Major
Marine Biology
Second Major
Spanish
Advisor
Wetherbee, Brad, M
Advisor Department
Biological Sciences
Date
5-2023
Keywords
Vertical Movements; Horizontal Movements; Movement Ecology; Shortfin mako shark; Shark; Three-Dimensional Habitat Use
Abstract
Highly mobile pelagic predators, such as tuna, swordfish, and sharks, often undergo changes in depth over a 24-h period and exhibit diel vertical migration (DVM), presumably to maximize foraging efficiency. Studies on the diving behavior of shortfin mako sharks (Isurus oxyrinchus) have been inconclusive as to whether this species consistently exhibits DVM. We investigated relationships between horizontal and vertical movements in shortfin mako sharks by using location and depth data collected from SPLASH tags and pop-up satellite archival tags (PSAT). A state-space model was applied to location data derived from SPLASH tags to generate continuous tracks and to quantify horizontal behavior in terms of "movement persistence", which classifies behavior on a scale of “resident” to “transient”, based on speed, angle, and distance between locations. Sharks engaging in resident behavior occupied shallower depths compared to transient behavior. Average depth and range of depths occupied by sharks were at least three times greater during transient compared with during resident behavior. Depth records from sharks tagged with PSATs indicated that longer-term use of the water column was similar to results obtained from SPLASH tags. Our results suggest that shortfin makos restrict their movements both vertically and horizontally, possibly to concentrate their activity within prey-rich waters. Restricted movements in the upper water column may be strongly associated with prey availability, explaining the lack of consistent DVM behavior exhibited by this species.