Chemical Messages Underwater
Fish detect the chemical cues that surround them using both their senses of smell (olfaction) and taste (gustation). Of these two, olfaction is considered the most important, especially over long distances. Fish use taste to assess things that are physically in contact with the taste cells. The underwater environment is a soup, full of chemical messages. Fish constantly intercept these messages, using their highly developed sense of smell, choosing which to respond to and which to ignore.
Do fish have noses?
Not exactly -- but they car smell. Unlike many land animals fish do not breathe through their nostrils; these are used instead solely for smell. Some species have two pairs of nostrils and actively pump water in through one pair and out through the other, so that the water passes over the sensory receptors responsible for detecting the chemical messages. They may then be decoded by the olfactory bulbs, which sit in the forebrain of the fish.
How well can fish smell?
A good sense of smell essential for many species of fish because vision can often be limited underwater by turbid, muddy water and shade from plants, such as lilies. The ability of fish to detect chemicals varies, but is strongest in those species that live with the least light. Fish in the deep seas are able to detect concentrations of chemicals as weak as one part per quadrillion -- put into understandable terms, this is about the same as being sensitive enough to detect a balloon in North America. Although this is an extreme case, an excellent sense of smell is common to many species of fish.The ability of blind cave fish to compete successfully for food in a tank containing sighted competitors is a case in point Although the cave fish cannot track the food visually, its excellent sense of smell means that it is able to build up a chemical profile of its habitat and use this to locate a meal.
How do fish use smell?
The ability of fish to orientate towards (or away from) the source of an odor is called chemotaxis. Each animal produces its own odor; if a fish detects the odor of a prey animal, it is likely to try to use this to track down that animal. The process of discovering the source of a smell starts when the fish, in this case, crosses the odor trail of the prey animal. Once the fish detects the presence of the smell, it circles to reconnect with the trail and then moves up the odor gradient, going from a weak smell to a stronger one, until it finds the source and devours it.
As well as finding food, fish are able to navigate using their sense of smell. The most famous of all fish species for doing this are salmon, but most fish are capable of detecting and moving towards the smell of a local, familiar habitat. Each habitat has a unique chemical signature and it is this that the fish respond to. This ability is extremely important -- fish that become familiar with a preferred area of their habitat know from experience where the best hiding places are and where and where food can be found.