At this time of year Atlantic salmon join our more familiar brown trout in the high jump contest taking place up our fast-flowing weir to spawn up stream. And just like athletes the fish can often be spotted warming up below their obstacle waiting for the optimum moment to make the leap. The salmon are capable of leaping 4 times their own body length, which when experienced brings to mind a time when this valley was a true wilderness and the salmon weren’t leaping a weir but a waterfall to avoid the clutches of the native bears. Without the bears, the salmon still have many predators to avoid including otters, herons and the most formidable, fisherman.
The natural breeding range of the Atlantic salmon is massive from rivers along the east coast of North America to Europe, including our own River Wharfe. Unlike it’s relative the smaller Pacific salmon who make similar journeys up the rivers they were born in, return to the sea to repeat the journey another year, most of our larger Atlantic salmon do not survive the journey inland. Salmon are such popular fish, each part of their life cycle has been named. After hatching in the spring, they are called alevins, they then remain in the relative safety of the gravel for around a month. After which they emerge as fry, These fry soon become parr, and live in our freshwater rivers, feeding on invertebrates. When they reach 3 or 4 years old, they become smolts and head to sea.
During their smolt stage, Atlantic salmon undergo a fantastic change from a freshwater species to a saltwater species, you may ask why do salmon go to sea? It is a perilous journey but the bite-size prey they have hunted in the rivers are no longer enough, to continue to grow they must head to the ocean. The energy content and abundance of food in the ocean is much higher than in freshwater, so fish are able to grow very big, very quickly on their diet of krill, sand eels and herring. This is important because larger fish are less likely to be eaten and the females will ultimately produce more eggs. As well as plenty of more prey however the oceans holds plenty more predators for the salmon, including larger predatory fish like Atlantic halibut, Atlantic bluefin tuna and sharks, seabirds such as the Northern gannett, seals and toothed whales.
The next leg of the great journey brings the Atlantic salmon back to our River Wharfe, as incredibly they return to the river in which they were born to spawn. Only this time the adult salmon must swim upstream, against the flow of the river! During this change from seawater to the freshwater of the river, Atlantic salmon lose all their teeth and grow new ones. Very few other fish can survive in such wide ranges of salinity, and would die if they moved between salt and fresh water the way that salmon do. Scientists believe that salmon navigate their way back to the river in which they were born by using the Earth’s magnetic field like a compass. Years previously, these impressive fish were just tiny eggs buried in the gravel of the riverbed, gently placed there by their mother gently swooshing her tail.
Comments