Atlantic salmon surf ocean currents home rather than swimming marathons
The image of salmon swimming thousands of kilometres across the ocean is largely a myth. Atlantic salmon ride sea surface currents in a loop that carries them back toward the river where they hatched. Only about 5 per cent end up in the wrong river, though exactly how they find the right one remains unconfirmed.
Life begins in fresh water, and the stages have names. Alevins live off their yolk sacs, fry leave the gravel to hunt, and parr prepare for the sea. That freshwater phase lasts anywhere from one year in rivers draining to the English Channel to eight years in Ungava Bay in northern Quebec. At around 15 centimetres the young fish smoltify: their spotted camouflage turns silvery, their hormones adjust to salt water, and instead of fighting the current they start swimming with it, usually heading seaward between March and June on an ebbing tide. At sea they appear to sense Earth's magnetic field through iron in their lateral line.
Unlike Pacific salmon, which die after spawning, Atlantic salmon can survive and return to spawn again, though only 5 to 10 per cent do. Those repeat spawners can grow huge. After two years at sea a typical fish is 71 to 76 centimetres long, but one netted in a Scottish estuary in 1960 weighed 49.44 kilograms, the heaviest on record. Some populations never go to sea at all, living entirely in lakes; in North America these landlocked fish are called ouananiche. As adults at sea they favour capelin, and they are considered more aggressive than other salmon.
The Latin name Salmo salar has been read as salmon the leaper, though resident of salt water is the likelier meaning. People destroyed much of its North American range early. Trapping beavers nearly to extinction by 1800, clear-cutting, log drives, mills and dams degraded streams, and in Nova Scotia's Gulf Region 31 of 33 salmon streams were blocked by lumber dams. By 1850, one historian estimated, over half the continent's historical salmon runs were gone.
The fish is a cold-water specialist sensitive to warming, and it has colonised Svalbard, at 78 degrees north, since the early 2000s as the climate there heated. Farming now supplies much of the world's salmon, with Norway producing more than half, although environmentalists criticise the methods.
Source: Atlantic salmon
