The salmon fishing writer Richard Waddington devoted many pages of his 1947 book Salmon Fishing: A New Philosophy to a laboriously articulated theory that wintering salmon positioned themselves in the mid-Atlantic in order to intercept common eels migrating as elvers or ‘glass eels’ from the Sargasso Sea off eastern central America. Salmon swam with these easterly migrations of juvenile common eels, he surmised, being sustained and fattened by them all the way back to their growing zones in the freshwaters of UK and Europe. It was somehow a beguiling idea.
The author died long ago, but depending on circumstances in his afterlife he may now have the chance to reconsider and chide himself for those errant thought processes. Knowledge is a grand corrective.
Knowledge was meant to be a defining feature too of the revered Scottish ecology writer, the late Frank Fraser Darling. But when it came to salmon he stumbled. Fraser Darling wrote that the fish that ascend rivers early spawn at the bottom end, and the late-season runners go to the top. In fact it is the other way round, which is why today’s efforts to resuscitate the spring salmon, assuming that like breed like, take hatchery fish from the upper parts of rivers, not the bottom end. The top reaches are where the springers are.
He made another assumption which is fantasy: that east-coast salmon seldom come back from the sea more than once whereas west-coast salmon can breed four or five times. Scale-reading, which ages salmon, and should have put matters straight, had been around for over fifty years when he wrote this.
For some reason salmon seem to befuddle commentators. The director of Scotland’s Marine Laboratory, therefore the senior government person in fisheries, portentously declared in a book on salmon published in 2000 that it was now well established that ‘a large majority of the fish returning to spawn in a particular river originated from that river’. Wow, really? But that was what the Scottish Ecclesiastic Hector Boethius announced in the sixteenth century. SALSEA misroutes down this way too: ‘Reducing man’s impacts on our salmon stocks may be the key to ensuring their survival’. When can we move on, please? It may be a matter of debate whether the Atlantic salmon is the greatest of fish, but it is certainly the one that leads to most confusion.
Before SALSEA it was thought from examining pelagic trawls that smolts at sea ate blown insects to start with and moved deeper down the water column as they got bigger to find crustaceans and small fish. When the diet stepped up to embrace fish, growth accelerated. SALSEA saw that the diet of young salmon changes with ocean conditions. Researchers concluded that they ate almost anything. Capelin (found on the Canadian shelf and off west Greenland), young blue whiting, lanternfish, five-bearded rockling, sprats and sand eels all formed part of their sustenance. Smolts would consume eggs, larvae and young fish and zeroed in particularly onto a sort of bug-eyed shrimp called themisto.
Interestingly, the herring and mackerel which were moving in the same area, though themselves bigger, selected smaller food, mostly crustaceans called copepods. Smolts punch above their weight in the food chain.
Influences on salmon routeing included predator avoidance and their own growth. Uncircumscribed by physical limits, some young salmon migrate all the way not only to the Arctic ice-shelf, but under it. Presumably, beneath the ice-shelf at least one direction is guaranteed free from lethal attack, and there are no trawlers.
SALSEA leader, Norwegian scientist Jens Christian Holst, has written: ‘From the very first scent of salt these fish are continuously hunted by marine predators.’ They face different predator assemblages at different stages in their migration to the wintering grounds. Directly offshore, and whilst they are in the process of adapting their saltwater/freshwater balance for marine survival, sea trout, sea bass and cod feature. There are the salmon gourmands, seals. Further offshore those enemies are joined in the attack by saithe, pollack and more seals, and even rays and skates. North American Atlantic salmon face similar families of predators, but from local species.
Moving further out, coast-huggers like sea trout cease to be trouble. Diving seabirds such as gannets enter the fray. Minke whales and fin whales join the list of toothed adversaries. Even sharks, cephalopods (molluscs including cuttlefish) and tuna can eat smolts. Dr Holst says, ‘the list could be continued at length’! He adds that fast growth is an obvious survival aim for smolts trying to prolong their lives at sea. One sees why.
In eastern Canada a survival peculiarity has been noted linking smolts with spent kelts. In the straits of Ben Isle kelts gathered outside the rivers they had descended until the smolts joined them. Both young salmon and older ones then moved northwards in convoy to the feeding grounds. What is happening? Are smolts being taught their passage by their elders? Is there any protective function in the presence of the kelts mixed with the next generation? Another chapter in the mystery of salmon migration opens up.
SALSEA also recorded what effects human actions were having on smolt runs. Escaped smolts from freshwater rearing cages in lakes and lochs run as nurseries for the salmon farm industry, or young feral salmon, were identified by their genetic markers and found in numbers. Their groups were looser in formation than those of wild smolts. Just how many feral smolts were found is a matter tenderly circuited.
For it is a potent finding. Salmon farm escapement is a highly political and controversial matter and now science can tear back the veil on the resulting profile of ocean fish populations. The relevance for salmon survival of the presence of farm-origin fish competing with wild ones in the sea for the same food is a question which needs answering.
Scale-reading using digital technology was another tool in SALSEA’s knowledge review. Reading scales is nothing new and the Inspector of Fisheries in Scotland, Peter Malloch, based on the River Tay, developed basic scale-reading theories over a century ago. Wider spacing between rings told of richer feeding. From scales readers could say how well fish had fed and grown at sea, what smorgasbord of young fish, fish eggs and larvae had been eaten, prefaced by how these fish had fared in their freshwater phase.
Scales are like dentine in human teeth: they read like tree-rings and tell a story. The scales fall from our eyes: the biography of a fish is available in its scale history. In contrast to many fish population studies done for Europe’s Common Fisheries Policy using predictive modelling by computers to allocate catch quotas (an innately unsatisfactory methodology), scales reveal what conditions were recently like. They offer real-time information, not academic projections for the formulation of shaky assumptions.
Long-time series of salmon scale histories existed in several places. The Copenhagen-based International Council for the Exploration of the Sea (ICES) made available its archive of thousands of tagged and recaptured salmon details based on scale-reading. SALSEA took this forward. New developments in digital analysis have added to the knowledge bank to be gleaned from scale readings.
Scales read on adult salmon when they came back to rivers are correlated genetically to young smolts caught in the smolt trawls. Some of these scales had been taken from the adult fish long ago. So young North-Atlantic smolts were being traced by their scales to fairly distant ancestors. In total, 23,000 scales from seven rivers in six countries were studied. From this sample smolts were mostly two years old, some one and three years old, and a few four years old. The further north the river of origin the older the smolt age; southernmost smolts were growing faster in their home rivers and undertaking the marine migration earlier. As it happens, they were often dying earlier too.
The research revealed that smolts preferred temperature bands of 9–12°C and salinities over 35 per cent. They avoided the shelf directly off western Norway, possibly because salinity is low, and aimed for the deeper, more saline water further west on the shelf-edge. The colder the water the