Chum salmon usually return from the ocean to spawn four years after they hatch. A high escapement return will usually be followed by another high return four years later. This four-year cycle is apparent in the data from the 1970s (note the peaks in 1971,75 and 79). Many factors may affect this cycle, such as water level fluctuations, diseases, predation and, of course, fishing pressure. "Escapement" means the number of spawning salmon that reach the spawning grounds each year.
