Chum (Oncorhynchus keta)
Almost all chum salmon in the Canadian portion of the Yukon River drainage are “fall” chum. Chum salmon spawn in rivers and streams along most of the west coast of North America, and along the Bering and Arctic coasts eastwards to, and including the Mackenzie River. The upper Yukon River stocks of this species may have the longest upstream spawning migration in the world with some stocks migrating over 1,678 miles (2,700 km). In more southerly rivers, adult freshwater migrations tend to be much shorter with spawning occurring closer to estuaries.
Juvenile chum salmon have until recently, only been captured on known spawning grounds. Juveniles have been observed in one spawning location from mid-April to mid-June. This apparently prolonged residence may be a result of staged emergence due to different thermal regimes during incubation, or a period of rearing after (or during) emergence. Investigation into downstream migration has just started, but it appears that the outmigration is extended.
There are two runs of chum salmon that enter the mouth of the Yukon River. The first to arrive are the “summer chum”, which enter the river mouth in early June and reach peak abundance around the third week of June. Summer chum generally spawn in the lower 497 miles (800 km) of the Yukon drainage and, until recently, were not believed to migrate into the Canadian section of the drainage. Summer chum salmon have been found in the Chandindu River. Other reports from Mickey Creek (tributary of the Fortymile River) and the Klondike River confirm the presence of summer chum salmon.
Adult fall chum salmon are distinguished from fall chum by later run timing, larger body size and a more silvery appearance at the river mouth. Spawning occurs primarily in the upper portions of the Yukon River drainage. Compared to summer runs, the fall chum are usually less abundant at the river mouth.
Upper Yukon River chum return as spawning adults to the river mouth generally from mid-July through early September after spending up to five years in the ocean. Peak migration timing of chum salmon entering the Canadian section of the drainage usually occurs mid-September. Predominant age classes of mature upper Yukon chum salmon include age-four (62%) and age-five (35%). Spawning has been documented in groundwater discharge areas where water of sufficient quantity is released into the gravel beds of waterbodies or -courses at a fairly constant flow rate and temperature (3-7 degrees C). While some chum spawn along cutbanks and in riffle areas of the mainstem Yukon, other chum prefer spawning in side channels and sloughs. Shore spawning has been documented in Kluane Lake. Peak spawning usually occurs during October through early November.
Juvenile upper Yukon River chum salmon emerge from the gravel during the spring (April/May). After emergence, they appear to spend little time in the natal area. By mid-June, most have moved away from the spawning areas. Little is known about the downstream migration of juveniles although it may occur close to the time of spring break-up.