ROSTOCK — A harbor seal named Filou learned to use its whiskers to distinguish the size of underwater vortex rings, a skill that may explain how seals track fish through murky water, according to research published in the Journal of Experimental Biology. The experiment was conducted by Yvonne Krüger, a biologist at the University of Rostock in Germany.

Harbor seals have about a hundred whiskers in the facial region and can extract various types of information from disturbances in the water using those whiskers, including the direction of an object's movement. Hydrodynamic trails generated by fish in the water column provide signals that harbor seals can follow to locate and capture their prey. Laboratory studies suggest that the animals may also be able to distinguish between the hydrodynamic movements of different types of fish.

"And with these whiskers, they can sense the water movements that are generated by fish," Krüger said. Her research suggests that the whiskers may help seals outmaneuver the evasive behaviors of fish they hunt, such as rainbow trout, a common prey species.

"Rainbow trout are able to camouflage their swimming direction by bending into a C-shape, and then they swim away in a different direction [than] they have been swimming before," she said. When a rainbow trout bends into a C-shape, it generates two underwater vortex rings moving in opposite directions — a smaller ring moving in the direction the fish is swimming and a larger ring moving the opposite way. Krüger hypothesized that a seal differentiating between the sizes of the two rings could identify the correct direction to chase a fish.

To test the theory, Krüger spent almost two years training Filou, a dark gray adult male harbor seal living at a marine science center in Germany, to select the larger of two artificially generated vortex rings underwater. "Filou likes to do everything correct. He likes learning new things. If he has one mistake, he gets frustrated," she said. "He's a nerd."

Filou successfully learned to distinguish between the two vortex rings even when the size difference was less than the width of a human thumb. Krüger said his ability to distinguish vortex ring sizes suggests he could follow the correct direction to hunt rainbow trout. When researchers covered Filou's whiskers with a nylon stocking, he could no longer distinguish between the two rings, indicating that the whiskers provided the necessary sensory information. Krüger and her colleagues suspect that harbor seals generally share this ability.

Robyn Grant, a sensory biologist at Manchester Metropolitan University, said the research is an important step in understanding how seals use their whiskers to extract information from hydrodynamic trails. "But this is a great start to tell us what they're capable of doing," Grant said. She added that understanding how harbor seals sense their surroundings may reveal how they could be affected by environmental changes, including extreme weather events that could mask critical hydrodynamic stimuli. The findings may also inspire sensors to help aquatic robots navigate for tasks such as underwater archaeology, mining, or biological surveys, she said.