The application of complementary satellite tags provided insight into vertical and horizontal habitat use for great hammerheads that would not have been evident using a single tag type. Critically, one of the three tagged sharks may have been presumed dead if we had used SPOT tags alone. As such, despite their demonstrated utility on other shark species that frequent the surface, fin-mounted SPOT tags may not be the best option for tracking great hammerheads. Despite a low sample size (n = 3), our findings are noteworthy as they characterize movements from immature animals tagged in an area where we know comparatively little about the movements of this species.
Data from the sPAT tags showed all three great hammerheads made daily vertical movements from the surface to depth. Given the consistent use of surface waters and the exaggerated height of the dorsal fin to which the SPOT tag was attached, why were so few SPOT positions reported? To begin with, it is possible that the SPOT tags simply malfunctioned; for example, no SPOT transmissions were ever received for GH 1. However, the SPOT tag on GH 2 transmitted to the ARGOS system, but not enough to generate a single position estimate, which requires at least two consecutive transmissions. This suggests that great hammerheads may not spend sufficient time (i.e., at least 90 s) on the surface to allow a fin-mounted SPOT tag to communicate with the ARGOS system long enough to estimate positions. Given the previously successful application of towed SPOT tags on this species [10], towed packages may present the best option compared to fin-mounted SPOT transmitters.
An alternative explanation for why our fin-mounted SPOT tags failed to generate position estimates for sharks at the surface invokes a swimming behavior recently described for this species. Using a combination of accelerometer loggers, animal-borne video, and observations of aquarium-held sharks, Payne et al. [16] document repeated and prolonged swimming by great hammerheads at rolled angles between 50 and 75°. Using a hydrodynamic model and wind tunnel simulations, Payne et al. [16] estimate a reduction in drag forces that equates to an energetic savings of ~ 10%. Hence, while all great hammerheads tagged in this study frequented the surface, perhaps they were oriented such that the SPOT tag was still below the surface and thus unable to transmit. Alternatively, biofouling could have precluded the saltwater switch on the SPOT tag from functioning. Recent recoveries of fin-mounted SPOT tags from a scalloped hammerhead (Sphryna lewini) and tiger shark (Galeocerdo cuvier) in the northern Gulf of Mexico at liberty for less than 6 months revealed extensive biofouling which had prevented the tag from transmitting any location estimates (Drymon and Wells unpublished data). Previous studies demonstrate that SPOT tags are highly effective for tiger sharks, with reporting rates as high as 100% [7], but it may be that the highly productive waters of the northern Gulf of Mexico promote rapid biofouling compared to more oligotrophic waters like south Florida and the Caribbean. Regardless of the mechanism, fin-mounted SPOT tags appear to be a poor choice for examining short-term horizontal movements or post-release survivorship for great hammerheads in the northern Gulf of Mexico, despite their frequent use of surface waters.
Our findings add to a body of knowledge highlighting shallow water habitat use by great hammerheads. Over a thirty-day period, GH 3 used waters 6 m or less during 93% of the time at liberty. This finding is similar to the results presented by Hueter and Manire [17], who noted that great hammerheads along the southwest coast of Florida were only seen at depths shallower than ~ 6 m. In the US bottom longline fishery, great hammerheads are most commonly captured in waters less than 20 m [6]. Roemer et al. [18] described six instances of extreme shallow water habitat use by great hammerheads, five of which involved prey capture. Great hammerheads are known to frequent the shallow waters off south Florida in pursuit of tarpon (Megalops atlanticus) [19, 20]. Given the abundance of rays, a preferred prey item for this species [21, 22] in the shallow waters of the northern Gulf of Mexico, consistent use of shallow waters by great hammerheads in this region, may represent foraging behavior, although investigations into the feeding ecology of this species would be required to confirm this.
Generally, larger sizes afford increased vagility; as a consequence, many marine animals have larger home ranges as adults [23]. Great hammerheads are one of the largest-bodied species of predatory sharks [20] and can travel long distances over short periods of time [24], including movements exceeding 1500 km [9]. Acknowledging our relatively short tracking periods, two of the three great hammerheads tagged in this study showed movement less than 35 km between tagging and pop-off locations. Counterintuitively, the smallest shark tagged (GH 2) moved nearly 400 km between tagging and pop-off locations; following what appeared to be clear eastward movement from Alabama into Florida, both fin-mounted SPOT and sPAT tags confirmed GH 2 west of the Mississippi River. Individual variation in habitat use has been demonstrated for other large predatory sharks, including tiger [25] and bull shark (Carcharhinus leucas, [26]). Thus, the individual variability in movement patterns illustrated in the current study appears to be common. Such high variability suggests that effective great hammerhead conservation and management plans will require movement data from individuals of all sizes and throughout the extent of their range. Interestingly, the longest horizontal movement in this study was demonstrated by a 187 cm TL female, likely the smallest satellite-tagged great hammerhead to date.
