The colorful Arctic charr, one of the migratory fishes we know was left behind after the last ice age, has adapted successfully to its home in the mountainous lakes of Britain.
The charr is a member of the same family as the trout and salmon, and its general features resemble those of its relatives. Its slender, streamlined body is covered with scales, its fins lack spines and it has the characteristic rayless, fatty fin on its back just in front of its tail fin.
The most obvious difference between the charr and the trout and salmon, however, is that the charr has much smaller scales.
The charr is one of the most brilliantly colored fish in our fauna. The back is usually olive or brownish and the belly is silvery white, orange, or even crimson. A deep red color signified males in the breeding season.
Pink or orange spots are scattered along the sides and the fins, while the ventral fins are reddish with the leading edge white in contrast. Coloring may differ from lake to lake with the seasons and as the fish grows, with males in the spawning season of winter or early spring being almost unbelievably bright.
Arctic charr can be found distributed in freshwater and near-shore marine waters ranging from North America and the Alps in Europe, northwards to the northernmost extent of land in Eurasia and North America.
Southerly limits in freshwater are extended by cold waters like deep lakes, particularly in alpine areas of central and western Europe.
Arctic charr is a habitat generalist and occurs in a wide variety of aquatic habitats like streams, rivers, and lakes across its range.
The charr is widely distributed in northern and western Britain mountain lakes. In Wales, it is found only in lakes close to Snowdonia, and in England only in the Lake District, while there are large numbers of Scottish lochs, mainly in the Highlands, with charr in them.
It also occurs on the Hebridean island of Lewis and Shetland. In Ireland, it is found mostly in the west coast loughs. The charr’s origin in the hundred or so lakes in which it occurs is of some interest, for it is believed that each population was isolated after the last ice age.
In Iceland, along the Norwegian coast, and Greenland and Canada, the charr is migratory, feeding in the sea but returning to freshwater to spawn, similar to the salmon.
During the ice age, charr were forced further south than where they are found today, and it is thought that they were migratory off the British coast, passing from the sea into rivers to spawn.
As the ice retreated further north, some are presumed to have lost their migratory habit and spent the whole of their lives in the rivers and adjacent lakes until they were trapped in lakes or rivers by the change in the level of the land about 8-10,000 years ago.
Since then, each population has adapted to the habitat within which it lives. Some lakes are filled with food while others are not.
Some are deep while others are shallow. Some are acid; others, on account of the rocks around them, are alkaline. These, and other factors, have all had an effect on the charr, which has had to adapt to survive in the surroundings in which they found themselves.
As a result, fish from one lake may look different from those from other lakes. By the early 20th century, as many as 15 different species or subspecies were recognized in the British Isles, based on where they come from.
Nowadays we ignore all the different names and treat the British charr as part of the species Salvelinus alpinus, which occurs across much of northern Europe and northern North America.
One of the most striking features of living in lakes is the small size of the charr. Few lake charrs grow longer than 12in (30cm), and only in food-rich lakes do they attain a weight of more than 3lb (1.3kg).
In areas where the charr migrates to the sea, it grows much larger, to a length of 39in (1m) and up to 271b (12.2kg) in weight.
Lake charr spawn in winter or early spring. In Lake Windermere, the winter spawners lay their eggs in the water of 3-10ft (1-3m) on gravel shallows in November and December.
However, spring spawners lay their eggs in 66-96ft (20-30m) of water between February and March. Some other lakes have two spawning seasons, but most have just one or the other.
Their eggs are relatively large (4-5mm in diameter) and are buried in the gravel, and develop slowly until they hatch in spring.
The size of the lake and the number of nutrients ensure that growth rates vary enormously from lake to lake. In large, food-rich lakes, charr spawns at an age of five to six years, while in food-poor lakes, they may be 10 or 12 years old before they mature. In the Scandinavian Arctic lakes, immature fish of 20 years have been found.
Charr eat whatever suitable food they can find in their home lake. This means their diet is confined to copepods, crustaceans, insect larvae, including midges, and freshwater shrimps. Large charr may eat other fish, but these tend to be scarce in the lakes inhabited by charr.
For all their beauty, charr are always on the borderline between survival and non-survival. Several lakes that were home to the charr do not have them anymore, and they have become locally extinct.
One of the main reasons for this is the discharge of domestic or farm sewage into the lakes, causing a huge change to the lake. Another serious hazard is the introduction of other fish species to their lakes. Pike, perch, and trout are harmful as predators, while other fish compete for the same food source.
Acid rain, due to pollution, has also affected the alkalinity in some lakes and is particularly serious in Scandinavia.
All over the northern hemisphere, the survival of populations of the charr is under threat. Although the species is not threatened, many lake populations are. As each of these has evolved in genetic isolation for thousands of years, the loss of one is significant, bringing the charr just a little bit closer to extinction.