Following the fish to find the edge: Delineating habitat boundaries using high resolution acoustic tracking data

Background/Question/Methods

Landscapes are characterized by a mosaic of multiple habitats resulting in numerous boundaries or “edges”.  These transitional environments are often ecologically distinct from the interior of the habitats resulting in vital rate changes across distances from an “edge”. Understanding how animals identify and utilize habitat edges is critical to understanding landscape-level dynamics.   However, defining edges is challenging and sometimes vague, especially for large mobile organisms that regularly cross multiple boundaries within a given landscape.  For these species, arbitrary definitions of “edge” are less useful than edge-interior delineation based on highly resolved spatiotemporal data on movement patterns of individuals along these ecotones.  We quantified the positions of 34 acoustically tagged red drum (Sciaenops ocellatus) within a mapped seascape comprised of sand, seagrass, marsh, and oyster habitats. Using 144,000 detections, we quantified position density (detections m^-2) in relation to the distance from the edge of seagrass/sand-flat interfaces as well as oyster reef boundaries.

Results/Conclusions

Along the seagrass/sand-flat boundary, position densities (detections m^-2) within the seagrass habitat increased up to 2 meters from the edge and remained relatively constant from this point up to 10 meters.  However, within the sand-flat, the density of detections gradually decreased from the edge up to 10 meters.  Conversely, within oyster reefs, position density was highest between the edge and 1.5 meters onto the reef followed by a decrease in density up to 4m onto the reef.  Interestingly though, as distances increased away from the reef, position density remained constant. The observed patterns suggest that within this estuarine seascape, red drum identify and utilize edges differently not only between but also within habitats. Identifying how an organism’s perception of edge varies provides ecologists with a better understanding of the landscape dynamics (i.e. foraging efficiency and predator prey interactions).  Additionally, changing environmental characteristics may impact animals in various ways depending on how they perceive individual habitats.  Degradation or fragmentation of oyster reefs may be less dramatic on red drum due to their preference of the edges, while loss of seagrass may be more impactful due to their preference of internal habitats.