Modelling environmental and energetic effects on feeding performance and distribution of shags (Phalacrocorax aristotelis): integrating telemetry, geographical information systems, and modelling techniques

The shag (Phalacrocorax aristotelis) is an important avian member of the inshore, marine community. It is a foot-propelled pursuit-diver, feeding predominantly on lesser sandeels (Ammodytes marinus), which it catches mainly on, or near, the seabed. A Geographical Information System (GIS) was used to integrate data on diving behaviour and feeding rates of shags with spatial information on three environmental variables: distance from the nearest breeding colony, water depth, and seabed sediments, the last-mentioned being used as an indication of sandeel distribution. The results are used to assess how differences in the marine environments in two areas of east Scotland might influence the feeding performance and distribution of shags. Output from the model indicates a highly dynamic situation in both areas, brought about by changing energy requirements of the birds through the season and variations in food supply, superimposed on a heterogeneous physical environment. In order to test qualitatively whether predicted daily feeding time was a good predictor of the observed distribution of birds on the sea, data from ship-based surveys were compared with spatial patterns of feeding times. For both study areas, the relative frequencies of where birds were recorded on the sea were lower in areas where the predicted daily feeding time was high. Limitations of the current model and future applications for this approach are suggested. (C) 1997 International Council for the Exploration of the Sea.