Short-term indirect effects of the ‘Prestige’ oil spill on European shags: changes in availability of prey

Abstract

In 2003, immediately following the 'Prestige' oil spill in Galicia, Spain, we studied the reproductive performance of European shags Phalacrocorax aristotelis at Illas Cies, the main breeding ground for the species in Spain. Over the 10 yr preceding the spill, we monitored the breeding and the dynamics of the Illas Cies population. We performed 1000 simulations of the population dynamics using the population parameters and their variability for the pre-spill period. The number of breeding pairs counted in 2003 was lower than any of the values predicted by the simulation models, suggesting that population parameters changed after the 'Prestige' wreck. Environmental conditions cannot explain the observed effects because weather conditions were far from severe in 2003. The analysis of shag casualities showed that despite the extensive oiling of Illas Cies and nearby waters it is unlikely that shags were killed in large numbers. Nevertheless, the shag feeding grounds around Illas Cies, which are mainly sandy bottoms, were continuously exposed to oil pollution throughout the shag breeding season 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, in 2003, there was a dietary shift with a lower occurrence of sandeel that, together with sandeel fishery data, indicate lower sandeel availability at foraging areas. In addition, reproductive performance in 2003 was significantly lower and chick condition was poorer compared to the pre-spill years. When all this information is taken into account, the picture that emerges strongly suggests that the European Shag population in Illas Cies is suffering a negative impact of an indirect nature mediated through a reduction on the availability of a highly preferred forage-fish.