Yaël: I've noticed that fish have little holes that look like nostrils. Does that mean they      can smell?

Don: Smell, or olfaction, as scientists call it, is an important sense for many fish, Yaël.      Those little holes that look like nostrils are called nares. Nares don't lead to the      throat the way nostrils do in mammals, but open up into a chamber lined with sensory      pads. Not all fish move water in and out through these nares in quite the same ways,      but key to a strong sense of smell for fish is the ability to move water rapidly over      these sensory pads. Some fish can pick up chemical signals when immobile by      pumping water through their olfactory system via tiny hairs called cilia. Other fish can      pump water by a muscular movement. Some fish, such as smaller species of      mackerel, have an olfactory system that requires them to swim in order to get water      moving through their nares. When the sensory pads pick up chemical signals, they      transmit them to the fish's forebrain, which interprets the signal and incites the fish to      respond appropriately.

Y: You mean like if the chemicals signal food, the fish will pursue the food. Or if the      chemicals signal danger, it'll flee.

D: You got it. But fish use chemical cues in all sorts of ways. For instance, a large group      of fishes, including minnows, release a chemical when they're wounded that incites      other fish to flee. And then there are salmon, which are known for a superb sense of      smell that enables them to sense the stream where they were born, so that they      may return to it to spawn.  

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