventional sensing systems track fish populations using high-frequency sonar, which is easy to control and produces an intense signal. But like a flashlight shining into a dark room, this kind of sonar can sense only a narrow beam of the dark depths.

Typically used on slow-moving research vessels, the sonar cannot capture the ever-changing motion of shoals. Instead, scientists must fill in the gaps of fish group behavior using theoretical, statistical and other models.

Makris and his team developed a system using low-frequency sonar, which propagates tens of kilometers in all directions the way ripples on the surface of a pond move away from a dropped pebble.

When the waves come into contact with an object or a group of fish, they bounce back and are collected by an antenna being towed behind the ship.

"What it's enabling people to do for the first time is to see instantaneously a 30-kilometer radius in 360 degrees," said Makris.

In experiments in waters south of Long Island, New York, the scientists saw some interesting traits in the fish populations.

For example, the fish often congregate in an hourglass pattern, a thin bridge connecting the two ends. The shape has been observed before in small schools, but this is the first time it was observed in such an enormous group.

They also noticed that the shoal was sharply bounded on one side to the edge of the continental shelf, where nutrients well up from deeper water. Makris thinks following the edge of the shelf could possibly be a form of navigation for the animals.

The one thing the technology can't do yet is determine the species of fish. And this could prove difficult if the vessel is imaging the shoal from tens of kilometers away, said Pitcher.

But according to Makris, the technique could be used in combination with other methods that determine fish species or could be used to track known shoals.