Boundaries

The ecoregion encompasses the river drainages of the Sea of Okhotsk coast south of the Taygonos [Tainykot] Peninsula (Gizhiginskaya Guba Bay) down to the Kiran River. The Taygonosskiy, Kolymskiy, Suntar-Khayata ranges, Yudomo-Mayskoye Nogor’ye Upland, and Dzhugdzhur Range divide the ecoregion with, respectively, the Koryakia [612], Kolyma [609], and Lena [608] ecoregions.

Topography

This is a mountainous coastal ecoregion, with elevations ranging from sea level to over 2600 m asl. Rivers of the coast of the Sea of Okhotsk flow down the Dzhungzhur Ridge and from the spurs of the Chersky and Gydan ridges.

Freshwater habitats

A majority of rivers in the ecoregion are short mountain rivers that lack developed deltas or estuaries because of the close proximity of ridges to the coast. The upper reaches of rivers in this ecoregion are usually shallow and flow in narrow valleys through stony river beds. The lower reaches of larger rivers usually split into branches separated by sand—pebble islands, and are overgrown by swampy vegetation. Along the northern coast rivers flow through plains and lowlands.

The ecoregion contains some notable large rivers — Okhta, Kukhtui, Kova, Gizhiga, and Yama — with lengths ranging from 300—350 km. These are particularly large compared to most of the other rivers that do not exceed 200—250 km in length.

Lake Khel-Degi lies in the upper Ini River drainage at an altitude of about 1000 m; its surface area is 875 hectares and mean depth is 21 m. Lake Chukcha is a thermokarst lake in the Kava River drainage.

Terrestrial habitats

This narrow coastal ecoregion lies within four terrestrial ecoregions (from west to east): Okhotsk-Manchurian taiga, East Siberian taiga, Northeast Siberian taiga, and Bering tundra. There are also pockets of Trans-Baikal Bald Mountain tundra and Chersky-Kolyma mountain tundra. Bering tundra in the eastern part of the ecoregion is dominated by larch forest.

Description of endemic fishes

Neiva (Salvelinus neiva)  is only known from the Uyeginskiye Lakes in headwaters of the Okhota River. It is a typical lacustrine char that differs from the sympatric dolly varden (S. malma) and whitespotted char (S. leucomaenis) by its resident mode of life and peculiar spawning coloration: red-brownish-yellow with large crimson spots on the flanks, orange lips, and reddish fins with white marginal rays.

Other noteworthy fishes

Graylings are among the major species in rivers and lake-river systems. They are generally absent in small streams up to 50-60 km in length. Grayling is represented by two species: East Siberian (Thymallus pallasii) and Kamchatkan T. mertensii. A varied pattern is observed among grayling species from northeast to southwest along the continental coast of the Sea of Okhotsk. The Kamchatkan grayling inhabits the northern part of Shelikhov Bay (in the Paren’ and Gzhiga rivers). The East Siberian subspecies has been recorded further west in a number of other rivers (Yama, Ola, Arman’). The Kamchatkan subspecies is then recorded from the Yana and Taui rivers, and further west the East Siberian grayling has been established in all rivers along the southwestern part of the coast.  

Salvelinus sp. 1 undescribed (Mak-Mak Lake in the Ola River drainage), Salvelinus sp. 2 undescribed (lakes Chistoye and Glukhoye), and Salvelinus sp. 3 undescribed (lakes in the Yama River drainage) are listed in Chereshnev (1990), but no data on morphology or biology are provided.

Ecological phenomena

The ichthyofauna of the ecoregion include all cold-loving species with wide ecological ranges. The majority of these are capable of inhabiting various types of water bodies, such as lakes, slow- and rapid-flowing streams, and areas near estuaries. Considering these circumstances, one should expect a more extensive dispersal of species and monotonous composition of fishes among different rivers. The observed distribution of species in the ecoregion, by contrast, is very uneven, mosaic, and cannot be accounted for by the climatic conditions of  the water bodies and ecological characteristics of fishes.

Justification for delineation

For ecoregions of the former Soviet Union, a species/genera/family presence/absence matrix was compiled for a hierarchy of hydrographic units, and cluster analysis and ordination techniques (Primer v.6 statistics software) were employed to assess biotic similarities among hydrographic units and to identify major faunal breaks.

The Okhotsk Coast ecoregion is defined by relatively short coastal rivers flowing to the east from dividing ranges located along the Sea of Okhotsk. It is mainly characterized by the absence of Coregonidae and Stenodus, a relative rarity of Prosopium cylindraceum, and a low number of Cyprinidae. In general, there are few true freshwater fishes. The diversity of forms of Salvelinus is high, and some of these have been reported as undescribed species. The ecoregion completely lacks elements of the Amur River fauna typical for the neigboring Coastal Amur ecoregion [615]. On the other hand, it is much poorer than the Anadyr River ecoregion [610] to the north due to limited ecological niches in rivers of the Okhotsk coast.