How do fish smell? Terrible! Well I thought I should get the obvious joke out the way first.

Fish have an incredible sense of smell. Scent is carried by molecules in the water and many species of fish (particularly eels) can detect a concentration of less than five "scent molecules" as they flows by.

Fish have elaborate nostrils and enlarged parts of the brain associated with smell. The nostril does not lead into the mouth as in mammals, but is usually designed with type of valve system.

The simplest is a forward facing flap that divides the nostril and deflects water into it when the fish is moving. Some fish have cilia that create a continuous current through a U-tube nostril. Others have a relatively thin membrane separating the nostril and the buccal cavity (area in the cheek or mouth) so that pressure changes in the buccal cavity associated with breathing create pressure to draw water in and force it back out.

Many fish use the sense of smell to detect pheromones, chemical substances released by an animal that influence the behavior of members of the same species. Fish may release pheromones during the breeding season or when alarmed. Pheromones are released by minnows as a fright-substance, that causes other minnows to display escape behaviour.

Smell also has the obvious function for detecting food but also the less obvious function of finding home. Studies suggest that species of salmon (family Salmonidae) use smell to guide them to their home streams during the breeding season (LAGLER, 1962).