European shag is the second most numerous seabird breeding in Galicia. The breeding population is quite dispersed although Illas Cíes held the most important colony, approximately 42% of the Iberian population and 2% of the world population. During the last decade the Iberian shag population showed an overall decline at an annual rate of 5%, attributed to elevated adult mortality in gill-nets, and was listed as endangered in the Red Book of the Birds of Spain. This species is an inshore and coastal feeder, feeding predominantly on pelagic and benthic fish, and resident throughout the year, so with great opportunities for oil exposure as long as the oil persists in their foraging areas, thus shags are good models to study the effect of the Prestige oil spill in the ecosystem.

Using this design, it was possible to observe that, colonies located within the path of the oil suffered greater declines (ca. 10%) compared with pre-spill trends and with population trends at unoiled colonies (Velando et al. 2005b), indicating that the Prestige oil spill affected European shag at the population level. Although a complex effect of the direct mortality on population was detected (Martinez et al. 2006), factors affecting the reproductive success were also evident. The breeding success was 50% lower in oiled colonies compared with unoiled colonies but the annual reproductive success did not differ among colonies before the impact (Velando et al. 2005a,b).
  The effect at population level documented in the European shags breeding at colonies within the oil path highlights that the effect on marine organisms are complex and long-lasting. Indeed, as a result, the future recovery of endangered shags could be jeopardized owing to continuous exposure to oil through food webs and reduced food availability as documented in this and other seabirds. There is a risk that the impacts of oil pollution on marine organisms may be underestimated by overlooking the effects of chronic exposures, delayed impacts or indirect effects driven by ecological processes.