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

A. Martínez-Abraín*, P. Santidrián Tomillo, J. Mouriño, S. Tenan, D. Oro

*Corresponding author:

ABSTRACT: We analysed the long-term (1975-2017) population response of a colony of a marine top predator, the black-legged kittiwake (Rissa trydactyla), to the population dynamics of sardine (Sardina pilchardus), its main local prey. The study site (Sisargas Islands) is located at the southernmost edge of the geographical distribution of the predatory species. Kittiwake counts of breeding pairs started with the discovery of the colony (1975), most 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 Seas (ICES) database. Both gull counts and fish landings were statistically analysed in search of regime shifts, by means of sequential t-test analysis. We found 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, two years after the prey shift. Multiple autocorrelation functions (ACF) were explored for the detrended time series of sardines and kittiwakes and results indicated an autocorrelation with a time lag of 2 years. Sardines have not recovered their densities ever since the collapse despite much reduced fishing effort, likely due to changes in large-scale atmospheric circulation in the Northern Hemisphere in the late 90’s. Kittiwakes at Sisargas have not recovered demographically either, remaining nearly extinct during the last ca. 20 years. Although we lack detailed demographic data for the study 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 finally discuss some behavioural social responses that may have occurred during and after the collapse to explain the dynamics of the study colony.