Acoustic monitoring of sixgill shark movements in Puget Sound: evidence for localized movement

Understanding the movements of species, particularly those that may exert strong influence on community structure or that may be susceptible to human perturbations, is critical to effectively conserve and manage populations. However, the study of movement behavior in marine fishes has been historically difficult and typically produces a limited amount of data (i.e., start and end points). We use an array of automated acoustic receivers to monitor autumn and winter movement patterns of sixgill sharks (Hexanchus griseus (Bonnaterre, 1788)) in Puget Sound, Washington, USA. Daily movement of sharks and maximum distance moved from tagging sites varied with size, with larger sharks having shorter daily movements and maximum distances from tagging locations than smaller sharks. Sharks were detected at the same site as the previous day 76% of the time. Movement away from the shark's tagging location increased slightly over the duration of the study, but most sharks occupied the same sites for most of the study. These relatively small and stable movement patterns could lead to localized, top-down impacts from sixgill sharks and suggest that local human perturbations, such as fishing or pollution, have the potential to negatively affect local populations of sixgill sharks.