Varying chick mortality in an organochlorine-“strained” population of the nominate Lesser Black-backed Gull Larus f. fuscus in the Baltic Sea

Abstract

Severe reproductive failure has contributed to a drastic population decline of the nominate Lesser Black-backed Gull (Larus fuscus fuscus) in its breeding grounds in the Gulf of Finland and the Bay of Bothnia. In intensive field studies in the central area of the Gulf of Finland, roughly 65-70% of chicks died in their nest in the 1980s and 1990s due to innate diseases. In the 1990s, the fledging rate was only 0.02. The diseases consisted mainly of liver degeneration and various inflammations, and most chicks died of sepsis. The hepatic concentrations of legacy organochlorines (OC) in dead chicks were high, and they correlated with the proportion of dead chicks in a brood. During the 2000s, however, the occurrence of diseased chicks in the Gulf of Finland decreased to 48%, which helped to achieve a fledging rate supposedly sustaining the population (0.52). At the same time, the trends in hepatic levels of certain legacy OCs in chicks decreased, especially the DDE, HCB, β-HCH, and iraws-nonachlor levels. In spite of skewed sampling (only dead chicks were available), our results indicate an enhancing health status among the present population, probably due to a less contaminated diet during the non-breeding period.