Sharp-nose lenok (Brachymystax lenok) and Blunt-nose lenok (Brachymystax tumensis)
A characteristic trait of the genus is its huge adipose fin — much bigger than any other salmonid fish. At first sight lenok resembles brown trout with short jaws and small mouths. Not for nothing are the English nicknames of the species “Manchurian trout” and “Siberian trout.” There are two distinct forms of lenok, which are considered to be separate species. The sharp-nose lenok with subterminal mouth has the scientific name Brachymystax lenok, and the blunt-nose form with terminal mouth is Brachymystax tumensis.
In many drainages, these two forms have different ecologies and are easy to distinguish. Only in the big Uda River most of the lenok in catches look like hybrids between the sharp-nose and blunt-nose species.
Sharp-nose lenok has pointed upper jaw that is longer than the lower one and subterminal mouth. It is a dark-olive or light-brown dotted fish. Its colors resemble the blunt-nose species, but the dark dots are normally more subdued. In addition, on the sides there are several crimson spots with diffuse outlines or red vertical stripes, which can look like parr-marks.
The sharp-nose lenok inhabits the Amur basin; it is also found throughout the Siberia from Kolyma in the east to the Yenisei in the west. The species is also found in the southwestern part of the Ob River basin – at the source of the Irtysh River. In many rivers, primarily in the Amur and Lena, two species of lenok live together. On the contrary, in the Yenisei, Kolyma and Indigirka Rivers there is only the sharp-nose species. In the Amur basin, as well as in the Tugur River, blunt-nosed lenok predominates; the number of the sharp-nose species is several times lower.
At the altitudes of over 500 m in the Yudoma and Uchur Rivers (Lena drainage) only the sharp-nose lenok is found. In some streams of the Lena drainage the blunt-nose lenok is also a common species, but it is found only at the lower altitudes. In the Amur and Tugur Rivers, the numbers of the blunt-nose lenok are higher comparing with the sharp-nose form. The maximum weight of the blunt-nose species is much bigger, up to 8–9 kg; there is information on 11-kg “monsters.” On the contrary, the sharp-nose lenok bigger than 3,5–4 kg is a rare exception. The average size of both species in catches is between 1 and 2 kg.
The biology of these two species is also different. In the streams around Khabarovsk and farther north, the sharp-nose lenok prefer swift mountainous stretches of the streams. Blunt-nose lenok populate mostly their middle and lower reaches, and in shorter rivers can be found even near the sea.
Sharp-nose lenok winter in mountainous or piedmont rivers, not far from their summer habitats. For the blunt-nose lenok much longer migrations are typical; in the winter many fish are caught in the main channel of the Amur River, which is too warm for any salmonids in the summer. The very beginning of the spring flood signals the fish to go back into the tributaries. Here lenok spawn and feed till the fall. In some rivers the length of these annual migrations is more than 100 km.
Lenok’s mouth is noticeably smaller than that of a trout of the same size. The basis of the lenok diet for most of the year are the invertebrates, mainly aquatic insects larvae. However, relatively small mouth does not prevent this ancient fish from eating quite large food objects, in particular, small rodents.
The range of blunt-nosed lenok consists of several unrelated parts. This fish lives in the headwaters of the Ob River, in some tributaries of the Lena River, in the Amur basin, in the Uda and Tugur rivers north of the Amur, as well as in many rivers of the Sea of Japan from the Samarga River to the Korean peninsula. The same lenok species inhabits the western rivers of the Bolshoi Shantar Island; it is also found in the north-west of the Sakhalin Island.
The blunt-nose lenok are olive-brown or red-brown fish with jaws of similar length. Body, dorsal, and adipose fins are covered by regular black dots of the size of its eye pupil.
The blunt lenok is much larger than the sharp-nose species; although the average weight of fish in catches is from 1 to 2 kilograms, they occasionally can be up to 10-11 kilograms.
The blunt-nose lenok mainly inhabits the middle parts of the rivers; in many drainages it is found in the lower reaches, near the sea. This fish migrates widely, appearing where abundant food can be found.
The blunt-nose lenok is often spending the winter in large rivers – in both of the piedmont and lowland types. By the beginning of the spring flood, mature fish are migrating upstream and enter tributaries for spawning. In the south of Siberia and in the Far East, the lenok propagation takes place in May, and in the north of the range – in June. After spawning, it stays for some time for feeding in the tributaries, then begins a downstream migration into the main river.
In the Lower Amur, the blunt-nosed lenok takes long trips. The summers it is spending in the tributaries – in the main river the water is too warm. In the fall, going down for the winter, some of the fish reach the Amur River, where they stay near the mouths of the tributaries and close to the sandy spits, together with the Amur pike. On the contrary, the sharp-nosed lenok is never caught in the main channel of the Amur River.
The ecological niches of both lenok species overlap; their diet is similar to large grayling — they can eat any animal food. The biggest share of a lenok rations consists of invertebrates, mostly insects and their larvae. The bigger fish also eat small fish and rodents.
Lenok is very sensitive to water pollution, for example, to the increase in its turbidity as a result of placer gold mining. In addition, it quickly disappears where the poachers practice continuous blocking of the rivers with fyke-nets. Its number in case of intensive sport-fishing also decreases rapidly.