Seals detect very tiny fluctuations of water flowing through their whiskers to track fish in dark seas.
A fish can cover hundreds of yards in half a minute, leaving a decaying underwater trail. Seals analyze the structure of the plumes, vortexes and jets to find out which direction the fin moved.
Using it’s streamlined whiskers to follow the structure of an underwater trail, a seal is able to stay right in the middle up to 35 seconds after the fish has stopped.
It turns out that sharks can detect small delays between their nostrils to turn toward whichever side picked up the scent. It’s not so much differences in the concentration of odor but directional cues based on both odor and flow.