This thesis describes a series of studies to investigate the predator-prey relationships between the Goosander Mergus merganser, a fish-eating duck, and populations of game fish in northern Britain. The distribution and numbers of birds during the breeding season, in northern England and Scotland, were investigated and variations in density, between and within rivers, studied. Relationships between densities in spring and summer survey periods were also investigated. Possible biases in survey data are discussed. Studies of the growth of ducklings reared in captivity allowed estimates to be made of food and energy requirements from hatching up to approximately the time of fledging, at c70 days. Seasonal changes in the body mass and body condition of adult and immature Goosanders were investigated and daily and seasonal food and energy requirements estimated. Time-budget data were collected to investigate how birds partitioned their time between various behaviours. Of particular concern was the temporal and spatial distribution of foraging activities between and within broad habitat types, viz. rivers, standing waters known to be roost sites, and non-roost standing waters. Feeding behaviour was also recorded. The species composition of the diet and the numbers of individual fish represented, were determined by the gut analysis of 54 birds received from various sources. Possible biases in this method are discussed. For salmonids, the size of individual prey items was investigated from regression equations of fork length on vertebrae length, based on a reference collection. Conflicting evidence of damage to fisheries from other studies chiefly in North America, is re-assessed in the light of current knowledge of the population dynamics of salmonids and of results presented here. The potential contribution of depredations by Goosanders to mortality at successive life stages of fish is considered.