What are lampreys?
Lampreys must be amongst the most rarely seen and poorly understood of all the fish species found in Scottish rivers. Their fossil remains show that they were around long before the dinosaurs and many familiar plant groups. There are three species of lampreys in Scotland: the brook, river and sea lamprey. They all belong to a group of animals called ‘Agnatha’ (meaning ‘without jaws’) which is the most primitive group of all living vertebrate animals. As the name suggests, lampreys lack jaws and instead have very primitive mouthparts surrounded by a large flexible lip that acts as a sucker. This curious feature provides the scientific name for the lamprey family, Petromyzonidae, which literally translates as ‘stone suckers’.
Lampreys resemble eels but have several unusual features not seen in most other fish. Instead of bones, they have a skeleton of cartilage. Lampreys also lack scales, and have a single nostril located on the top of the head in front of the eyes. They do not have gill covers but do have a series of seven sac-like gills which open directly through holes on each side of the head. All lampreys have a long, cylindrical body, like that of eels.
The sea lamprey is by far the largest of the three species with adults reaching a length of one meter and weighing up to 2.5kg. Adult sea lampreys are a brownish-grey colour with extensive black mottling, but close to breeding time their colour lightens to a golden brown. The river and brook lampreys, which are closely related to one another, are far smaller. They are both similar in colour, with adults having a dark olive or brown back and a lighter, silvery belly. The river lamprey is intermediate in size between the other two species with adults usually measuring around 30cm and weighing about 60g. Brook lampreys are the smallest with adults only attaining a length of 15cm, yet there are even smaller dwarf populations with adults from one such population on the Isle of Skye often measuring less than 10cm.
Part of the reason for the Atlantic salmon’s renown is undoubtedly the epic migration it undertakes from the river it grew up in, to distant oceanic feeding grounds, the return to its ‘home’ river and then on to spawn in the gravel beds of its birth. Salmon from Scottish rivers mainly leave between April and June, when they are called ‘smolts’. Once in the sea, they begin an arduous journey to feeding grounds around Greenland and the Faroes. There they feed, along with salmon from North America and Scandinavia, on a diet that includes shrimps, squid, sprats and sand eels.
Salmon remain at sea for an average of one to two years, although some stay longer. Those returning after one year are known as ‘grilse’. Losses at sea are high – they are taken by a variety of predators including humans, seals and large fish such as sharks, but sufficient numbers are able to return to the river to produce the next generation.
Little is known about the habits of salmon at sea but in order to complete such an impressive migration and return to the river of their birth, salmon are thought to use several methods of navigation. In the oceans they may use currents; closer to the river mouth chemicals in the water are likely to be important, and when ascending the river further, chemical and visual information guides them to their spawning grounds.
Salmon in Scottish rivers are unique in that they return from the sea almost all year round. Those that return early in the year tend to have been at sea for more than one year and will be migrating to spawning grounds in the uppermost reaches of their ‘home’ river. Once back in the river, adult salmon generally do not feed, resulting in a prolonged fast and a loss of 40% of their body weight by the time they spawn. Yet they retain sufficient reserves of strength to ascend waterfalls up to 3m high, and can frequently be seen negotiating these spectacular and seemingly impassable obstacles, such as Rogie Falls on the Black Water and the Falls of Shin, Sutherland, during the summer.
When returning from the sea, both male (‘cock’) and female (‘hen’) salmon have silver sides and a silver/white belly, but as spawning time approaches the male in particular will change markedly both its colour and appearance. Such male fish can be known as ‘Tartan Fish’ due to the mixture of red, brown and purple colours which develop on the sides and back. The male also develops a prominent upturned hook (‘kype’) on the lower jaw.
The spawning grounds to which the salmon are returning are typically gravel beds in relatively shallow, fast-flowing water. Salmon reach their spawning grounds during October to December when the female will excavate a depression in a gravel bed by lying on her side and repeatedly swishing her tail. This action lifts the gravel and carries it downstream. Then, when the spawning site (or ‘redd’) is about 15cm deep, she releases her eggs into it and they are immediately fertilised by an accompanying male. The female then fills in the redd, burying the eggs. During spawning the male fish aggressively chases away other fish. The splashing associated with spawning behaviour can sometimes be seen, especially when it happens in very shallow water and the backs of the fish are exposed.
After spawning the male fish remain near the redds in order to mate with any other females; however, they eventually become very weak and die. The majority of female fish also eventually die, but a small proportion are able to migrate back to the sea to feed (‘kelts’) and survive to spawn again.
River runners – Scottish Natural Heritage