of was a fish that negotiated survival for eight re-runs, which was netted off Newfoundland and aged by scale-reading – a cross-cut scale being interpreted like the rings of a tree. This one must have been up near the rabbit in terms of prolific genetic legacy! Similar return rates have been recorded in some of the glorious rivers of New Brunswick. On the eastern side of the Atlantic, a fish analysed in Wales had returned to the redds five times.
It is in the northern oceans that depleted salmon rebuild their condition. If you were to catch and eat these salmon before they had reconstituted themselves, they would be oily and disagreeable, like cod or mackerel after spawning.
For a long time pundits tried to work out where salmon winter; it was akin to the mystical quest for the end of the rainbow. Somewhere a fish as long as your finger grew exponentially to become a fish as long as your arm. Where was this fabulous larder? It is known now that a proportion of British salmon winter off western Greenland, where Greenlanders in small boats net them close to the coast. These salmon stay more than one winter. They are a minority here; many larger salmon from North America fatten off Greenland too. A little further north, the salmon of the Russian Kola Peninsular and the salmon connected to rivers on Norway’s long coastline winter in the North Atlantic off Norway.
The salmon of eastern North America winter in the Labrador Sea and on the northern Grand Banks, as well as western Greenland. It can be overlooked: the distance across the Davis Strait starting from northern Newfoundland is only 600 miles. Greenland’s seas are a neighbourhood bouillabaisse. The fact of highly variable runs of east-coast salmon in North America is reckoned by scientists to be related to the environment, food supply and ocean temperature; so much is reasonably obvious. But unlike on the eastern side, where salmon-run prediction is not attempted, Canadian and American forecasts on fish runs are based on what is found in the sea off Newfoundland and Labrador when the water reaches four degrees. In the 1980s and 1990s, between Labrador and western Greenland sea temperatures were suppressed by Arctic waters pushing south. This in turn saw declining growth rates across species like capelin and cod, delayed spawning times and generally reduced salmon runs.
Not only are weather and climate unpredictable variables at sea, but living matter in the sea is infinitely complex. Any child at the seaside has noticed that if you bottle seawater the life seems to fade away. This is because the sea is a dynamic environment in a state of continuing flux. Filled with plankton, microscopic living particles of plants and animals, the sea is a bubbling broth. It has been calculated that the amount of organic material in an acre of sea equates to the vegetation on an acre of average land. Planktonic abundance fuels the vitality of seawater and is the foundation for a pyramid of creatures feeding from one predator level to the next.
Adult salmon are near the top of this pyramid. Fast swimmers, they evade other fish. Vulnerable to being cornered by seals and acrobatic sea mammals when in semi-confined areas like estuaries, they are generally too swift for capture. The rich soup of planktonic life becomes in turn the feed for krill, capelin, herring, shrimps and molluscs, which are all part of salmon’s ocean diet.
Important elements like phosphorus and nitrogen determine marine productivity. These either wash onto the shelves that are the underwater extensions of landmasses, or are pushed from below in the deep ocean by upwellings as ocean currents mix, driven by the Earth’s rotation. Areas of the ocean vary enormously in their productivity, the North Sea and the Grand Banks being shallow expanses and exceptionally fertile, as contrasted to vast parts of the mid-Pacific where the water, as you peer into it with the sun behind, is startlingly clear precisely because there is so little plankton and suspended material. It is as void as sterilised bottled drinking water. In other places millions of plant cells can occupy one cubic foot of seawater.
This partially explains what has been called the ‘explosive growth’ which salmon display after leaving their freshwater nurseries as six-inch smolts.
Marine biology and marine research have made quantum leaps in recent time. Two areas of rapidly advancing research concern life in the bathymetric deeps, where life-forms have been discovered way below depths at which it was formerly thought any life at all could exist, and secondly in the pelagic or surface skin of the ocean. Although we now know that salmon obtain food from depths of as much as 800 metres in the dimmed realms of the sperm whale, and can stay at 400–600 metres for as long as 24 hours, it is in the surface ocean layer that smolts have to survive when they leave rivers.
Their departure is called the smolt ‘run’. The small fish leave their natal river for the great unknown when prompted by rising temperature. A government fisheries department in Scotland used infra-red images at night on a tributary of the River North Esk to watch smolts assembling for the ‘run’. The technology was not perfect; rising water levels lost the images of fish, but the presence of smolt-traps further downriver showed that smolts did indeed continue running in high waters. For the bigger picture the technology served adequately. It showed that fish shoaled in small parties of three to six, they used the core of the current for propulsion, and they descended rivers pointing seaward.
Tagging with microchips has established another new finding. Smolts enter the sea in a mass to minimise predation. Having travelled downriver in small schools, they pack to go to sea, then closer to the sea becomes an assembly point. The reason is the same as why other small fish shoal – it enhances an individual’s survival chance to be one of many congregated in a dense mass.
A salmon river is occasionally blessed with egg-bearing gravels all the way up its sinuous length. The Miramichi in Canada is an example of a spawning bed over a hundred miles long. Hen fish will sweep out redds and lay their eggs in them from the narrowest streams at the top to the wide, gravelly wash-out bars near the river-mouth. To coordinate the smolt runs, however, the development of eggs into fry and parr and then into smolts in the headwaters of the river must be earlier in the season, so that when these young shoals of salmon go seawards they do not miss the camouflage of other shoals of smolts which have matured later and which are waiting nearer the river-mouth.
Accordingly, in northern Scottish rivers parr begin to go silvery, and turn into smolts or ‘smoltify’, as early as March in the headwaters and as late as May lower down. To prepare them for ocean life, away from the shady corners and dark shadows of natal streams, they develop an ocean livery. Their skin grows a layer of silver guanine crystals. These crystals arranged in verticals rows act as mirrors, camouflaging the little fish by reflecting its surroundings.
The keen perception of Dr Richard Shelton, formerly head of the government’s marine laboratory in Pitlochry in Scotland, noted that the only parts of the smolt to remain unaltered are the black edges to the fin and tail. He believes these are helpful visual aids to other smolts in keeping the pack together, whilst not giving too much definition away to predators.
To help the smolt register where it originated, and to find its own personal natal stream later as a fully grown fish, the hormone thyroxine is raised temporarily to allow the small brain to take in vital extra survival information.
The cohort of smolts journeys down the river, making the little flips and splashes familiar to springtime anglers, and reaches the sea to coincide with feeding opportunities. The oldest smolts reach the salt first and the youngest, maybe only a year old, go last, an order which is reversed when they return as adults. A critical feeding assignation is with the outburst of sand-eel larvae on the sandbanks.
The similar appearance of different smolts is deceptive. Those from the southern edge of range, France and Spain, are a fraction of the age of individuals from colder northern rivers. Whereas many southern European smolts are just over a year old, those from Arctic Norway, Greenland and Ungava Bay may be seven before they risk life at sea. From Iceland and Scotland smolts have dwelt from between two and five years in the river. The age difference reflects the length of the growing season, further north, less time.
It is an extraordinary thought that physically similar fish,
