Comparing the horizontal and vertical approaches used to identify foraging areas of two diving marine predators

International audienceAccurately locating the foraging areas of diving marine predators is central to understanding their ecology and implementingconservation and management regulations. This study compares horizontal and vertical approaches of identifying seal foragingareas. We analysed GPS locations and dive data obtained from GPS/GSM tags fitted on eight grey seals (Halichoerusgrypus) and nine harbour seals (Phoca vitulina). In the horizontal dimension, we used the First Passage Time to identifyArea-Restricted Search (ARS) behaviour. In the vertical dimension, we used two dive criteria indicating benthic foragingbehaviour: dive shape and vertical descent speed. The two approaches were spatially compared using the Index of Differencesin Spatial Pattern. We found that the two approaches highlight similar hotspots when pooling all individuals of thesame species. However, the degree of overlap varied considerably at the individual level. Some individuals performed mostof their likely foraging dives (vertical dimension) in areas where they also displayed ARS behaviour (horizontal dimension),while others performed these dives both in and outside ARS zones. We suggest that comparing foraging areas detected fromhorizontal and vertical approaches (1) can strengthen the confidence in the efficiency of approaches to accurately spatializethe actual foraging effort of a diving predator at the scale of a colony (sampled with several individuals); and (2) providesmore comprehensive insights into potential interindividual differences in foraging strategies as some divergent individualstrategies may not be detected using only horizontal movements