Speaking Salmon

How?

Have you ever wondered how?

• Batches of spate-tumbled smolts arriving in the estuary find each other to form a school? There are very few of them in relation to the volume of water, so relying on sight with an underwater range of no more than 10 metres is not a viable option.  Of course they will form schools, because that’s the most efficient way to hunt and survive attacks, and evolution has long since eliminated the inefficient and vulnerable solo artists among their antecedents.
• Having got together they stay together reliably, day and night; near the surface and in the dark depths; all whilst manoeuvring at high speed in 3 dimensions?
• Having reached the Greenland Basin, they find their lunch?  There’s lots of food about, but there’s much more ocean.  The odds of accidentally colliding with a bunch of capelin, prawns, sprats or baby squid are very poor.  Furthermore the young salmon would expend a great deal of precious energy and muscle protein in the search process: your magnificent 3SW fish would return as a scrawny 5 pounder, if at all.  Again, relying on eyesight alone doesn’t work: the probability of a salmon spotting a 100 metre square collection of prey fish on each transit through a 1 Km block of water is less than 1% (it doesn’t improve with repetition because both parties move).  And that’s in broad daylight.  The chance of getting Christmas dinner by visual collision in the prevailing northern latitude darkness falls to 0.01%, which is a recipe for starvation.
• And along the way they avoid being eaten themselves?  The species has not survived as long as it has by being unnecessarily vulnerable or being taken by surprise.

So what's the trick?

Sound and vibration in water

The underwater sound environment is entirely different to that in which we live in air.  Accordingly, when thinking about the underwater world we have to dump our experience and preconceptions.  Simply, salmon don’t ‘hear’ like us, because they don’t have ears: they sense, using large arrays of receptors.  However, the one thing we have in common is background noise: the ocean is far from silent.

The key features of sound in water are that it:

• Is about 800 times more intense than in air, because the water is incompressible and therefore a much more efficient transmitter.  In addition the surface layer reflects sound back into the water.
• Travels far further than in air: relatively minor events are detectable at ranges measured in kilometres, but the level of background noise is relatively very high because it is drawn from a much wider area.
• Goes about 4.4 times faster.
• Is influenced by the composition of the water.  The ocean is not uniform and has layers with differing salinity/density, temperature and motion.  The trapping of sound within layers is a common phenomenon in salt water, whereas it’s rare in air and usually only encountered in conditions of extreme atmospheric inversion.
• Becomes increasingly complex as distance extends, which presents processing challenges for the receptor.
• Works best at lower frequencies (50-700 Hz) equating to wavelengths greater than 1.5 metres.

Some of the most detailed knowledge of sound in water exists in the closed world of defence anti-submarine warfare.  It began in World War I with simple hydrophones; expanded rapidly in World War II; and culminated in the height (or depth) of the Cold War with the imperative to detect, locate and track very quiet ballistic missile submarines.  Billions have been spent on the science and technology.  Much of what the advanced navies know is highly classified because their national survival might depend on this knowledge.  But we do know that underwater warfare specialists receive extensive training in the identification of naturally generated sounds and their differentiation from those originated by nuclear reactors, turbines, pumps, generators, shafts, propellers and bearings.  Schools of prawns (“a bit like Rice Crispies”), herring, mackerel (“sawing”), squid and shad (“ticking”) all contribute recognisably to the cacophony.  For the sailors it’s enough to know those are natural.  For the fish biologist the meaning of the sounds takes on special importance.  Over the past 20 years affordable computing power and increasingly sophisticated signal processing software have delivered a revolution in capture and analysis.  One of the leaders in this field is Dr Rodney Rountree, Adjunct Professor at the University of Massachusetts, who specifically researches inter-fish communication.  Some of what follows is drawn from his pioneering work and is hereby acknowledged.

Knock! Knock!  Who's there?  Haddock!

Why do fish need to communicate?

• First, if their evolved survival strategy requires communal coordinated activity in schools, they need to be able to find each other and stay together thereafter.  It is axiomatic that the latter requires continuous signalling.  Research (1976 - Pitcher) has demonstrated the ability of fish to rejoin a school reliably and quickly even when blinded.
• Second, they must be able to raise the general alarm if there’s a threat approaching in order to be able to trigger a whole-school evasive action.
• Third, to inform their fellows of the detection and location of food.
• Fourth, to attract a suitable mate.  It always helps a relationship to be on the same wavelength!

How do they do it?

For conventional ‘round’ fish (which includes salmon) the air in the swim bladder provides a useful ‘sounding’ mechanism: mouths, gill plates and tongues may also assist in both generation and modulation.  Crustacea, squids and invertebrates have different mechanisms and thus produce different tones and frequencies.  Some species of prawn can generate very loud sharp “snap” sounds by means of diaphragm and shell movement.

Picture by British Sea Fishing

Here is an example of possible talking haddock and what may be their routine schooling signal drawn from Dr Rountree’s work.  Click on the spectral diagram to listen to the recording of the knocking sound issued by haddock in the Gulf of Maine.  In the zoomed and filtered display the knocks are clearly visible as the orange peaks; and in the smaller display above you can see the frequency composition of a sample of knocks, noting the sharp rise and slower decline, and the distribution of intensity between frequencies.

If you look at the recording of mating haddock, you will note a different signal profile, and particularly its extended duration and frequency concentrations, which together are capable of conveying more detailed information. If this is indeed what Dr Rountree suggests, then the reasoning above indicates that haddock need a vocabulary of at least 4 distinct sounds. There are 3 in the ‘possible haddock’ category on Dr Rountree’s website.  I suspect that the “Attack!  Dive! Dive!” alarm is harder to capture.  Nevertheless, the data Dr Rountree has accumulated clearly shows the existence of fish chatter with distinct characteristics.

Hello Salar

Given the fact that the salmon is a round bodied schooling predator, then it is reasonable to infer that evolution may have given it the capacity to communicate in at least 3 of the 4 modes outlined above whilst it is at sea.  The need to communicate in fresh water is less clear cut and perhaps less pressing owing to closer proximity and the use of smell (a later post), but in the absence of any data we can’t rule it out.

Of course the ability to detect, process and interpret sounds is arguably even more important than transmission.  For a predator the capacity to detect, identify and locate prey species through their schooling signals is a critical capability.  Taking the example used at the start of this article of transit through a 1 Km block of water, if the salmon can detect prey signals at say 50 metres, then the probability of intercept rises almost 25-fold.  The probability increases with the square of the increase in detection range, so once it approaches 100 metres the success rate starts to look distinctly good for the salmon, provided that detection includes a respectable directional component.

We don’t know what the salmon’s detection range may be for any species, including its own mutual communications.  They’re too rare and widely distributed in the ocean to be a viable research target for Dr Rountree and others.  But we do know that nature has endowed the salmon with a remarkable sensor suite in the sound and vibration domains.  The two key areas are the hard upper surfaces of the head and the lateral line, which appears to be nature’s version of the towed array passive sonar (in addition to its other functions in motion and direction sensing etc).  By virtue of its length of the lateral line provides not only a large number of detectors, but also, a directional capability from the timing of the arrival of signals onto each detector.  The salmon needs both head and lateral sensors because they have different functions, which become clearer when you realise that the utility and efficacy of the lateral line is much reduced when the salmon is facing directly towards the target.  This leads to the reasonable supposition, supported by indicative research (2001 - Coombes, Braun, Donovan) that the lateral line is primarily for surveillance, detection and location; and the head for homing target acquisition in the final pursuit and attack phase up to the point at which the prey enters the field of view.  During their research, fish whose lateral lines were artificially stimulated turned directly to attack the frequency generator head on.

The importance of the lateral line is evidenced by the intense concentration of nerves and synapse channels, which you can see clearly amidst the motion sensing ‘hairs’ when taking a salmon apart.  The darker material along the flank gets progressively wider and deeper as it approaches the head, indicating the accumulating volume of material.  It’s especially clearly visible in a smoked fillet.  Put simply, in any fish, the bigger the dark area, the greater the sensor bandwidth (i.e the capacity to transfer data to the brain). The salmon is very well equipped in this department.

It’s also interesting to cut up other species to observe the scale and density of the lateral discolouration.  As an avid sea angler in my childhood, and being generally inquisitive (aka bothersome child), I dealt with a wide range of species.  What I observed was that generally slow moving, solitary and sedentary flat fish like flounder, turbot and plaice had relatively small lateral line nerve concentrations.  In contrast, the round-bodied, mobile, high speed schooling predators like bass and mackerel had especially large arrays.  Indeed, relative to its size, the mackerel’s lateral array was remarkable, a feature that is possibly determined by the nature of its prey.  Pollack, which tend not to cover large areas when hunting, were less well equipped.  

Looking across the field, the salmon is near the top end of the scale of lateral sensitivity, signal processing and employment, consistent with its long and comprehensive evolution as a voracious predator and agile survivor.  It clearly depends on sound and vibration for critical functions that are essential to its survival in the ocean.

So what?

So much, so interesting, but what is its relevance to the angler?

If certain frequencies can stimulate a salmon to attack oceanic prey, can we exploit this in fresh water?  In thinking about this it helps to grasp what 300 Hz sounds like in air : for comparison Middle C is 261 Hz.  It is certainly much higher than the dull thrum of commonplace line vibration in fast water, which is in the range 10-30 Hz.

Back in the 1950s and 60s, several companies, notably Vibro and Mepps, marketed spinners - vibrating spoons - that they claimed would stimulate takes by salmon.  As this preceded legislation on descriptions and advertising standards, they were not required to furnish the evidence to underpin their claims.  I’m not aware that either spoon was markedly better than the competition such as devons and tobies (some of Abu’s lures were also claimed to have frequency stimuli).  The difficulty I observe is how a spinner rotating at about 3-6 Hz (rpm) can generate consistent vibrations in the 1-500 Hz band that mean anything to fish. The lures do generate vibrations across a broad spectrum, but there's no way of knowing what the fish make of the noise.  Needless to say, the makers of bass lures in the USA make extraordinary claims for the stimulative properties of the frequencies generated by their lures.  You can even buy powered lures, complete with batteries, chip controllers and vibration chambers (“chatter-bait”).  I note, however, that their competitors remain in business, which suggests first that these lures don’t come with a performance guarantee.  It also shows that American bass anglers are even more fixated than their British salmon peers.

For the fly fisherman, frequency generation is problematic.  There are no moving parts in a fly, so you’re wholly dependent on the effect of water passing over or through the fly’s body.  The external flow will generate vibrations, but their frequency and power will vary with the speed of the water and with any discontinuities or apertures in the body over which it passes (the ‘bottle top’ effect).  However, for the apertures to be effective vibration generators you would probably have to dispense with all of the fly’s body dressing to obtain the requisite smooth surface flow.  The internal flow effect is similarly influenced by speed and additionally by the size and shape of the aperture and passage through which it travels.  The small frontal diameter of most fly designs poses a severe limit on the options: water doesn’t pass rapidly through small holes if it can easily disperse sideways.  Unless and until someone does some research and underwater measurement I shall remain sceptical.

So what else?

If we can’t exploit it directly, how might the underwater sound environment influence salmon behaviour?

Mechanical noises

Given the sensitivity of the salmon’s faculties, we might reasonably infer that it would find extremely noisy environments difficult and therefore avoid them if possible.  If one of the roles of the sensors is to detect predator threats, then a salmon will feel insecure if that capability is disabled by extraneous noise.  The most recent research in this area (Bristol & Exeter universities) was into the predator awareness of sticklebacks in enhanced noise environments, which clearly showed changes in behaviour.  We don’t know whether it applies to salmon, but I should be surprised if it didn’t.

Of course much depends on the frequency and modulation of the sound.  Just as we can’t hear the ‘silent’ dog whistle (or much else besides at my age), there will be noises that fall outside the salmon’s working range.  For example, the thump of HGV tyres over the expansion joints on the A9 Tomatin Viaduct, which the bridge piers transmit directly into the Dalnahoyn Pool below, are at a very low frequency.  However, the productivity of the pool suggests that those sounds don’t trouble the salmon at all, despite their evident loudness in air.  The same appears to be true of railway and smaller road bridges.

The fact is that there are relatively few sources of 2-700 Hz vibration that are likely to impact salmon rivers.  One exception is diesel engines, commonly used in pumping, power generation and propulsion, which emit vibrations right across that range.  The engine, the transmission system and the application device will all emit vibrations at different frequencies across the critical range.  For example, one make of 4 cylinder diesel engine, which has many common auxillary applications, when running at 1850 rpm under load displays 4 frequency peaks centred on 400 Hz.  If diesel engines and the associated machinery are employed and mounted with inadequate damping on metal structures that stand in the water or on the adjacent rock or soil bank, then they will transmit their vibrations directly into the water.  This will raise the level of underwater background noise to what may be uncomfortable levels for the salmon’s highly sensitive receptors, irrespective of whether the human perception is of no more than a hum.

The second exception is electric motors, commonly used in water and sewage processing.  The vibrations emitted by electric motors vary with loading.  However, the driven device (e.g. pump) and the transmission system is likely to be the greater source of undesirable frequencies.  Again, the mounting, damping (or lack of it) and location are all critical factors.

Taken together this suggests that fishing near water processing and industrial sites undertaking continuous mechanical activities (e.g. sawmills) may be less productive as salmon are unlikely to remain in the area.  It would be interesting to examine the factual historic data and subjective anecdote relating to catches (or lack of) in their vicinity.

The Noisy Angler

The moment you step into a pool the salmon's formidable sensors will detect your activity, even if you have felt soles and a light step. However, they don't know it's you or what you're doing, because in evolutionary terms humans haven't been angling long enough to achieve any genetic impact on salmon. Unlike the calls of whales, seals and other fish, salmon anglers' noises aren't in the salmon signal library. Certainly they wouldn't be able to connect the crunch of your studs on the gravel and the clink of your wading staff on the rocks with the drama of being caught, except perhaps if they'd been caught shortly before by another heavy-footed fisherman.

On the assumption that you don't make enough noise to jam the salmon's receptors and processors you will otherwise have to try hard to disturb them greatly. However, bearing mind that being 'on the take' is at best only an unusual and transient state for a salmon, it won't help if you make enough noise to divert their attention and thereby stop those unknown aberrations going on in a taking salmon's brain. It also helps to remember that the salmon's sensors are closely connected via their directional capability to vision, in that an unusual sound may cause a salmon to focus on its source. If they see you coming their way, they'll take flight. Put simply, if you're noisy it increases the chance of you being seen, so please don't wade near lies or the running line through a pool.

Obviously sharp sounds within the salmon's working frequencies are most likely to catch its attention. Don't do anything hasty and place your feet steadily and carefully: the scrabble of misplaced studs is very loud underwater. Don't rush into the water and through the shallows: the noise you make in 6" of water will be transmitted as a jumble of sounds across the whole pool. The vibrations from your progress along the dry gravel may also precede you. Avoid big slooshing strides through the water: you may be mistaken for a bear (without the smell) which remains in their library despite its extinction in the UK.

It's sensible to put a rubber ferrule on the end of your wading stick to avoid sharp clinks on rocks that will travel a long way through water. I buy a stock of them at the local hardware store before each season and plan on losing 3-4 stuck between rocks. The best ones have parallel sides and a plate in the bottom that stops the shaft wearing through the rubber.

Remember that loud noises above the water can be transmitted into the water. Research has indicated that some fish can detect the calls of predatory birds, so unless your name's Domingo or Jenkins I'd advise against singing along to the I Pod tracks. Anyway, the salmon may not share your musical tastes.

The bottom lines are remember that the salmon hears at least 400 times better than you do; good water craft starts the moment you get out of the car; and mind your language.

The Noisy Ocean

With the huge growth in international trade, shipping and offshore activities, the levels of underwater noise in the oceans has increased dramatically.  A major Dutch research work published in 2010 (Slabberkoorn et al) highlighted both the the sources of increasing noise and its effects.  There is a respectable summary on the BBC Science site.  There is also a growing body of evidence of the effect of excessive oceanic noise on migratory fish, albeit no-one has yet looked at salmon.

In that respect the biggest problem confronting British salmon is offshore oil and gas related activity in the North Sea.  In oceanic terms the North Sea is small, shallow and exceptionally industrialised.  The exploration and infrastructure construction phases of operation are very noisy, but they are transient.  The less obvious but potentially more serious issue may be the continuous transmission of machinery vibration in the sub 1 KHz range  from piles, legs, pipes and other infrastructure into the water.  What seems relatively quiet to us above the water may be both noisy and unpleasant for fish. Those vibrations will travel great distances from their origin, mix and combine with others and accumulate into a cacophony of discordant noise that has the potential to drown inter-fish communication and jam their sensors.  This may make it difficult for migrating smolts to form and maintain the schools that are essential for their survival; maturing fish to locate prey; and returning adults to protect themselves from predators.  Perhaps the root problem may not be marine mortality of salmon per se, but rather their marine confusion.

For those who fish the Dee there is the very specific problem of Aberdeen harbour, which is one of the busiest marine environments in the world.  The large number of diesel powered vessels, the frequency of their transits and continuous manoeuvring in such a confined space, together generate exceptional levels of underwater noise.  It would be a simple and quite cheap research exercise to characterise Aberdeen’s noise environment by frequency and volume; correlate the product to known fish sensitivities; and establish control samples in other harbours and estuaries.  Unfortunately for the salmon anglers, Aberdeen University’s current primary research interest in this field is focused on the effects of noise on seals and dolphins rather than their prey.  However, in advance of any investigation, I remain persuaded that there may be a connection between the high levels of activity in Aberdeen harbour and the reduced salmon catches on the Dee, especially of the more hesitantly running spring fish.



Happy Christmas