Three-dimensional kernel utilization distributions improve estimates of space use in aquatic animals

Tracking data have previously been used to define animal movement patterns through two-dimensional (2D) kernel utilization distributions and separate analysis of vertical locations. Here we describe the use of three-dimensional (3D) kernel utilization distributions to estimate the volumetric space use of individuals based on tracking data and to estimate the overlap in activity space between individuals. Data from European eels (Anguilla anguilla) from Norwegian coastal waters were used to compare the information conveyed by 2D and 3D activity space estimates and the utility of this approach for aquatic species. The use of 3D kernels produced detailed representations of space use in A. anguilla that permitted examination of depth use in a geographic context. Comparison of 2D and 3D home ranges showed that 2D analyses overestimated the amount of overlap between individuals by 13%-20%, because individuals sometimes occurred in the same location but used different depths. Hence, the 3D approach provided more comprehensive representations of animal movement in three dimensions while producing a metric that can be used for testing hypotheses relating to scientific descriptions of activity space, habitat use, and movement parameters.