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

Accurately locating the foraging areas of diving marine predators is central to understanding their ecology and implementing conservation and management regulations. This study compares horizontal and vertical approaches of identifying seal foraging areas. We analysed GPS locations and dive data obtained from GPS/GSM tags fitted on eight grey seals (Halichoerus grypus) and nine harbour seals (Phoca vitulina). In the horizontal dimension, we used the First Passage Time to identify Area-Restricted Search (ARS) behaviour. In the vertical dimension, we used two dive criteria indicating benthic foraging behaviour: dive shape and vertical descent speed. The two approaches were spatially compared using the Index of Differences in Spatial Pattern. We found that the two approaches highlight similar hotspots when pooling all individuals of the same species. However, the degree of overlap varied considerably at the individual level. Some individuals performed most of 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 from horizontal and vertical approaches (1) can strengthen the confidence in the efficiency of approaches to accurately spatialize the actual foraging effort of a diving predator at the scale of a colony (sampled with several individuals); and (2) provides more comprehensive insights into potential interindividual differences in foraging strategies as some divergent individual strategies may not be detected using only horizontal movements.