The lateral line sense is sufficiently directional to enable blinded fish to find and capture prey. A very low concentration, perhaps a single molecule in some cases, is necessary to excite nerve impulses from the olfactory epithelium. Fish have elaborate nostrils and enlarged parts of the brain associated with smell. The nostril does not lead into the pharynx as in mammals, but is usually designed with some sort of flow-through system. The simplest is a forward-directed flap that divides the nostril into incurrent and excurrent openings and deflects water into it when the fish is moving relative to the water.
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 so that pressure changes in the buccal cavity associated with breathing create an aspiratory effect to draw water in and force it back out. Male finds female in many deep-sea fishes. Homing Salmon and other migratory fish follow scents in the water to find natal streams. If their nostrils were plugged before release, about half went up the wrong fork. Taste buds in fish are not confined to the tongue or buccal cavity as they are in most tetrapods.