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.