Boudewijn T.J. & Dirksen S. (1995) Impact of contaminants on the breeding success of the Cormorant Phalacrocorax carbo sinensis in The Netherlands. ARDEA 83 (1): 325-338

In The Netherlands Cormorants reached a minimum number of breeding pairs in the mid-1960s. Since the end of the 1970s the population is increasing again, and many new and rapidly expanding colonies have been settled. However, in a colony in Dordtse Biesbosch, situated in the centre of the sedimentation area of the rivers Rhine and Meuse, breeding success was very poor during 1987-1989. In order to establish the factors responsible, a study of breeding biology was carried out in 7 colonies, representing a wide range of contamination levels in surrounding feeding areas. Because literature on breeding biology of the species is relatively scarce, this paper firstly presents data on the breeding biology from a colony of Cormorants in a relatively clean area, producing average numbers of fledglings. Secondly, the different stages of breeding are compared for all colonies studied, together with several other parameters. From these data, possible factors causing the large differences in reproductive success between the colonies are evaluated. The low breeding success in Dordtse Biesbosch (c. 0.5 fledged young/pair) and, to a lesser extent, in some colonies upstream along the river (c. 1.2-1.7 fledged young/pair) can be considered the cumulative effect of (1) a later onset of egg-laying and a related decrease in clutch size, (2) a reduction of eggshell thickness, probably to a level low enough to cause breaking of part of the eggs, (3) a high embryo-mortality, resulting in a reduced hatching success and (4) a high mortality of young, especially in their first weeks. It is concluded that organochlorine contaminants are the most likely cause: p,p'-DDE (through eggshell thinning) and PCBs (through direct toxic effects on the embryo).