|80 billion cubic km of water|
How many salmon?
The purpose of this post is to provide something to ponder in the dog days of the close season. To help fill the long dark evenings it is much longer than my normal offering, and anyway it's quite a complex subject. There's no way I can make it shorter, so please be patient and stay with me. That said, I make no claim on great scientific knowledge, nor am I a fish or cell biologist. By training I am a mathematician and economist. What follows are not answers but thoughts for discussion drawn from my studies: I shall be most grateful for your comments.
|The Broad Bean|
Brain section - Pacific salmon
|(Photo - Seymour Salmon)|
|No strawberries please|
The Monarch Butterfly
A much-researched North American species of butterfly provides some useful insights that may help our understanding of the salmon's migration and the possible role of genetic printing. The Monarch is native to the north eastern states of the USA, where it thrives in the warmer months. Like many insects it only has a lifespan of a few weeks, so successive generations are born and die during the course of a summer. The winter temperatures in Vermont too low for the Monarch to survive, so well before the start of the ski season the fourth generation heads south west to Mexico, 2,800 miles away. The destination is not just Mexico (an enormous target) but a very precisely defined group of mountains only a few miles in extent, where countless millions of Monarchs congregate to breed and die before the cycle restarts and they return to Vermont in the spring.
|The journey back to Vermont|
How it navigates to its destination is another issue. It is reasonable to suspect that the sun provides its primary orientation, but a simple sun-following regime would cause it to follow a daily series of semi-circular tracks across the USA, with an average heading of south rather than south west. And of course it needs to head away from the sun on the return journey. There is something much more complex in play here - not least involving time and azimuth correction - that far exceeds the Monarch's brain capacity. On that basis the navigational data must be stored elsewhere in its body.
Salmon and Spatial Awareness
When I was a boy my grandfather told me that the salmon found its way home to spawn by detecting the special smell of its parent river. This was and remains a widely held theory. However, despite the extraordinary power of the salmon's sense of smell, I observed that this theory suffered from a gaping hole in its logic, in that it covered only the return leg of migration whilst leaving the smell-free outward Atlantic journey wholly unexplained. I was indeed an insufferable 10 year old and over the years my curiosity has not waned. More recently I examined some Norwegian research data from an experiment in which they disabled the noses of a sample of salmon, which nonetheless found their way home to spawn. The theory of smell being the key to navigation just doesn't stand up to scrutiny. Navigationally its importance appears to be in the final river stages of the return migration. We therefore need a better theory that covers both legs of migration and works at an oceanic scale.
For the next step in the reasoning we return to North America and the migration of Pacific salmon. Owing to the huge value of the commercial catch, the sea life of these species has been far more intensively researched than their Atlantic cousins. Whilst perusing the scientific journals I came to the conclusion that the salmon may know where it is - what's described as spatial awareness.
The trigger for that thought was Vancouver Island. It's 300 miles (500 Km) long and lies between migrating salmon and a very large number of spawning rivers. There is no reasonable way that a salmon approaching from the Pacific Ocean can smell its parent river from the far side of such a big obstacle. The river water is too diluted in the ocean to be detectable, and in any event the ocean currents flow southwards away from the salmon's direction of travel. If the salmon makes the wrong choice between turning left or right when it meets Vancouver Island it could add up to 500 miles (800 Km) to its migratory journey. Bearing in mind that the main run of each species is extremely concentrated, the errant salmon risks missing the boat and hence the possible extinction of the DNA it strives to preserve. However, the research published in the Journal of Current Biology in March 2013 indicated that the salmon made reliable decisions for getting around the Island, which suggests they have some degree of spatial awareness. The researchers surmised that it might be based on a mixture of ocean currents and magnetic orientation.
What might be the foundations of any spatial awareness? For an ocean going fish like the salmon we can rule out the use of physical features and landmarks employed by humans, primates and birds in their localised travels. They just don't exist, and even where they do the salmon's range of vision is too short for this to be a viable navigational foundation: you can't navigate over hundreds of miles with a field of view of 30 feet. The sun may be helpful but it's transient during the day and between seasons; unreliable, in that it can be obscured by clouds; and invisible underwater every night and for 3 months of the year at the northern latitudes of the salmon's feeding grounds. There has to be something else that differs from place to place and so allows the differentiation of location. Only 2 universal forces unaffected by water come to mind - magnetism and gravity. The science of the latter challenges my brain and we don't know enough about its spatial characteristics (until the space survey reports in about 10 years' time) to make decent inferences that may be relevant to salmon. Accordingly, I'll turn to magnetism, about which we now know a great deal more by courtesy of the MAGSAT project's mapping of the magnetism of the earth's surface crustal layer.
|(Diagram - Morlands School)|
However, we need a compass to give us that indication as we don't have a magnetic sensor system within our bodies. There is, however, evidence to suggest that some species do have magnetic materials in their horizontally arranged spinal structures, which might arguably provide a means of orientation. The most often quoted example is dairy cows, which when not grazing appear to orient themselves North-South, except when power lines influence the local magnetic conditions. If this is correct then we may be observing natural orientation (direction), which is one essential component of spatial awareness. But it doesn't meet the other component - location - which leads us to the second phenomenon, crustal magnetism.
|Crustal magnetic map of the North Atlantic|
|Linear magnetic phenomena near Iceland|
The white gaps indicate polarity reversal
Note that the local magnetic field is offset
from the whole-earth field by about 75 degrees
When you look at the detailed maps there is a profusion of clearly identifiable local magnetic features distributed throughout the Atlantic salmon's range. Indeed, there is a complete family of near parallel magnetic-tectonic lines in the area from the UK past Iceland to Greenland. Their magnetic intensity declines with distance from the centre-line of the formation, and the polarity reverses between stripes. The Reykjanes Ridge magnetic feature runs all the way from just south of Iceland to the feeding grounds where the Gulf Stream meets the Continental Shelf. For the return journey, looking back towards the UK from Iceland you can observe unique local magnetic signatures and patterns, both coastal and inland.
|GEE navigation 1943|
How you found Berlin at night in a Lancaster bomber
There has been ample consideration of the salmon's ability of otherwise to detect and respond to the earth's magnetic field, but this could only ever explain orientation. The MAGSAT programme has now given us detailed imagery of local magnetic conditions, which could provide the basis for the salmon's embedded map, and thereby complete the spatial awareness riddle. Certainly you can construct a logical hypothesis from the evidence that if salmon have magnetic sensitivity, then crustal features and the whole-earth field could give it the means of knowing where it is at sea, day or night.
Life's more problematic in the river because magnetism has very long wavelengths (about 2.5 Km for the whole earth field), which means it's not precise, give or take a few miles. That's fine at sea when looking for Greenland, going around Vancouver Island or finding the meeting of the Gulf Stream and continental shelf, but even the Tweed is only 200 yards wide. Of course you can use your nose for the closing stages. But differential magnetism may good enough to lead you back towards where you spent the first part of your life, facing into the current 95+% of the time, with your horizontal spine recording the local magnetic map and adjusting your inherited DNA. If, however, you were brought up in a hatchery and release pond with all sorts of metal in their construction, then it might just be that your magnetic map is less clear. Could this be why reared salmon are less reliable navigators (one Tyne hatchery fish turned up in Canada and another in the Mersey)? That's a debate for another article in the next close season. Until then, I wish you a Happy New Year, in the hope that the 2014 season will bring a major improvement on 2013.