Going Home Makes Scents to a Salmon.

Migrating Coho Salmon, Oncorhynchus kisutch.

On a cool autumn afternoon, I stepped outside of my friend's house and witnessed a phenomenon of nature I have never seen before. In a stream flowing through the back yard, the bodies of spawning coho salmon, Oncorhynchus kisutch, shone blood-red in the bright sunlight. These fish battered themselves mercilessly against the stream bed, digging shallow nests in which to lay their eggs. Against all odds, they had survived the rigors of ocean life, returning to their birthplace to spawn.

There are eight species of Salmonidae (salmon) that are native throughout many parts of the western United States, including the Seattle area. Salmon begin their lives in freshwater stream beds, where they stay from several months to as long as two years, depending upon the species. After they have reached about an inch in length, the young fish travel downstream to open ocean, where they remain for two to seven years, until they reach sexual maturity. Oceanic salmon live in large groups, or shoals, that wander throughout the northern Pacific Ocean and into the Bering Sea, as far as 1000 miles or more away from their birth streams. Yet, despite their wanderlust, reproductively mature salmon overcome enormous challenges to return to their natal streams to reproduce.

How do salmon find their way back home to spawn? The cues used by salmon to find their way home from the ocean are not fully understood. But many scientists believe that a combination of geographic features, temperature, magnetic, celestial and chemical cues, hearing and other factors are involved.

However, when migrating salmon make the transition from marine into fresh water, it appears that they rely primarily upon their sense of smell (olfaction) to locate their birth streams. This hypothesis was first tested in 1954, when Wisby and Halser, two Canadian scientists, captured migrating salmon and plugged their nostrils before releasing them downstream. Those salmon with plugged olfactory pits (a salmon's nose) took the wrong fork in the stream about half the time, which is to be expected if they were randomly choosing. Those fishes without plugged olfactory pits returned to the correct stream fork 100% of the time.

Amazingly, salmon memorize and later recognize distinctive odors that characterize their particular spawning streams. But scientists still do not know exactly which molecules are used as olfactory cues by salmon. It is thought that these scents are probably a unique combination of rotting vegetation, insects, fish and dust released from local rocks and soils. However, there is some evidence that amino acids (basic building blocks of all proteins) dissolved in the water, may also play an important role in salmon olfaction.

Salmon are very sensitive to odors, particularly their predators' smells. For example, author C. Herb Williams, in the November 1978 issue of Pacific Search, described a study in which a solution containing one part of human skin dissolved in 80 billion parts of water was dumped into a river. The scent from this solution was sufficient to stop migrating salmon for as long as half an hour. Further, experiments by Canadian scientists show that salmon will slow or stop their migrations when certain human smells are present in the water. Additionally, trout -- another salmonid -- show distinct flight responses when a fisherman washes his hands upstream.

Even though it is poorly understood at present, olfaction plays a critical role in the life cycle of salmon. They travel thousands of miles during their lifetimes and yet they rely upon their sense of smell to locate their natal streams, passing by hundreds of suitable rivers occupied by other salmon populations on their way home. Their ability to make such fine distinctions is truly remarkable.

Thanks to my colleague, Nat Scholz, a newly-minted Ph.D. from the Zoology Department at the UW and now a famous scientist at NOAA, for answering my questions at the last minute for this story. Thanks to the Save Our Wild Salmon Coalition for the photograph at the top of this webpage, to Cory Wisnia for the "salmon movie" graphic, and to Atlantic Salmon of Maine for the wallpaper.

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[15 December 2001]