Oil Spills & Seabirds Acute Mortality Sublethal effects Indirect effects Population level


Acute mortality  

The Prestige wreck was the worst possible scenario to seabirds on Iberian coast. The accident occurred in over-wintering season when seabirds tend to congregate in large numbers on Iberian coast. In addition, the oil released during the erratic course of the Prestige tanker oil, and further after sinking, created an extensive oil slick that wind and currents pushed it into a vast coastal area, from northern Portugal to Brittany (France). During wintering, many seabirds spend much of their lives in contact with the sea surface, thus they are not able to escape an oil front, such as created by the Prestige.



Oil destroys the waterproofing properties of feathers and once wet, oiled birds often succumb to hypothermia, smothering and drowning. Moreover, birds often ingest toxic hydrocarbons while preening which results in poisoning and death. In the Prestige oil spill, dehydration and exhaustion seem to be the most likely cause of death, as shown a study based on necropsies of beached carcasses (Balseiro et al. 2005). Accordingly mass mortalities of 23,000 seabirds were documented during the days after the spill (Garcia et al. 2003). However, the number of oiled birds recovered after a spill is only the tip of the iceberg. The total number of birds oiled needs to be estimated because a large proportion of the corpses do not come ashore. Indeed, much of the toll from the Prestige, in terms of oiled seabirds, happened in the open ocean, and thousands of birds' corpses sank before reaching the shore. Using results of a drift block experiment (simulating the drift of died auks at sea) to estimate the number of unreported casualties, unprecedented numbers of seabird deaths were estimated, with 150,00 to 250,000 casualties (Arcos et al. 2004). The most affected species were diving seabirds, especially guillemots, razorbills and puffins. Recovery of leg rings showed that many guillemots and puffins were from colonies in western Britain, from south Wales to the Scottish islands (Garcia et al. 2003). Since Prestige oil spill occurred in seabird wintering areas remote from breeding colonies, the effect on mass mortality on seabird population parameters it is difficult to quantify. Nevertheless, a long-term study on demography of common guillemots at Skomer Island, Wales, indicated that oil spills, including the Prestige wreck, doubled the annual over-winter mortality of adults breeding in this colony (Votier et al. 2005).



Drift block experiments served to estimate the number of casualties




Although Prestige oil spill mostly affected wintering seabirds, hundreds of European shags, a resident endangered seabird, also perished (Velando et al. 2005). Although the spill occurred 3 months before the onset of shag breeding, this species is largely sedentary and shags remain around breeding colonies throughout the year. Unfortunately, a strong female skew in adult mortality was detected in shag casualties during the Prestige oil spill (Martinez et al. 2006). By the time the spillage took place (November 2002) shags at sea were most likely females, and males were already at breeding places. The result of the female skewed mortality implied a deficit in mates with whom males could reproduce and hence a decrease in reproductive success at population level. Analysing the population dynamics of European shag including the demographic effects of sex-specific mortality, a study (Martinez et al. 2006) revealed that the mortality of shags caused by the Prestige oil spill implies a reduction of 11% in the breeding population.




Effects of shag mortality in annual population growth rates with respect to pre-spill dynamics (Martinez et al. 2006)

Arcos J.M., D. Alvarez, P.M. Leyenda, I. Munilla & A. Velando 2004. Seabird mortality caused by the Prestige oil spill: preliminary insights from a drift blocks experiment. Proceedings of the 8th International Seabird Group Conference "North Atlantic Seabird Populations: 10, Aberdeen.
Balseiro, A., Espi, A., Marquez, I., Perez, V., Ferreras, M.C., Marin, J.F.G., Prieto, J.M., 2005. Pathological features in marine birds affected by the Prestige’s oil spill in the north of Spain. J. Wildlife Dis. 41, 371–378.
García L., Viada C., Moreno-Opo R., Carboneras C., Alcalde A. & González F. 2003. Impacto de la marea negra del "Prestige" sobre las aves marinas. SEO/BirdLife, Madrid.
Martínez-Abraín, A., Velando, A., Genovart, M., Gerique, C., Bartolomé, M.A., Villuendas, E., Sarzo, B., Oro, D., 2006. Sex-specific mortality of European shags during an oil spill: demographic implications for the recovery of colonies. Mar. Ecol. Progr. Ser. 318, 271–276.  
Velando, A., Munilla, I., Leyenda, P.M., 2005. Short-term indirect effects of the Prestige oil spill on a marine top predator: changes in prey availability for European shags. Mar. Ecol. Progr. Ser. 302, 263–274.
Votier S.C., B.J. Hatchwell, A. Beckerman, R.H. McCleery, F.M. Hunter, J. Pellatt, M. Trinder & T.R. Birkhead 2005. Oil pollution and climate have wide-scale impacts on seabird demographics. Ecol. Lett. 8: 1157-1164.
Alberto Velando. Universidade de Vigo


Última modificación: 26 de noviembre de 2007