At this time of year one of nature's most amazing migrations is coming to end in UK rivers - the Atlantic salmon are returning to spawn.
Salmon (Salmo salar) are termed anadromous; hatching in freshwaters and migrating to saltwater. Brown trout (Salmo trutta) are another UK species that are anadromous. The life cycle and migrations of such species are still not fully understood but what we do know fascinates scientists and wildlife enthusiast alike.
By November salmon have returned from their oceanic feeding grounds, typically the Norwegian Sea, waters around Greenland and other sub-arctic areas, to the river in which they themselves were hatched. They return to their rivers with amazing accuracy but exactly how they navigate is still a mystery. Some think it is a combination of magnetic fields, detection of chemicals in minute amounts, thermal gradients, even using stars for guidance. Salmon lay eggs in the cool, highly-oxgenated waters upstream in a river. This means returning adults may have to swim up some major physical obstacles such as waterfalls. Salmon can jump incredibly well - jumps of 12ft have been recorded!
Once in a suitable area of the river, male salmon develop a hooked lower jaw with pronounced teeth known as a kype. They then defend areas of the riverbed and the females entering that area. The female chooses an area of gravel where there is good water flow through to dig a nesting area, known as a redd. She lays several hundred eggs and the male fertilises them. The female then covers the eggs with gravel. Salmon are not truly semelparous (mating once and dying) like their cousins the Pacific salmon but are only weakly iteroparous (mating more than once). Around 90% die after their first spawning but some (usually females) can return from the sea up to four times.
Salmon eggs are large and have a long incubation period, typically a couple of months. The young salmon hatch (known as alevins) and remain in the gravel still absorbing yolk provided by the female. Eventually they emerge from the gravel in spring and feed for themselves. During the next two years, the young salmon (known as parr) remain in the river to feed, grow and develop the characteristic spotty markings. After 2 years they undergo smoltification - a physiological process in which they develop to be able to survive the marine environment. The smoults then migrate downstream and out to sea to feed for a few years before returning to spawn in the same river.
Not all males migrate out to sea; some known as precocial males remain in the river their whole life. They develop functional gonads but remain parr-sized and keep juvenile colouration. These males avoid defending a breeding territory like the large anadromous males but instead sneak fertilisations with females.
Humans have long affected British salmon populations. Fisheries once were set up in estuaries to trap all returning salmon - decimating populations. Although this practice no longer occurs, humans continue to influence these fish. Dams built along rivers can impede their return to spawning grounds. In some areas fish are intercepted at the base of dam and placed in hatcheries to spawn. In other parts 'fish ladders' are used - a series of steps up the side of a dam to allow salmon/trout to migrate upstream as normal. There are even 'fish lifts' on some dams - machines which detect fish swimming into a box at the base of the dam, then hoist them up and empty them upstream.
