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.

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)