The powan, Coregonus lavaretus (L.) is endemic to only two British waters, Loch Lomond and Loch Eck, Scotland. This thesis describes the seasonal and longer term growth processes of the two populations, concentrating on growth in length back-calculated from scales, factors affecting recruitment and mortality, reproductive cycles, and seasonal deposition and mobilisation of storage products, particularly lipid. The interrelationships of these cycles is discussed. The populations differ in their diet and duration of feeding, and it is shown that most of the inter-population differences in seasonal cycles of growth relate to these feeding differences. The Loch Eck population is the more variable. In addition to adult and immature powan, a third category is identified, termed adolescents. These are fish which are entering their first reproductive cycle. Immature and adolescent fish are analysed separately and compared with the adults. There are some differences in seasonal cycles between the juveniles and adults, mainly in relation to the presence or absence of the reproductive cycle. A preliminary histological study of the ovaries of adolescent females is carried out. Comparison of historical data with the results of the present study shows that there has been little change in the Loch Lomond powan in the past 200 years. Both lochs are coming under increasing human pressure, and conservational measures urgently need to be taken if the powan populations are to survive.