Sharks Don't Have Super Noses, So How Do Salmon Smell Their Way Home?

When Scientists Measured Shark Smell, It Matched Other Fish

Tricia Meredith, then at Florida Atlantic University, tested five species of sharks and rays by measuring how they reacted to dissolved amino acids, the small building blocks of proteins that many fish detect as the smell of food [1]. According to this news summary, the most sensitive sharks detected about one drop of scent dissolved in a billion drops of water [1]. That is a very faint smell. But it is roughly the natural background level of amino acids floating around in coastal seawater [1]. Meredith’s own summary was that a shark cannot smell a drop of anything in an Olympic-sized pool [1].

The news article that reported the study also gives a reason why a shark might not need to be even more sensitive: it could then have trouble telling traces of a possible meal from the background [1]. That is the article’s explanation, not a measured result. The finding does not say a shark’s nose is poor. It says sharks sit about level with other fish, and the legend of a super-nose goes beyond the measurements.

One more detail shows why nose size is not the whole story. In sharks and rays, the inside of the nose has thin, plate-like folds called lamellae that add surface area, and tiny secondary folds on them add a lot more. A 2019 study of 13 species found these folds differ in shape and size from species to species [2]. More folded surface might sound like a sharper nose, but I did not find a source here that links folds to sensitivity, so this note does not claim one.

A Fish Nose Is a Pair of Holes Made for Smelling, Not Breathing

Look at the head of a fish from the side, and in front of the eye you may spot small openings. They are not for breathing. Sharks, for example, breathe with gills on the sides of the head, while two nostrils at the front pull water into a nasal chamber [1]. As water flows through, smell molecules dissolved in it reach the sensing cells. So a fish nose works as a small water-testing device.

This is why amino acids matter in the story. They dissolve in the water, and they are one of the kinds of substances fish detect as the smell of food. The same kind of molecules turn out to be the key to the salmon’s trip home.

Salmon Come Back to Their Home Stream, and Smell Seems to Guide the Last Part

In 1946, a biologist named Arthur Hasler was hiking a mountain trail in Utah when a scent from his boyhood suddenly brought back a flood of memories. He wondered whether salmon could do something similar: each stream might have its own smell, young salmon might memorize it before heading to sea, and years later they might use that memory to find the right stream [3]. With a graduate student, Warren Wisby, he set out to test the idea.

In a 1953 field test, a BYU magazine article reports that all but 8 of 73 normal fish went back to the branch of the stream where they were first caught. Among 70 fish whose nostrils had been plugged with cotton to block their sense of smell, 28 entered the wrong branch [3]. Blocking their sense of smell made the fish more likely to choose wrongly, which supports the idea that smell is part of how salmon choose.

Two cautions. First, this is evidence for the final choice at the stream, not for steering across an ocean, and I did not find a source here on how salmon navigate the open sea, so this note leaves that out. Second, the experiments I could read are on several different species, so one result should not be applied to every salmon.

Each Stream’s Smell Is a Recipe of Amino Acids Made by Life on the Riverbed

What is a stream’s smell made of? A research project led by Hiroshi Ueda at Hokkaido University, in Japan, reports that the key ingredient is a mix of amino acids dissolved in the water. According to the project summary, this mix is mostly produced by biofilm, the thin layer of microbes that coats stones on the riverbed, and it stayed steady across a river basin, across seasons and across years [4]. A smell that kept changing could not serve as a memory cue, so that steadiness matters.

According to the same project summary, Pacific salmon such as chum, masu and sockeye learn the amino-acid mix of their home stream as they migrate downstream. Years later, when mature, they are reported to identify it by smell and choose their home stream with high probability. Pink salmon did not choose it in these tests [4]. I could read only this short summary, not the full papers, so I cannot give the exact percentages here.

Think of the recipe like a perfume blend. Many streams contain the same ingredients, but the amounts differ, and the blend is what identifies a stream. That is a way of picturing it, not a quote from the research.

Young Salmon Can Learn an Artificial Smell, Which Shows the Memory Is Learned

If the memory is learned, a lab-made smell should work too. Hasler and a colleague named Scholz exposed young fish to synthetic chemicals before they migrated. According to the BYU article, after the fish went to sea, they returned 18 months later to the lake that had been scented with the chemical [3]. A 1979 review by Hasler describes experiments of this type with farmed coho salmon, rainbow trout and brown trout, using the chemicals morpholine and phenethyl alcohol [5].

This suggests that what a young salmon stores is a chemical pattern, and that it is not limited to natural amino acids. In a real river the pattern is steady from year to year, so it works as a reliable address. Putting the results together, a shark and a salmon may have noses of similar sensitivity, and the difference is in how the smell is used: a salmon learns a river-specific recipe and keeps it for years. That is a reading across several studies. None of them compared sharks and salmon directly.

Try It Yourself: Look for the Nostrils on a Fish

Next time you are at an aquarium, or see a whole fish at a market, look at the front of the head. Can you find the small openings in front of the eyes? Do you see one pair, or more? Compare two kinds of fish. Do the openings sit in the same place on both?

At an aquarium, looking through the glass is enough. If you touch a fish at a market, ask an adult first, mind the sharp fin spines, and wash your hands afterward.

If you want to dig deeper, read the AIP news article [1] and then the Hokkaido project summary [4]. Ask yourself: which numbers come from measurements, and which are explanations by the writers?

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