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

Guy Harvey Oceanographic Center Facility

The pelagic environment represents a three-dimensional landscape; however, shark movement studies often focus on only two dimensions (i.e., horizontal movements). Combining data sets on horizontal movements with vertical movements provides a much more detailed view of movement behaviors and habitat use. The shortfin mako shark (Isurus oxyrinchus) is a wide-ranging species found in temperate and tropical waters worldwide. Although horizontal and vertical movements of this species have been examined at various scales, there has been little work uniting these movements. To investigate the relationship between horizontal and vertical movements of juvenile mako sharks in the eastern North Pacific Ocean, 35 mako sharks (114 – 245 cm FL) were double-tagged between 2003 and 2008 with Pop-up Archival and Transmitting (PAT) and Smart Position or Temperature Transmitting (SPOT) tags within the Southern California Bight. We examined the daytime depth distributions of these individuals after their horizontal movements were first classified by thermal habitat based on water column thermal structure, and into one of two behavioral modes (area-restricted search or transiting) using a switching state-space model. Despite high inter- and intra-individual variability, thermal habitat and behavioral mode influenced depth distribution. With warming thermal habitats, transiting sharks increased the proportion of their time below 50 m. Differences in the proportion of time spent in waters greater than 100 m between the two behavioral modes were also most pronounced in warmer thermal habitats. Further, maximum depths increased when sharks were engaged in transiting behavior, except for in the coldest habitat. These results suggest an expansion of vertical habitat use when sharks switch to transiting behaviors, which may increase the probability of locating prey resources, and that the degree of habitat expansion may be influenced by temperature.