News & Technology

25 January 2017

Seals hunt down hidden fish by sensing their breath in the sand

IT’S a slasher film scenario playing out in nature. Harbour seals use their whiskers to follow underwater currents rippling away from the gills of fish so they can track down and kill their next victims.

Until now it wasn’t clear how seals locate and catch bottom-dwelling flatfish that hide beneath the sand. “We have solved a long-standing riddle,” says Wolf Hanke, the biologist at the University of Rostock, Germany, who led the study. “These flatfish are very cryptic. They burrow in the ground and they’re covered with sand or silt, but the seals still grab them.”

The only way for the fish to avoid being eaten would be to hold their breath until the seal swims on – although it’s unclear if flatfish do this when they spot the threat. “When partially immersed in sand on the ocean bottom, many flatfish leave their eyes sticking out, and with 360 degrees of vision they would very likely see a seal swimming towards them,” says Alex Schreiber, who studies flatfish at St Lawrence University in Canton, New York.

Flatfish constitute up to 70 per cent of the diet of harbour seals, so being able to detect the fish is key to the predator’s success. After attaching a camera to a seal and watching it pinpoint flatfish despite having no visual clues to their position, Hanke and his colleagues set out to figure out how the seals do it.


They monitored three male seals in the Baltic Sea in a netted enclosure, one fitted with eight nozzles that could release water at an angle and rate designed to mimic the breathing patterns of flatfish. Each nozzle could be individually activated.

The seals were blindfolded for half the trials. When they could see, all the seals found the activated nozzle at least 90 per cent of the time. Even blindfolded, they still identified the correct nozzle with at least a 75 per cent success rate (Journal of Experimental Biology,

“To see a seal following an underwater wake is amazing,” says Colleen Reichmuth, a marine biologist at the University of California, Santa Cruz. “It’s as if the animal is pulled by an invisible thread.”

The researchers also covered the seals’ faces and whiskers with a mask. In those trials, the animals failed to find any of the active water nozzles, and instead mostly spent their time drifting on the surface.

This article appeared in print under the headline “Seals detect breathing of hidden fish prey”