A river pulsing with life
The Fraser River, the longest river within British Columbia, hosts one of the largest sockeye salmon runs in the world. The heart of this ecosystem is the sockeye salmon, Oncorhynchus nerka, a species known for its dramatic, cyclical population dynamics. Total adult returns of Fraser River Sockeye are highly variable, ranging from 2 to 28 million fish annually. This fluctuation is not random; it follows a distinct four-year cycle. One year in the cycle, known as the "dominant" year, sees an immense return of millions of salmon. The other three years consist of one "sub-dominant" run and two "off-cycle" years with much lower numbers.
The cause of this boom-bust cycle lies in density-dependent competition in the salmon's freshwater nursery lakes. Most Fraser sockeye mature and spawn at age four. During a dominant run, the massive number of spawning adults produces an even more massive number of offspring, or fry. These fry migrate to nursery lakes, such as Shuswap or Quesnel Lake, where they spend a year feeding on zooplankton. The sheer number of fry from a dominant spawn overwhelms the lake's food resources. This intense competition results in smaller juvenile salmon, known as smolts, which have a much lower survival rate when they migrate to the ocean. The subsequent crash in the returning adult population allows for the cycle to begin again. With fewer spawners, the next generation of fry faces less competition, grows larger, survives better at sea, and produces another boom.
An epic 4,000-kilometer journey
The life of a Fraser River sockeye is a 4,000-kilometer round-trip. It begins when a female deposits between 3,000 and 4,000 eggs in the gravel of a tributary stream, such as the Adams River. The eggs incubate over the winter, and the young fish, called alevins, emerge in the spring, nourished by a yolk sac. Once the yolk is absorbed, the fry, now about 3 cm long, move into a nursery lake for a year.
After a year in the lake, a physiological change called smoltification prepares them for saltwater. They journey down the Fraser River and into the Pacific Ocean, where they will spend two to three years feeding and growing in the Gulf of Alaska. Guided by a keen sense of smell and the Earth's magnetic field, the surviving adults navigate back to the Fraser River. They stop eating and undergo a dramatic physical transformation: their bodies turn a brilliant crimson, their heads become green, and males develop a humped back and hooked jaw. They battle their way upstream, returning to the exact stream where they were born to spawn and then die, completing their life cycle and transferring important marine nutrients to the inland ecosystem.