Mass-movements of the trout of Llyn Tegid have been inferred from scale structure and catch records for the lake and tributary streams. About 60 per cent of the trout enter the lake from tributary (nursery) streams when two-years-old, 20 per cent when one-year-old and 20 per cent when three-years-old. Within an age group, the performance of this lakeward migration appears to be related in some way to the attainment of a certain size. In the lake from age I to age IV trout have a survival rate of 29 per cent per annum. A summer movement from the lake shallows to deeper waters has been detected, most fish leaving the littoral in April-June and returning during the winter from September onwards. Sexually maturing fish leave the lake to spawn in the tributary stream during October and November.