Vertical Movement Rate Estimates for Atlantic Istiophorid Billfishes Derived from High-Resolution Pop-Up Satellite Archival Data

Development of pop-up satellite archival tag (PSAT) technology has greatly increased our knowledge of billfishes (Arnold and Dewar, 2001). The acquisition and subsequent fisheries-independent transmission of PSAT data via the Argos system has revealed behavioral traits of these comparatively rare species, whose survival depends on the ability to range over large geographical areas (Goodyear et al., 2008). While PSATs are capable of monitoring light level, depth, and temperature at relatively high resolution (i.e., ≤ 60 s intervals), battery power and data volume restrictions can limit data transmissions to summary information only. Occasionally, PSATs are recaptured with fishing gears or recovered after washing ashore (e.g., Gunn et al., 2003; Hoolihan and Luo, 2007). In these instances, the complete archived high-resolution data are accessible from the PSAT memory. Analyzing these data for change in depth (over time) provides an opportunity to derive coarse estimates of vertical movement rates, in lieu of a dedicated speed sensor. Knowing these swimming rates provides the potential to better understand the vertical and thermal habitat use, and bioenergetic optimization of billfishes. Similar swimming capability may also be implied to horizontal movements, helping to elucidate home range and migration patterns. In addition, swimming speed information is a necessary component for building foraging models.