European shag showed the clearest impact at the population level (Velando et al. 2005a). Cascading effects in the ecosystem after an oil spill include trophic cascades in which predators reduce abundance of their prey; this was documented for the European shag breeding at Illas Cíes (Velando et al. 2005b). Over the 10 years preceding the spill, the reproductive performance and population dynamics was monitored in this locality. After the Prestige oil spill the shag feeding grounds around Illas Cíes, which are mainly sandy bottoms, were continuously exposed to oil pollution as revealed by the high levels of pollutants in sediments, plankton, and other organisms. In the pre-spill years, shags showed low seasonal dietary variation, feeding mainly on sandeels. Nevertheless, after the spill, there was a dietary shift with a lower occurrence of sandeel due to lower sandeel availability at foraging areas as confirmed by sandeel fishery data. This dietary shift provoked a reproductive impairment, thus reproductive success after the spill was reduced by half compared to pre-spill data.

  Recent data indicates that the recovery of European Shag population is still constrained by this reduction on the availability of a highly preferred forage-fish. Accordingly, in 2007, the shag population breeding at Illas Cíes did not show signs of recover and was 70% lower than pre-spill counts (Alvarez & Velando 2007).