Integrating vertical and horizontal movements of shortfin mako sharks Isurus oxyrinchus in the eastern North Pacific Ocean

Abstract

Resources in the pelagic environment tend to be patchily distributed, often resulting in animals engaging in adaptive behaviors to maximize foraging success. While these behaviors have been examined in horizontal and vertical dimensions separately, there has been limited work integrating these functionally related movements in sharks. We investigated how vertical behaviors change in relation to horizontal movements in the pelagic, highly migratory, shortfin mako shark Isurus oxyrinchus. Data from 30 sharks (114 to 245 cm total length), double-tagged with Pop-up Archival and Transmitting (PAT) and Smart Position or Temperature Transmitting (SPOT) tags within the Southern California Bight were analyzed. We examined shark daytime depth distributions after their horizontal movements were first classified by water column thermal structure (thermal habitat), and into 1 of 2 behavioral modes (area-restricted search or transient) using a switching state-space model. Despite high inter- and intra-individual variability, thermal habitat and behavioral mode influenced depth distribution. Within thermal habitats, sharks spent similar amounts of time near the surface in both behavioral modes, although transient animals spent more time in deeper waters within some thermal habitats. Comparing among thermal habitats, sharks performing transient movements in warmer waters spent more time at depth. Sharks experienced an expansion of vertical habitat use when they switched to transient behaviors, possibly to search for prey, and the degree of habitat expansion may be influenced by temperature. These results suggest that in a 3-dimensional habitat, such as the pelagic environment, prey searching behaviors in the horizontal and vertical dimensions are linked.