Population modelling of European shags (Phalacrocorax aristotelis) at their southern limit: conservation implications

Velando, A. and Freire, J., 2002. Population modelling of European shags (Phalacrocorax aristotelis) at their southern limit: conservation implications. Biological Conservation, 107(1), pp.59-69.


The European shag (Phalacrocorax aristotelis) population at Cíes Islands (the most important breeding area in its southern limit) increased rapidly from 1986 to 1992, and afterwards the population suffered a slight decline. This study analyzed population data obtained from ringing recoveries and reproductive monitoring between 1993 and 1997. The reproductive success was highly variable and associated with adverse weather events. Adult survival rate was very low compared with other colonies, probably due to high accidental capture in gill-nets. In recent years, the fishing effort with gill-nets increased in the study area. Sensitivity analysis of parameters showed that the population is more affected by changes in adult survival than in reproductive success. When dynamic simulations were run with an increase in shag mortality of 5% above the present level, population extinction occurred in all simulations. In contrast, when a reduction of mortality of 5% was introduced in the simulations, the population increased in all cases. The main lines of action to study and protect this population should be: (1) ringing schemes to obtain better estimates of survival variability; (2) studies on the interaction of feeding areas and fishing vessels; (3) regulations on gill-netting; and (4) the incorporation of population models as an adaptive management tool to synthesize assessment work and management scenarios.

Population trends and reproductive success of the European shag Phalacrocorax aristotelis on the Iberian Peninsula following the Prestige oil spill.

Velando, A., Alvarez, D., Mourino, J., Arcos, F. & Barros, A. Population trends and reproductive success of the European shag Phalacrocorax aristotelis on the Iberian Peninsula following the Prestige oil spill. Journal of Ornithology 146, 116–120 (2005).


In 2003, immediately following the Prestige oil spill in Galicia, Spain, we studied the population trends and reproductive performance of European shags (Phalacrocorax aristotelis) at oiled and unoiled colonies. This bird is an important member of the nearshore marine community, breeding in the area affected by the Prestige oil spill. The European shag feeds around the breeding colonies throughout the year, making it a useful indicator of environmental change. Before the oil spill, population trends were similar between oiled and unoiled colonies. Nevertheless, 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. In 2003, the breeding success was 50% lower in oiled colonies compared with unoiled colonies. The data available from pre-spill years suggest that the annual reproductive success did not differ among colonies before the impact. European shags breeding at colonies affected by oil showed a negative initial impact from the Prestige oil spill. The reduction in reproductive success at oiled colonies may be due to sublethal effects of oil exposure or low food availability after the oil spill.

Long-term reproductive impairment in a seabird after the Prestige oil spill

Barros, Á., Álvarez, D. & Velando, A. Long-term reproductive impairment in a seabird after the Prestige oil spill. Biology letters 10, 20131041 (2014).


Large oil spills are dramatic perturbations on marine ecosystems, and seabirds are one of the worst affected organisms in such events. It has been argued that oil spills may have important long-term consequences on marine organisms, but supporting evidence remains scarce. The European shag (Phalacrocorax aristotelis) was strongly impacted at population level by the Prestige oil spill, the biggest spillage in the eastern North Atlantic. In this paper, we report on the long-term consequences on reproduction of this coastal seabird, using temporal and spatial replicated data (before–after–control–impact design). Our study revealed long-term reproductive impairment during at least the first 10 years since the Prestige oil spill. Annual reproductive success did not differ before the impact, but after the impact it was reduced by 45% in oiled colonies compared with unoiled ones. This is a rare documentation of long-term effects after a major oil spill, highlighting the need for long-term monitoring in order to assess the real impact of this type of disturbance on marine organisms.

Climate Influences Fledgling Sex Ratio and Sex-Specific Dispersal in a Seabird

Barros, Á., Álvarez, D. & Velando, A. Climate Influences Fledgling Sex Ratio and Sex-Specific Dispersal in a Seabird. PLoS ONE 8, e71358 (2013).8): e71358. doi:10.1371/journal.pone.0071358

Climate influences the dynamics of natural populations by direct effects over habitat quality but also modulating the phenotypic responses of organisms’ life-history traits. These responses may be different in males and females, particularly in dimorphic species, due to sex-specific requirements or constraints. Here, in a coastal seabird, the European shag (Phalacrocorax aristotelis), we studied the influence of climate (North Atlantic Oscillation, NAO; Sea Surface Temperature, SST) on two sex-related population parameters: fledgling sex ratio and sex-specific dispersal. We found that fledgling sex ratio was female skewed in NAO-positive years and male skewed in NAO-negative years. Accordingly, females dispersed a longer distance in NAO-positive years when females were overproduced, and on the contrary, males dispersed more in NAO-negative years. Overall, our findings provide rare evidence on vertebrates with genetic sex determination that climate conditions may govern population dynamics by affecting sex-specific density and dispersal.