The smell of home

salmon migrating
Fish, like this salmon, rely on a sense of smell to find their way ‘home’ to reproduce — if it means traveling trying to cross misplaced roads and other obstacles.

Fish have a sense of smell 500 to 800 times more acute than humans, and some species use that “super-power” to guide them to the right stream when it is time for spawning. What is it that they smell in the water?

Somewhere in the brain, smell and memory have a close connection. A whiff of a particular something can take any of us back to childhood for a brief second. But imagine a compulsion to follow that smell for days, always moving towards it in ever-increasing concentrations…back to home.

A trout’s sense of smell is estimated to be 500 to 800 times more acute than that of a human. Fish “smell” by drawing in water through a front opening or nare. The water passes through the nasal sac inside the snout where the olfactory gland can detect odors in concentrations of just a few parts per million.

One function of this amazing sensitivity appears to be to lead the fish home when it comes time to reproduce. Adfluvial populations of trout spend most of their life in rivers or lakes, moving to streams only to mate and spawn. Soon after fish hatch from the eggs, the smell of their natal creek is imprinted on the young fry. That particular home smell remains as a memory that lasts for as many years as it takes to reach sexual maturity.

Fisheries biologists can manipulate this imprinting to help reintroduce populations into a particular stream. In the 1990’s, hatchery-raised Kokanee fry were imprinted at the crucial moment with a synthetic chemical before being released into Lake Roosevelt to grow to maturity. Several years later during spawning time, the same chemical was dripped in the target creek to attract those adults to their new home. More commonly, biologists use in-stream incubators to imprint the desired smell (and location) onto the fish’s memory.

While research is ongoing as to what particular set of soluble chemicals evoke “home” for a fish, the most common theory is that it is the rocks, the vegetation, and other aquatic organisms that create the scent to which the fish return. Each stream’s odor is unique because each watershed that feeds it is unique. Imagine a row of glasses, each filled with water from a different tributary to your local river. Our native salmonid species could identify the source of each glass blindfolded, and perhaps tell us a bit about how our watershed management is affecting the flavor and smell. Fish notice when fertilizers and weed-killers are washed into creeks, or motor oil, or chemical de-icers, or (ugh) malfunctioning septic systems. With our weak human sense of smell, we can only imagine the degree of sensory input a fish receives from its surroundings. But imagining it may encourage us to work more diligently to “leave no trace” while recreating, to plant (or protect) streamside vegetation, and to restore functioning wetland areas. We may not smell the results ourselves, but for a fish… the nose knows.

Carol Mack
WSU/Pend Oreille County Extension