The following excerpt is from the book The Fisherman’s Ocean by David A. Ross, Ph.D. Reprinted with permission from Stackpole Books, Mechanicsburg, Pennsylvania.
[dropcap]A[/dropcap]ll animals rely on their various senses – vision, hearing, and smell in particular – to orient, defend, and feed themselves. The creatures living in the ocean have developed unique ways of using these senses, ways much different than those of land animals. Using their senses, fish are sensitive to subtle changes in the movement of water, to temperature and salinity changes, to light and sound, and to other, less obvious, influences. Sound travels faster and usually further in water than through air, so many fish are sensitive to vibrations and have excellent hearing.. On the other hand, the ocean is relatively opaque, making it difficult for fish to see, so they must either adapt to low light conditions, or get by with senses other than vision. Also, light is rapidly absorbed by water, and colors quickly change with depth and distance. Wind, which carries odors on land, moves about ten times faster tha ocean currents, which carry odors in the ocean. The ocean also has none of the many landmarks or visible signs that are so common on land.
Scientists and anglers have much to learn about the sensing abilities of fish to our own, theirs are often impressive. For anglers, knowledge of the senses of fish and how they work, and their limitations, can help increase fishing success.
THE SENSE OF SMELL
Smelling is one of the best-developed and most important abilities of fish. A fish’s sense of smell is far more acute than that of humans and most animals. For example, we know that dogs have an excellent sense of smell; some breeds are more than a hundred times more sensitive than humans, and can detect odors or chemicals in the parts-per-million range. Some fish, however, can detect odors and chemicals in part-per-billion range or more, which is more than a thousand times better than most dogs and close to a million times better than humans.
Using its sense of smell, a fish can differentiate between categories of chemicals, as well as between specific compounds withing these categories. For instance, fish not only can detect amino acids (a type of organic compound), but can respond differently to specific types of amino acids. Fish detect chemicals in water by using their senses of smell and taste. Sometimes it’s not clear which sense is at work.
The Smelling Process
Most fish have small nostrils (called nares) on each side of their snout, just in front of their eyes. many fish, even those with an excellent sense of smell, have nostrils that are inconspicuous or not visible. (A fish doesn’t breathe through its nostrils like many other creatures, but rather uses them exclusively for smell). Sharks tend to have relatively visible nostrils, a feature that has earned them the reputation of being extremely dependent on their sense of smell. Actually, , sharks do not have a well developed sense of smell, and this sense is important to their survival, but not necessarily any more so than it is for other species of fish.
The smell “sensor” within the nares is called the olfactory organ, which contains numerous odor-sensing cells. For fish to smell, water must move rapidly over these cells. This can happen as the fish swims or faces into the current to force water over the sensory cells. Many game fish species have two nostrils on each side of their head – one for incoming water, – which facilitates a greater water flow over their olfactory cells. Fish with double nostrils generally have an especially excellent sense of smell.
Next Wednesday’s Fish Facts Ross will cover The Value of a Sense of Smell.