On a cool autumn afternoon, I stepped out of my friend’s house and witnessed a phenomenon of nature I had never seen before. In a stream flowing through the back yard, I saw the bodies of spawning coho salmon, Oncorhynchus kisutch, gleaming blood-red in the bright sunlight. These fish battered themselves mercilessly against the stream bed, digging shallow nests in the gravel where they were depositing their eggs. Against enormous odds, they had survived the rigours of ocean life, and had returned to their birthplace to spawn.
Salmonidae (salmon) are native throughout many parts of the western United States, including the Seattle area, where I watched these fish. Salmon begin their lives in freshwater stream beds, where they live from several days to as long as two years, depending upon the species. Prior to migration, the young fish enter a physiological stage known as smoltification, which prepares them to survive in salty marine waters. Then the salmon migrate to the sea and remain there for two to eight years, eating and gaining weight 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 spawn.
How do salmon find their way back home to spawn? The cues used by salmon to find their natal streams are not fully understood. But many scientists think that a combination of geographic features, temperature, magnetic, celestial, and chemical cues, sounds and other factors are involved.
It has been “common knowledge” for more than 100 years that when migrating adult salmon enter fresh water, they rely primarily upon their sense of smell (olfaction) to locate their birth streams where they will breed. But this hypothesis was not formalised until 1951, when two zoologists, Arthur Hasler and Warren Wisby, who were then at the University of Wisconsin-Madison, proposed that salmon have such a keen sense of smell that they can distinguish their home stream from the thousands of other suitable spawning streams by identifying and following their natal stream’s scent.
This hypothesis was first tested in 1954, when Halser and Wisby captured migrating salmon and plugged their nostrils (olfactory pits) before releasing them downstream. The salmon with plugged olfactory pits took the wrong fork in the stream about half the time, which is what you would expect if they were randomly choosing. Fishes without plugged olfactory pits returned to the correct stream fork nearly 100% of the time.
According to Hasler and Wisby’s Olfaction Hypothesis, young salmon imprint upon the unique scent of their natal streams when they are smolts. The olfactory hypothesis proposed that each stream’s drainage area has a unique combination of rotting vegetation, insects, fish and dust released from local rocks and soils. But which molecules provide the salmon with their informational cues? Maybe one such molecule type is one or more amino acids: there is mounting evidence that individual amino acids (those basic building blocks of all proteins) dissolved in water may provide important information to salmon, which they detect through olfaction.
As any fisherman (and most fish-keepers) can tell you, fish are very sensitive to odours, particularly the scent of their predators. In the November 1978 issue of Pacific Search, author C. Herb Williams described a Canadian study where a nearly homeopathic solution containing just one part of human skin dissolved in 80 billion parts of water was dumped into a river. Astonishingly, the scent from this solution was sufficient to stop migrating salmon for as long as half an hour. Additional experiments by Canadian scientists show that salmon will either slow or stop their migrations when certain human smells are present in the water, and trout — another salmonid — show distinct flight responses when a fisherman washes his hands upstream.
This offensive scent was identified as the amino acid, serine, which — because human skin contains serine — has led to some fishermen to refer to this as “the serine problem”. The amount of serine in human skin depends upon the sex, age and race of the individual — interestingly (and appropriately), the worst “serine offender” are adult white human males.
But the olfactory acuity of salmon can be temporarily dulled or permanently damaged. For example, a 2008 study in rainbow trout showed that a 96-hour exposure to a pesticide mixture with the same concentrations as what is found in at least some British Columbia rivers reduced their responses to natural odours. Olfactory interference or damage also resulted after exposure to sublethal concentrations of heavy metals, such as copper, that are common in urban runoff, as well as drops in pH, which result from acid rain. So common are these water pollutants that they may be important contributors to the threatened and endangered status of many salmon stocks.
Even though it is poorly understood at present, we know that olfaction plays a crucial role in the life cycle of salmon. These fish rely on scent for nearly every aspect of their lives, from locating food to avoiding predators. They travel tens of thousands of kilometres during their lifetimes and rely upon their discerning sense of smell to bring them home again.
Sandahl, J., Baldwin, D., Jenkins, J. & Scholz, N. (2004). Odor-evoked field potentials as indicators of sublethal neurotoxicity in juvenile coho salmon (Oncorhynchus kisutch) exposed to copper, chlorpyrifos, or esfenvalerate. Canadian Journal of Fisheries and Aquatic Sciences, 61 (3), 404–413 | doi:10.1139/f04–011
Tierney, K., Sampson, J., Ross, P., Sekela, M. & Kennedy, C. (2008). Salmon Olfaction is Impaired by an Environmentally Realistic Pesticide Mixture. Environmental Science & Technology, 42 (13), 4996–5001 | doi:10.1021/es800240u
W.J. Wisby & A.D. Hasler (1954). Effect of olfactory occlusion on migrating silver salmon (O. kisutch). Journal of the Fisheries Research Board of Canada, 11: 472–478