Pigeon Lake, Alberta, is a shallow eutrophic lake with a sandy basin, gentle contours and an area of 40 square miles. It contains whitefish, pike, yellow walleye, perch, burbot, white suckers and spottail shiners. The whitefish have been commercially exploited for many years and catch statistics are available from 1918.In 1941 a greatly increased catch of whitefish was permitted. Large annual yields continued until 1946; in 1947, in spite of considerable effort, a very small catch was made. Since this collapse fishing was prohibited in two years and light in two years. The lake now contains a normal whitefish population.Samples of the commercial catch during this period showed that the average age of the fish fell from 5.1 to 2.3 years, then, after collapse, increased to 5.7 years. Growth rates increased greatly, then decreased to the original level. Age at maturity decreased from five to two years.Calculations of the number of fish each year-class contributed to the fishery reveal that the collapse of the fishery was not due to overfishing; the weak year-classes which caused the collapse had parent year-classes of normal abundance. It is suggested that egg destruction by strong winds may have caused the weak year-classes.