Spatial dynamics of satellite-tracked blue sharks, Prionace glauca, and the role of oceanographic fronts as resource hotspots

The Atlantic Ocean is subject to high levels of longline fishing with blue sharks, Prionace glauca, being the most commonly caught species. Despite high levels of fishing pressures, the spatial dynamics and behaviour of blue sharks are poorly understood. Recording the individual movement patterns of fish using electronic tags is however becoming a central method for understanding how individual animals respond behaviourally to changes in environment and what habitat preferences may exist. Hence, the robust identification of how large marine predators use the ocean is essential for management and conservation. Near real-time satellite linked tags were fitted to 12 blue sharks in the North Atlantic Ocean between 2006 and 2011. A total of 14,543km of tracking data was obtained over a combined period of 476 days. Using novel remote sensing techniques to detect oceanic features objectively, shark movements were compared with the locations of thermal fronts. Front-affiliated swimming movements were observed for several sharks, but the use of bathymetric clues may also be important in determining movement patterns. Frequent seasonal fronts associated with southern Portugal coincide strongly with the space use of juvenile sharks and may be influential regarding nursery ground habitat preferences. In addition to a whole array of potential clues, movement patterns are directed by finer scale thermal gradients that are not detectable using the current model. Despite the apparent complex relationships between blue shark-movements and environment, areas with distinct bathymetric and frontal signatures could be regions in which to concentrate conservation efforts