The Salmon: The Extraordinary Story of the King of Fish. Michael Wigan

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СКАЧАТЬ turn up on fishmonger’s slabs somewhere, people notice. Protection at that stage is a practical possibility. Smolts, in contrast, are needles in the haystack of the ocean.

      SALSEA makes a stride in knowledge about salmon’s ocean phase. However, it did not satisfy all those awaiting its findings. Managers of salmon sport fisheries were looking for answers to their own pressing questions.

      They complained that original promises on the development of the genetic map actually fell far short. Some tracking has limned in a few details, but the big picture remains largely unknown. They make the point that, interesting though genetics might be, the practical application of using the information on the average fishing river is limited. Say you discover there is a different genetic stock in one branch of the river – intriguing – but how can you manage the fishery, aside from keeping the tributaries in good health, to accommodate that information hidden in the DNA?

      Most cogently, critics point to the report’s failure to nail salmon farming as the destroyer of wild fisheries through its lethal by-product of proliferating parasitic sea lice. The million-strong swarms of sea lice created by salmon-cage aquaculture adhere to smolts as they leave fresh water and kill them, thereby throttling wild salmon survival. They say the report’s equivocal and incomplete findings will leave politicians an escape route from firm and decisive action in favour of more time-consuming and inconclusive research, measures once sarcastically dubbed as designed, ‘to maintain the momentum of procrastination’.

      They have a point. Some of the more arcane disputes about discrete genetic stocks in different branches of one river, and efforts to keep them pure, are undermined by the historical fact that river stocks have been intermixed long ago. All over Britain salmon have been moved from hatcheries and tipped into rivers wherever owners of fisheries wanted to beef up fish numbers, or revive them. It has been going on for over a century. It is the same in other salmon countries, too. Genetic purity is a myth, which is surprising given that genetic purity of stocks is the new mission for salmon theorists.

      In Scotland re-stocking only with stocks from that river, and even from a specified part of the system, is now official ‘best practice’, to the frustration of many wizened fishery managers. The new knowledge about discrete strains is not being used in the most intelligent way.

      It has always been hard for the genetics messiahs to deal with the fact that on the British west coast river where the Beatles originated, the Mersey, the water has been re-populated with salmon entirely by the vagaries of Nature, its own native stock having been wiped out. The Mersey now has Creole salmon of mixed origin coming from at least thirty different rivers. Who can object? ‘Nature hates a vacuum’ is true for salmon as for all else. Genetic straying has re-populated a major river.

      The Thames is another melting-pot culture. Its tentative existence as a salmon river once again owes its brilliant success to stocks from many different places. Reflecting its diverse human mix London’s passing salmon population is polyglot too.

      I saw a dramatic illustration of the basis for genetic straying whilst rafting in British Columbia late one summer. At day’s end our three rafts headed for a tributary with a nice shelving sand-bar to moor up on for the night. We crunched onto the beach and were stunned to find huge salmon lying dead on the water’s edge.

      There was a biologist aboard. He looked closely at the fish and saw that their gills were clogged. They were king salmon, the big boys, and there were around forty of them stranded down the river-edge for a few hundred yards, all just above the tributary’s junction with the main river. The biologist said the fish were all within a day of spawning. So a valuable stock or ‘year-class’ of a rare and wondrous fish lay wasted about us never to breed, eradicated in the last moment of its evolutionary purpose. Why? The brown water was still silty from a landslide further upriver. A natural event had wiped out the big fish in this tributary for one breeding season.

      What gave the event an added twist was the furious debate taking place in west Canada’s media that summer about threats to king salmon, their precarious status, and the need for firmer protection laws. Nature had thrown a joker onto the gaming table and we were staring at it.

      But it was also where genetic straying and fish unfaithful to their natal imperative step in. Suppose, as we know is possible, that one pair of king salmon had gone up a tributary close by. They bred there. That strain of salmon thereby dodges fate and escapes elimination. In due course some of the progeny from that union relocate themselves as adults in breeding livery back in the original natal stream, and re-populate it.

      Straying is Nature’s way of spreading risk. The same is true, surely, about the differing ages at which young salmon go to sea. If some ‘smoltify’ and migrate in their second spring, and some in their third, the risk of total elimination is spread. On big rivers in Scotland like the Tay, different grilse runs climb the system from early spring to the autumn. They are all fish which spent only one winter in the sea, the definition of grilse. Bookies call it hedging bets.

      Peter Malloch might have had a lot to say on some of the purist preconceptions about river-stocking only with site-specific strains which have crept into modern management. It was Malloch who realised a century ago that salmon sometimes remained at sea a long time, and that not all fish were grilse as had been assumed before. He understood the salmon’s admirable diversity. The migratory fish turn homewards to reproduce, swimming south. It is presumed, but only so far tentatively claimed, that they follow the same passage, but going the opposite way, as that which they used as teenagers – another neat theoretical twist made available by modern science which this aristocrat of salmon analysis long ago would have appreciated.

      Left behind in the marine larder are the others, the non-movers, growing and growing. Or not growing: some salmon a long time at sea are not especially big. Maybe they too are hedging their bets, passive actors in an evolutionary insurance policy.

      A salmon’s eventual size is determined by its length; it can be fat or it can be thin, but without length it can never match the biggest. Girth is the feeding which beefs up the body length. Some of these wintering salmon spend two years in the vicinity of the Arctic, some three, some four, and some even prolong their Arctic sojourn to five years.

      Turning southwards as the new year awakens, they ultimately acknowledge the ritual of reproduction, or so it is assumed. Scale-reading shows that after January sea-feeding picks up again following the short winter check. Thus, the fish achieve peak condition prior to the demanding migration south.

      Today’s existence is tougher for salmon, for as the northern hemisphere has warmed, dragging the food supply northwards, the return journey south lengthens. Migrating salmon are starting further from home.

      Krill may move, natal streams do not.

      How long they take to swim home is unknown. Scale-reading differentiates between sea-time and freshwater-time, but we do not yet know in which sea, at what time. Could we? If scales were better able to track diet perhaps we could pin down more accurately how long the journeys take. Sand eels are on known sandbanks and capelin live in the north – the diet could position the predator.

      However, there is another lateral-thinking way, a technique not yet fully developed for practical use. We need tags that could measure the angle of the sun at midday, then, as polarised sun filtered through the seawater, it would be possible to calculate latitude. Temperature and the intensity of the light are recordable now; the next step is a reader for polarised light. The tags used on smolts register temperature and depth, but so far they cannot measure the angle of the light. This tantalising technical advance may be not far off.

      What is hard fact is that salmon are fast movers. When they do reach rivers they can travel incredible distances in a day’s journey, proven by the presence on them of sea lice. These are saltwater parasites СКАЧАТЬ