Temporal patterns in abundance of Triaenophorus crassus Forel in relation to patterns in abundance of species composition of pike (Esox lucius), lake whitefish (Coregonus clupeaformis Mitchill), and lake herring or cisco (Leucicthys tullibee Richardson) in 35 commercially fished Manitoba lakes were studied for the period 1973-1983. Patterns in fishing effort, annual production, differences in body size of lake whitefish, lake sizes and their geographical locations and differences in value between lake whitefish and walleye (Stizostedion vitreum vitreum) were also examined. It was concluded that changes in species composition, annual production levels, fishing effort, and differences in body size of lake whitefish affected abundance of T. crassus in lake whitefish. High flow rates from Churchill, Nelson, and Hayes watersheds were correlated with high annual catch levels. Overfishing or underfishing may have occurred in some of the lakes which may have affected recruitment and density and abundance of T. crassus due to an increase or decrease in smaller lake whitefish. Most of the lakes were located in the north and north-central regions of Manitoba. The southern lakes had lower abundances of T. crassus. Patterns in abundance of T. crassus could not be explained by lake size or location of lakes according to watershed. Although walleye was more valuable than lake whitefish, there was no evidence that they were fished preferentially, nor was there a direct correlation between harvest of lake whitefish and walleye and abundances of T. crassus. The general trend was towards a change in lake classification to a lower category of lake whitefish i.e. higher abundances of T. crassus. However, inconsistent sampling of lakes for T. crassus, particularly lakes classified as high grade (i.e. with low abundances of T. crassus) such as Patridge Crop, Natawahunan, Guthrie, and Sabomin, contributed to the difficulty in predicting long term trends.