Predators have a variety of strategies for locating prey. Some look, some listen, others even track their quarry electrically. Now it appears that some aquatic hunters find food by following the wake left by swimming fish. According to findings presented yesterday in the early online edition of the Proceedings of the National Academy of Sciences, the European catfish uses a prey's wake much like wild dogs follow terrestrial trails.

Earlier investigations into prey location among fish had focused on predominantly visual predators. Many fish-eating fish hunt at night or at depths where vision is nearly useless, however. German biologist Kirsten Pohlmann of the University of Konstanz and her colleagues thus hypothesized that such fish might be using chemical and hydrodynamic cues in the wakes left by prey to locate their next meal. To test this, the team studied the strategy of the nocturnal European catfish as it stalked swimming guppies in a test aquarium illuminated only by infrared light, in which catfish cannot see.

Analyses of the swim paths of predator and prey support the team's hypothesis. For one thing, the catfish followed the rather convoluted trails left by the guppy, as opposed to intercepting its path and making a direct attack. The team also found that the catfish usually attacked from behind. Whether taste, smell or hydrodynamic stimuli are providing the signals used by the catfish during wake tracking remains to be seen. Whatever the case, the researchers assert, "we suspect that exploitation of these cues is common among large animals that track moving prey through water."