Among-year and within-population variation in foraging distribution of European shags Phalacrocorax aristotelis over two decades: implications for marine spatial planning

Marine spatial planning aims to deliver sustainable use of marine resources by minimizing environmental
impacts of human activities and designating Marine Protected Areas. This poses a challenge where
species’ distributions show spatio-temporal heterogeneity. However, due to logistic constraints and challenging
timescales many studies of distribution are undertaken over few years or on a restricted subset of
the population. Long-term studies can help identify the degree of uncertainty in those less comprehensive
in space and time. We quantify inter-annual and sub-colony variation in the summer foraging distribution
of a population of European shags Phalacrocorax aristotelis, using a tracking data set
comprising 320 individuals and 1106 foraging trips in 15 years from 1987 to 2010. Foraging distribution
over the study period was concentrated in three areas. Data from one and two years captured an average
of 54% and 64% of this distribution, respectively, but it required 8 years’ data to capture over 90% of the
distribution. Foraging range increased with population size when breeding success was low, suggesting
interplay between extrinsic and intrinsic effects. Furthermore, females had foraging ranges on average
36% greater than males. Finally, sub-colony segregation occurred in foraging areas up to 4 km from the
colony and in the most distant locations (>10 km), whilst there was considerable overlap at intermediate
distances (6–10 km). Our study highlights important considerations for marine spatial planning in particular,
and species conservation in general, notably the proportion of the population distribution identified,
the prevailing conditions experienced and the need for balanced sampling across the population.