Fig. 1. An example of variability in the abundance of a naturally spawning salmon population: the number of chum salmon (Oncorhynchus keta) returning to Fishing Branch River, a tributary of the Porcupine River in British Columbia (graph from Fisheries and Oceans Canada: http://www.taiga.net/coop/indics/chum.html). The escapement estimates for 1971 and 1975 should be 312,800 and 353,280, respectively.