Oiling of live gulls as a tool to monitor acute oil spill effects on seabirds

Abstract

Oiling of seabirds is a serious ecological threat to marine ecosystems worldwide (Wells 2001). Because they spend much of their lives at sea, seabirds are particularly prone to marine oil pollution exposure, with shortand longterm population consequences (Peterson et al. 2003). In seabirds, the degree of exposure to oil is dependent upon ecological and behavioural traits related to habitat use that may differ among species, populations, age classes and sexes (Day et al. 1997, Votier et al. 2005, MartinezAbrain et al. 2006). Due to their high vulnerability and exposure, seabirds have been used as bio-indicators of marine oil pollution for many decades (Furness & Monaghan 1987, Burger & Gochfeld 2004). The most important monitoring method draws from beached bird survey schemes, as they are thought to provide important evidence to assess geographical and temporal patterns in chronic oil pollution at sea (e.g. Seys et al. 2002, OSPAR Commission 2005). Here we present a novel non-intrusive approach that can be useful to assess oil effects on seabirds based on the monitoring of oiling rates in live free-ranging gulls. The few attempts to date of systematically recording oiling rates in live marine birds have been used to evaluate temporal changes in the exposure of Common Terns Sterna hirundo and Black Skimmers Rhynchops niger to chronic oil pollution (Duffy 1977, Gochfeld 1979, Erwing et al. 1986). To assess the feasibility of this method, we established a monitoring scheme to record oiling rates in live gulls in a coastal area affected by the Prestige oil spill disaster in Galicia (northwest Spain). The spill started in November 2002 and, as a result, thousands of marine birds, mostly wintering auks (Alcidae) from Great Britain and Ireland, died in the following months, and other seabird populations were negatively affected (Velando et al. 2005, Votier et al. 2005). We found that the temporal pattern of visible oiling in live gulls was coupled with the temporal occurrence of dead and impaired oiled seabirds at beaches.