Foraging ecology of goosander (Mergus merganser) and red-breasted merganser (M. serrator) ducklings in north east Scotland
Fish populations were sampled by electrofishing at a variety of sites on the Rivers N. Esk (1992-3) and Dee (1993-4) for comparison with sawbill duckling diets from the same rivers. Sampling methods are described and discussed in relation to other methods and in terms of possible biases. Within electrofished samples species distribution were similar on the two rivers, but more fish were caught per unit effort on the N. Esk. Catch results are described in terms of fish numbers and catch weights of each species from different sections of river and from different sampling times over the summers. Special attention is paid to the distribution and size frequency of salmon, because of their importance in the fish communities of the two rivers and as food for sawbill ducklings. The diets of goosander and merganser ducklings on the study rivers are quantified in terms of species of prey in the diet, size/age classes of fish and relative proportions of these species and classes. Estimates of diet are made from the stomach contents of samples of ducklings. Species composition of the diets are then related to the distribution of fish in those parts of the rivers in which the ducklings forage. Sizes of fish in the diets are then related to the sizes of fish in the duckling foraging areas, taking into account the changes in fish size with location and overtime, and the size (and approximate age) of ducklings. For comparison with diets, fish length data from electrofished samples was interpolated by regression between pairs of consecutive samples and adjacent river sections. This provided expected mean river fish lengths for the dates and locations at which broods were sampled.