The ocean has depth: two- versus three-dimensional space use estimators in a demersal reef fish

Most aquatic animals employ 3-dimensional (3D) movements to fully exploit the resources of the environment they inhabit. Many of these animals, however, are impossible to observe directly, making it necessary to use indirect methods of observation such as biotelemetry in order to study them. Despite technological advances with tracking equipment enabling movement to be assessed in 3 dimensions, many studies restrict their analyses to traditional 2-dimensional (2D) space use. We compared 2D and 3D (1) core and home range size, (2) home range overlap and (3) changes in space use in relation to biological and environmental variables of a large, demersal reef-dwelling fish species, the eastern blue groper Achoerodus viridis, tracked using passive acoustic telemetry. Mixed effects models were used to determine differences between the core/home range sizes and home range overlap between the sex of the fish and the breeding and non-breeding seasons. The 2D analyses were unable to detect differences in core and home range sizes between the sexes that were successfully identified by the 3D analyses. 2D analyses only detected differences in home range overlap between the breeding and non-breeding seasons, whereas the 3D analyses found seasonal differences according to the sex of the fish. Two-hourly 2D space use estimates failed to detect differences in space use between fished and protected areas that were detected in the 3D analyses. This study demonstrates that to truly understand how animals use the space they inhabit, we must assess their movement in the full spatial context of their environment.