Reproductive recovery of a common eider Somateria mollissima population following reductions in discharges of polycyclic aromatic hydrocarbons (PAHs).

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed toxic compounds in marine ecosystems, but the effects of such pollutants on populations of aquatic birds are poorly known. This study examined the reproductive biology of a marine duck, the common eider (Somateria mollissima), in a Norwegian fjord that received high input of PAHs over several decades. Numbers of breeding females and post-hatch duckling mortality were compared between the years before and after termination of PAH discharges (1989/1990). Moreover, 5 years after the pollution was terminated, reproductive parameters were compared between the polluted fjord and a nearby unpolluted area, to investigate long-term effects. The environmental impact of the pollution peaked in the 1980s, and during this time the number of breeding eiders was halved in the study colony. The duckling mortality was very high, peaking in 1991 when about 8 % of all hatched ducklings were found dead in or near the nest. Since 1993 the ratio of dead ducklings per breeding female stabilized at about one third of the level before the termination of the PAH discharges. Moreover, between 1994 and 1999 the numbers of breeding females increased by 50 %, and in 1995 females in the polluted colony were in better condition, laid larger eggs and had shorter incubation periods than females in the unpolluted area. This study indicates that in the 1980s, PAH pollution affected the reproduction of the eiders in the polluted fjord, although other natural factors may also have influenced eider reproduction. A few years after the pollution level dropped, reproductive output improved strongly, but the number of ducklings dying in the nest was still relatively high compared to unpolluted areas.