Wednesday 14 January 2015

The Importance of Smell

Over the past 2 years I've written a lot about how and what the salmon sees, and the appearance of flies underwater.  If you've not read them, the  key posts are Windows on the World and Here's Looking at You.  Of all the salmon's remarkable senses, sight is important to the angler because in the final stage in the process of fishing it alone triggers the unknown stimulus (or malfunction) that causes a fish to respond to a fly.  But it does not create the preconditions that lead up to that moment.

We see it all
Near & Far
Light and Dark

As humans we have intense dependence on sight. Compared to our limited sense of smell and hearing dulled by urban life, our eyesight is pretty miraculous. In our youth in daylight we can instantaneously focus, render and resolve objects across a span from 50 cm to 5 km, within widely differing levels of contrast. A large chunk of our brain is dedicated to image processing and storage, including the visual clues for navigation and situational awareness. However, this remarkable level of visual capability (and our consequent relative diminution of our other senses) leads us astray when thinking about salmon and causes us to disregard the ways in which its other senses impact upon behaviour.

River Dee mid-April 11am clear water, bright day
Sun elevation 37 degrees
Visibility < 6.5 m
Compared to our air, water is a poor visual medium and rapidly becomes much worse as soon as the river rises or the angle of elevation of the sun falls below 5-6 degrees.  Even in the clearest rivers like the Dee, the physics of light in water limit visibility to less than 10 metres in optimal conditions. A salmon can detect a moving object at that range but will not identify it until it is much closer. As a result of its environment the salmon is inherently short sighted because it doesn't need to see a long way.  Given those limitations, nature has enhanced the capabilities of the salmon's other senses.

In my last post, Speaking Salmon, I described the salmon's extraordinary sensitivity to sound, vibration and motion; and its role in schooling, hunting, threat detection and situational awareness. Those capabilities are optimised for the open ocean: once the salmon enters the river their effectiveness is constrained by the confined space. In contrast, its sense of smell works relatively better because the river's current helpfully delivers the sample in a continuous stream.

A Superlative Nose

Strictly it isn't smell at all, but chemoreception. However, for simplicity and clarity I'll stay with familiar terms like nose and smell.

Evolution has given the salmon one of the most sensitive noses on the planet. The large olfactory chambers are connected to the brain via high capacity channels (clearly visible at 11 and 1 o'clock from the brain cavity). A significant amount of the salmon's brain capacity is devoted to processing smell information.  This combination of detector and processor gives the salmon the ability to discern smells at the level of parts per billion.  Put in a familiar context, that equates to detecting one pub measure of whisky in 25 tons of water, and correctly identifying its brand.  

River Findhorn
Garden Pool at Tomatin

A salmon could detect the anomaly of three measures of whisky in the volume of water in the pool shown in this picture, and if so conditioned, react to it.

The salmon's sense of smell is not as vulnerable to 'jamming' in the same way as its vibration sensors, because each scent exists as a discrete element and is lodged in its memory library. However, new and unfamiliar smells will lack meaning and thus may cause indecision and create the need for inspection and confirmation.

Exceptional Capabilities

The salmon's sense of smell delivers exceptional capabilities:

  • Navigation. Contrary to popular belief, smell is not the critical foundation of the salmon's migratory navigation. Norwegian researchers have shown that salmon can find their way back with their noses blocked. There's a fuller explanation of navigation in The Annual Miracle. However, once the salmon approaches the estuarine outflow, smell provides confirmation of its destination, which becomes progressively more precise as it ascends the river until additional factors come into play.
  • Threat Detection. Smell delivers an acute, 24 hour, long-range early warning system that is conditioned and programmed from birth, and superimposed on embedded cellular information inherited from previous generations. All fish-eating predators discharge trigger scent chemicals, especially in their excrement.  The salmon can detect upstream threats in the river (across the range from herons, via otters to bears) over great distances.  Even though a 15 lbs salmon is not vulnerable to attack by a heron, the fear embedded in infancy remains. Fish that are physically ruptured by attack also discharge signal chemicals. Lying on or near the flow line will give a salmon the best information more rapidly and reliably than in a backwater.  A 50 gram dollop of heron poop will be quickly detected by every fish hundreds of metres downstream. We don't know how the salmon will react, but may reasonably surmise that their anxiety level will increase.
  • Friend Detection. The salmon's body is adjusted to living in salt water. Once it enters fresh water, osmosis causes the less dense fluid to enter its body, causing over-hydration. The salmon compensates for this by excreting large volumes of urine. The smell of the urine gives the salmon a means for detecting and identifying the presence of other salmon upstream. The information it derives from the smells will include numbers, sex, stress levels and genetic-family groupings.  Salmon are more likely to be alert and run confidently if the information coming downstream indicates a secure and friendly environment above.   Conversely the lonely springer is afforded no such comfort, which might explain their more tentative progress. This feature may also add some credence to the popular belief that the presence of numerous kelts in a river may encourage fresh fish to run.
  • Family Detection. In researching this post I discovered research has demonstrated that even humans can detect their children and siblings by smell. Given the salmon's
    A dividend of
    hormonal aggression
    far more powerful sense of smell and their excretion of hormone-charged urine, they will be able to identify family members accurately at distance.  
    The research confirms this ability to discern relationships in fine detail, and suggests that it is significant in the final pre-spawning period.  Salmon are social schooling fish that are happiest in familial company. Even though smolt-to-adult survival figures may be quite low, it is probable that small groups of siblings will return and group together.  They will be more relaxed and confident that way. However, as hormone levels rise later in the season, the presence of non-family competitors leads to friction and aggressive behaviour, primarily amongst males (see the Morning Glory post). Keep your eyes peeled, and if you spot such behaviour, fish to exploit it. In sum, it's likely that fish in a pool will seek out and congregate with any 'relatives' from the same family group or spawning tributary (with small DNA variations) that they detect, whilst avoiding competitor groups. As a result, later in the season as pools become more crowded, you get more fish displaced into sub-optimal lies that are correspondingly edgy and potentially more likely to do something daft like taking your fly.  
  • Exploration and Curiosity. Falkus' observations of salmon behaviour indicated that they would close with unfamiliar objects in a manner suggestive of sniffing. However, I've found no research evidence to suggest that minor traces such as human scent, pheremones or otherwise, on a fly have any perturbing effect whatsoever.  Owing to the brevity of our relationship with the salmon, we're firmly in the 'unknown' category, and the female fishing advantage remains unproven. Personally I think they succeed because they don't commit the sin of trying to cast 10 yards up the far bank, and consequently lay out a straighter, neater and thus more effective line.
  • The Downside - Aversion, Delay and Indecision.  Such high nasal performance has a price, but the dividend goes to the angler. By the time a smolt departs to sea it will have accumulated a comprehensive library of smells that will remain effective on its return. It will not, however, cover all eventualities, especially if you consider the random nature of rain outwash from roads, agriculture, farmyards and so forth. Any sample of river water will include some scents the salmon has never previously encountered, including tributaries other than the one in which it was born and raised. Differentiating and resolving this information
    A typical smell decision point
    small beck on the right

    requires time: nature is rarely hasty in such matters, and a fish will stop to work things out before proceeding. The picture shows an example of a typical decision point, where a small but often vigorous beck enters the main stream from the right. In the previous 2 kilometres it passes through 2 farmyards and under the main A road to Hawes. The stock in the fields through which it passes changes with the seasons: cows in summer, sheep at other times. The running line through the pool enters at far left by the bushes and crosses to the right before the big oak. As a result the smell pattern changes abruptly as the salmon enter the flow effect of the beck, which may be transmitting unfamiliar smells that require resolution. That process will cause the fish to pause, however briefly, and an alert fish at the short halt offers a much improved taking chance, so observe and locate any such decision points on your beat before you reach them.


As humans we tend to relegate smell and thereby forget its importance to the salmon. If we take a little time to understand the salmon's exceptional sense of smell; its consequences for behaviour; and the opportunities it creates for us to catch them, then we may just improve our chances. So always look for any smell decision points when you walk the beat and fish to them. Later in the season, if you see anything that looks like hormonal behaviour, fish to it energetically. So, as ever, observe, think, act: please don't be a passive angler.

The season opens soon: Hurrah!


  1. This is an exceptionally well written, and most interesting, article.
    Have you researched whether artificial smells, such as the bait additives used by pike fishers, attracts salmon into taking? I have bought a bottle of pilchard oil, and intend to dip a fly or two in it this year. Stoater.

  2. Many thanks for your kind comment.

    Of course the big difference is that the pike is an active feeder, whereas the salmon is not. However, as a scavenger the pike is naturally attracted to the smells emitted by injured, dying and dead prey, so any angler who can approximate those odours may well succeed.

    When feeding at sea the salmon uses vibration and sound for surveillance and sight for target acquisition. It attacks at such speed that smell would barely register. When feeding in fresh water in its youth its feeding was largely sight-based and reflexive.

    I've run experiments presenting salmon with exact replicas of their sea diet to see whether they would react (see the Fast Food and Broad Beans post): they don't. Thus even if you could replicate a suitable prey smell, I fear all you may secure is smelly fingers!

    Tight lines nonetheless.

  3. Really intertesting.

    I used to smoke, and often wondered if this had an adverse effect on my fishing for either trout or salmon.
    Any thoughts?
    Not long to go.


    1. Nick, many thanks for your kind comment. I've got no research to back this up, but would suggest that the salmon species' relationship with man and tobacco together (only about 100 years) is too short for any threat linkage to be made. You can observe the same fact with stalking roe deer: they've known man for a million years and bolt at his scent; but they don't react at all to tobacco smoke (I used cigars to keep the insects off). On those grounds I think your fishing would be unimpaired. Indeed, many of the legendary fishermen of the 1950s were serious smokers, mostly as a result of their war service, and what they smoked would have left your choices for dust - Capstan Full Strength anyone?