How do you think seals catch fish?
Well, let’s guess. Humans rely greatly on vision to catch prey. Sharks can smell blood in the water from a mile away. Or since they’re in water, it could be taste.
How would anybody find this out?
A couple of scientists from Germany blindfolded seals and had them follow and find a small toy submarine. The blindfold ruled out tracking by vision, and the plastic of the submarine meant that no fishy smells could come into play.
Even when the submarine turned a corner before the seal was released, the seal followed the path of the sub exactly. If they had been listening, they would have gone straight to the sub.
The yellow curve is the path of the submarine, the red curve the path of the seal. The green (straight) line is the expected path of the seal if it were using sight or sound. The picture in the paper was much better, but I am rather proud of my seal despite its slight turtle-like appearance.
So what’s left is touch. That’s right, touch. The highly sensitive whiskers of seals can follow the trail of turbulence left in the water by a swimming fish. Covering a seal's whiskers with a sock prevents the animal from being able to find the sub (or fish).
It's difficult to think of seeing your world in touch, but many animals rely on senses other than vision. I wonder how it would feel to echolocate...
Dehnhardt, G. et al. Hydrodynamic Trail-Following in Harbor Seals (Phoca vitulina). Science 293, 102-104(2001).