Chinook salmon (Oncorhynchus tshawytscha)
Chinook salmon spawn in streams and rivers along the west coast of North America. The Yukon River is one of the most northerly of the major Chinook spawning rivers, hosting some of the longest upstream migrating salmon stocks in the world. Some headwater stocks migrate over 1,840 miles/2960 km to reach their spawning grounds in the Yukon and northern British Columbia.
In the Yukon River drainage basin, Chinook fry emerge during spring and early summer from the gravel where they were spawned. Emergent juveniles are found in still-water areas such as the advancing margins of rivers and micro-habitats near shore. Some fry leave the rivers of their birth, or “natal” streams, soon after emergence. The timing of entry appears to be related to overall annual water temperatures. Juveniles may be carried downstream into larger rivers by the spring freshet.
Young-of-the-year start to enter non-natal tributaries in late June and may migrate significant distances (over 6 miles or 10 km) upstream. High densities may be found immediately downstream of physical obstructions such as beaver dams. Sampling has indicated major emigrations or flushing of fish from smaller tributaries during summer high water events. They are absent or present in low densities in highly turbid waters. In small, moderate gradient non-natal streams, juveniles are most numerous in small pools. Similar in-channel pools are used in larger streams. In large rivers, juvenile chinook are easily captured along margins and in mixing zones below where tributaries join the Yukon River. They have been captured in near shore waters in lakes and near the mouths of tributaries. Juveniles may be abundant in tributaries not used for spawning.
Populations of young-of-the-year (age 0+) juveniles in non-natal streams generally have a larger average body size than those in natal streams. Generally, body size increases with distance from the stream mouths. Predators include northern pike, burbot and inconnu and birds such as mergansers, kingfishers, gulls and loons. Chinook fry grow rapidly, building up reserves of fat for their first winter in freshwater, known as the “rearing” phase.
Successful over-wintering of 1+ juveniles has been documented only in streams and smaller rivers, but is expected to occur in larger rivers and lakes. Overwintering in small streams appears related to local ice formation and groundwater availability. Areas with heavy glacial-fluvial material have high potential to store and release groundwater during the course of the winter. Overwintering in lakes has not been documented. In spring, a portion of the over-wintering population remains in small streams until early- to mid-June.
Surviving juveniles migrate to the Bering Sea to start their ocean life usually after spending one year in freshwater. Some never leave the freshwater environment. These “residuals” are almost invariably male, and reach spawning condition at a small size. Movement of 1+ juveniles from small streams appears related to annual thermal regimes. Rarely are 1+ juveniles captured in the Canadian portion of the Yukon River drainage after July 15. Most captured later than July 15 have been residuals from spawning streams.
Adults return to the mouth of the Yukon River from mid-to-late May through early July after spending 2 to 6 years in the ocean. Peak migration timing of Chinook salmon entering the Canadian section of the drainage usually occurs mid-to-late July with spawning from late July through mid-September. The average age composition of mature, upper Yukon Chinook salmon show interannual variation, however a recent average is 59% age-6, 27% age-5, 9% age-7, 5% age-4, <1% age-8, and <1% age-3.
Chinook salmon use a wide variety of spawning habitats including small streams, larger rivers and lake outlets, or waters intermediate to these. Streambed substrate requirements, redd structure and spawning behaviour have not been specifically studied in the Yukon River drainage. There is no apparent use of groundwater for redd location. Emergence from the egg occurs as early as September in the central Yukon River. Emergence from the substrate has been little studied, but may vary by up to six weeks (April to June) in nearby waters having different thermal regimes.