The cherry salmon occurs in small numbers in Sakhalin rivers and is the first migratory Pacific salmon to appear in coastal waters, normally in mid- to late May in southern Sakhalin. Mass migration upstream occurs during the second half of June – mid-July with spawning occurring in late July – early August,
although this timing varies across the island (SakhNIRO 2000). Before spawning, the cherry salmon spends about two-eight weeks in the river, lingering in holes in the main channel and enters tributaries directly before spawning. An important factor in stimulating the spawning migration is a rise in water level and increase in river current velocity (e.g. as associated with spring-early summer floods), which permits fish to reach their spawning grounds, which are situated largely in the upper reaches of rivers. On the southwestern coast and in Aniva Bay, migration generally occurs mid-May to mid-June, while in northern Sakhalin (middle reaches of Tym river) mass migration occurs from the second half of July into the first half of August and spawning from late July to early September. In the rivers feeding into
Terpeniya Bay (e.g. Poronai River), the migration starts approximately a month earlier than in the Tym and spawning occurs early July to mid-August (Gritsenko 1973).
Normally, cherry salmon spawning areas are isolated from the spawning grounds of other salmon species. In southeastern Sakhalin (in the tributaries of Belaya river - the Naiba river basin) it spawns in areas of pebbles, with rocks and some sand and relatively high current velocities. In northern Sakhalin, spawning grounds are situated in the upper reaches of the main rivers and its tributaries. These sites tend to be small watercourses with an alternation of pools and riffles. Spawning redds are situated in areas where water depth is 10-25 cm, current velocity 0.2 to 0.8 m/s.
Alevin hatching occurs after 35–50 days and by about day 80-85, juveniles start moving into the water column where they begin actively feeding. They leave the redds by the second half of April (later in northern Sakhalin). Shoals of juveniles then disperse into the upper and middle reaches of the rivers where prefer sections of watercourses with a rapid, turbulent current and cover provided by bushes and log jams (SakhGu 2000). During the summer, young fish feed on a varied diet of airborne and aquatic insects as well as the eggs of pink and chum salmon. The duration of the river-dwelling stage of this species appears to be dependent on diet and generally during the second year in freshwater the juvenile population divides into smolts, which undertake migration to the open sea, and parrs that remain in the river for another year. Some individuals achieve sexual maturity without leaving the rivers. These are dwarf males, which sometimes occur in large numbers in the upper tributaries and which actively take taking part in reproduction. The cherry salmon
migration in northern Sakhalin occurs at an older age (three years) compared with that in many other regions of the range, where the bulk of juveniles leave the rivers during the second year of life (SakhNIRO 2002). Cherry salmon smolts, among which females prevail, spend a large amount of time in the near shore zone where they feed on crustaceans and small fish, before moving offshore into open waters.
From a commercial perspective cherry salmon is of minor importance and is only caught in relatively large amounts in south-western Sakhalin during May- June, in the course of the pink salmon migration. Between the late 1950s and the late 1980, its catch normally did not exceed 5-10 % of the totals caught during the pink salmon migration, although in some years, it constituted up to 40 % in an area from Slepkovsky Cape to the Lopatin Cape (SakhNIRO 2000). In the course of the pink salmon fishery in other parts of the Island, cherry salmon may be present as a bonus catch in small amounts, which is explained by its lower numbers and the fact that the bulk of fish have already entered the rivers for spawning.
4 POTENTIAL EFFECTS AND IMPACTS
This section describes the potential effects of the river crossing works and the impact they may have on the environmental receptors that are identified in Section 3. An effect is defined as a change in conditions that results from the river crossing works, and may be direct or indirect. An impact is the response exhibited by key environmental receptors to this effect (positive or negative). Effects and impacts upon ecology are considered below for each of the crossing methods carried forward for further consideration in the strategy, namely the wet cut crossing method (Section 4.1) and Horizontal Directional Drilling (HDD) (Section 4.2). Aerial crossing and dry cut methods have been discounted based on the arguments presented in Section 2.4.1 and 2.4.4. The effects and impacts of other activities, water intake and use of chemicals are discussed in Section 4.3.
The following sections discuss the main physical changes related to in- channel construction works and go on to describe the ecological effects associated with these potential changes. Given the importance of the
watercourses that are to be crossed by the pipeline to salmonid fish, particular emphasis is given to the potential effects of suspended sediment
concentration increase and substrate change on salmon spawning behaviour and habitat.