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, 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