Diurnal migration of adult peamouth chub was studied in Nicola lake. Results are taken from over two hundred gill net sets in 1959 during which time more than five thousand chub were netted. A migration surfaceward and shoreward occurred in evening and a reverse migration in morning. Diurnal migration was characteristic of the summer. Chub during winter had constant close association with the bottom at which time bottom organisms were the main food item. Onset of diurnal migration in summer was accompanied by increase in importance of plankton and chironomid pupae in the diet. Adults of all sizes and both sexes performed similar diurnal migrations but young-of-the-year chub showed the reverse migration, inhabiting shallow shore waters by day and dispersing to deep water by night. Inversion in behaviour occurred at about one year of age. Adult migration was unrelated to oxygen or temperature changes but timing of migration and depth distribution of chub was closely associated with light intensity, adults avoiding brightly illuminated areas. A diurnal change in diet accompanied the migration. In August food items changed from plankton during day to chironomid larvae and pupae in evening and night. Adaptive value of the migration is discussed in relation to physical and biotic characteristics of the environment. / Science, Faculty of / Zoology, Department of / Graduate