MEPS 489:235-244 (2013)  -  DOI: https://doi.org/10.3354/meps10440

ABSTRACT: While many seabird species in the North Atlantic have declined over the last decades, the Norwegian population of the European shag Phalacrocorax aristotelis has increased. In the present study, we assessed the impact of food availability and climate on the shag population by analysing 25 years of data (1985 to 2009) on breeding numbers in 3 large colonies: 2 in the Norwegian Sea (65°N and 67°N) and 1 in the Barents Sea (70°N). Predictor variables were ICES abundance estimates of young saithe Pollacius virens, the most important forage fish for shags in the Norwegian Sea, and for the Barents Sea colony also total stock size estimates of Barents Sea capelin Mallotus villosus. As proxies for climate variation, we used the North Atlantic Oscilliation index (NAO) for the last and the preceding winter (lagged by 1 yr). Finally, the annual population size of the study colonies in the preceding year was included in the models to control for potential density-dependent effects. The predictor variables explained 46 to 67% of the variation in annual growth rate in the colonies. In the Barents Sea colony, the shag population growth rate was only associated with capelin abundance, whereas for the Norwegian Sea colonies, there were strong positive relationships with 1 yr old saithe and a negative effect of the lagged NAO winter index. The latter effect may be a result of unfavourable weather conditions with high winds and precipitation in winter increasing mortality among non-breeding age classes of shag. Our study is the first to demonstrate a close correlation between stock estimates of the primary forage fish for European shags and shag breeding numbers. This suggests that the population growth rate and diet of shags may be used as cost-efficient and reliable indicators of major shifts in saithe stock recruitment.

KEY WORDS: Phalacrocorax aristotelis · Norwegian Sea · Barents Sea · NAO · Saithe · Breeding numbers