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, at the same development stage, vary in age by so much. There are no obvious parallels in bird or mammal biology. Possibly there are comparable patterns in other fish, but none comes to mind. Salmon evolution is supreme adaptation.
What happens after the entry into seawater has recently been tracked in an internationally funded programme codenamed SALSEA-Merge. SALSEA is the most remarkable research on a fish at sea in recent time. The European Union, Canada, the USA, the Total Foundation in France, the Atlantic Salmon Trust in the UK and a variety of universities and agencies combined to fit out the RV Celtic Voyager and two other vessels with proper equipment, and then put the right people on board who knew what questions to ask and how to get answers.
The results fill in another corner of the lifestyle jigsaw.
The research boats spent three years catching around 27,000 juvenile salmon from 466 different locations in the North Sea, the Norwegian and Irish seas, and generally in the north-east Atlantic around the Faroe Islands and Iceland. Information from 284 out of Europe’s 1,700 salmon rivers from nine countries was used in genetic sampling and analysis. The biggest sample came from Scotland, followed by Norway. By targeting the most productive and the largest rivers, the SALSEA team reckons that 80 per cent of Europe’s productive salmon area was embraced by the research effort. The resulting picture then is a clear story about European salmon, including Russia and Scandinavia, up to 2011.
The novel side of the analysis was the use of genetics. Only recent science allows researchers to find out where a fish comes from. Work on Ireland’s River Moy had already shown what a sprinkling of other rivers had found too – that inside their catchments salmon stocks can be divided up into genetically discrete populations. It was true of a minority of rivers, but demonstrated the impressive complexity of salmon adaptation.
The Atlantic salmon in some form or other has been occupying European and North American rivers for 60 million years. In that immense time it has developed local strains to adapt to local conditions. Then the last Ice Age ended, with a thaw that peeled back the ice-covered land over all Britain north of London. A mere 15,000 years separates us from the frigid conditions that dominated before. In cosmic terms it is a blink in time. Salmon saw pristine territory opening up in front of them, and occupied it.
Rivers in Scotland have plenty in common with other rivers in salmon range. They are spring-fed. These springs can come from hundreds of feet below ground and each one has a differing chemical composition. Also, the springs bubble from the ground loaded with different temperature readings, dependent on their depth.
Variations between springs account for differing populations of salmon. For the scent in each stream, and the mineral contents, differs from that of its neighbour. Salmon have brilliant olfactory senses, being able to pick out the most dilute odours from home-stream chemistry even through a fog of additives and man-made complexities. The fish’s ancestors have used that water and over time adapted to it.
Sometimes that adaptation will translate into an identifiable genetic type. SALSEA went to sea armed in advance with the genetic map of many rivers. The researchers were hunting smolts, young salmon entering an alien saltwater world pregnant with feeding and with threat. What galvanised researchers to go to all this effort? The answer to that question is both simple and complex. At the simple level, it is because the salmon is important enough to justify it – it is a glittering symbol of environmental wellbeing. The complex answer backtracks in time.
In the 1960s European rivers had seen prodigious runs of salmon. The silver bonanza from the spring tides re-ran the programme of fresh shoals arriving through the year and it seemed as certain as the sun dropping in the west that from the start of spring these great leaping, wild, sparkling fish would go on and on revitalising rivers which had gone doggo for the winter.
Then a decline commenced. Fewer and fewer salmon came back in the 1980s, and then the 1990s. The canaries in the mine, or anglers, found their enticing presentations drifting across the stream undisturbed.