Foerageergebieden van Aalscholvers Phalacrocorax carbo in Drenthe en Groningen

Observations of foraging and resting Cormorants were plotted in squares of 5x5 km in the provinces of Groningen (2499 km2) and Drenthe (2681 km2), using the maximal number if more than one count per week was available (Fig. 1). For each 5x5 km-square, the area of open water was calculated from topographical maps. Numbers of Cormorants showed a positive correlation with the area of open water, canals, streams and wide ditches (Fig. 2). The density of Cormorants on lakes and pools was more or less constant at 6.0 birds/100 ha, and ditto at 3.2 birds/100 ha on canals and streams. Cormorant numbers in September were on average 1.7x higher than the monthly average for the whole year. Taking into account the distribution and area of open water per 5x5 km-square, and using formula (1) in which P = area of lakes and pools in each atlas-square in ha and L = ditto area of water in canals and streams, the expected quantitative distribution has been calculated for both provinces (Fig. 3), It was shown that the calculated distribution closely resembled the actual distribution based on pooling observations. The distribution in the early 1990s is not much different from the one in the 1980s. Consequently, favourite feeding areas must have increased in importance, given the overall increase in numbers. All large roosts were located within 10 km of the nearest open water. Smaller roosts were usually 10-20 km apart, again showing that Cormorants will not range further than c. 10 km from the roost.