An analysis of the yield of brook trout (Salvelinus fontinalis) and rainbow trout (Salmo gairdnerii) per unit of fishing effort at Furnace Brook, Vermont, shows how modern statistical methods can be utilized to determine the significance of changes in the relative abundance of animal populations. The data cover the 5 years from 1936 to 1940 and the analysis of covariance employed shows that there was a significant decrease in the catch per unit of effort in 1939 which was attributed to the effects of severe floods caused by the hurricane in September of the previous year. This method of analysis, although useful for supplying a measure of relative abundance, fails to provide an estimate of the actual stock on hand.