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