Delayed predator-prey collapses: the case of black-legged kittiwakes and Iberian sardines

We analysed the long-term (1975-2017) population response of a colony of a marine top predator, the black-legged kittiwake Rissa tridactyla, to the population dynamics of sardine Sardina pilchardus, its main local prey. The study site (Sisargas Islands, Spain) is located at the southernmost edge of the geographical distribution of the predator. Kittiwake counts of breeding pairs started with the discovery of the colony (1975), likely close to the actual year of first colonization. Sardine landings by age class (1978-2016) were taken from the International Council for the Exploration of the Sea (ICES) database. Sequential t-test analysis revealed that a regime shift of the oldest sardine age class (age 6+) took place in 1991 and that kittiwakes experienced a regime shift in the number of breeding pairs in 1993, 2 yr after the prey shift. Multiple autocorrelation functions for the detrended time series of sardines and kittiwakes indicated an autocorrelation with a time lag of 2 yr. Despite much reduced fishing effort, sardine densities have not recovered since the collapse, likely due to changes in large-scale atmospheric circulation in the Northern Hemisphere in the late 1990s. Kittiwakes at Sisargas have not recovered demographically, remaining nearly extinct during the last ca. 20 yr. Although we lack detailed demographic data for the studied kittiwake population, we suggest that massive breeding failure and subsequent dispersal to higher-quality patches might explain the rapid non-linear collapse in breeding population density. We discuss some behavioural social responses that may have occurred during and after the collapse to explain the dynamics of the study colony.