NOTE Kerstetter et al.: Shark predation and scavenging on fishes equipped with satellite archival tags 



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The only other natural predators of large pelagic 

 fishes are various species of sharks. Several species of 

 lamnid sharks maintain elevated body temperatures, 

 including the shortfin mako {Isurus oxyrinchus) and the 

 white shark (Carcharodon carchariasl, both of which are 

 found in the area of Georges Bank (Cramer, 2000) and 

 the Central Pacific (Compagno, 1984). Several shortfin 

 makos were caught by the same longline vessel during 

 the week following each white marlin PSAT deployment 

 (WM1: n=4, 95-189 cm FL; WM2: n=3, 94-199 cm FL) 

 (Kerstetter, unpubl. data). The opah tag record closely 

 resembles the relatively constant temperature noted for 

 lamnid sharks, despite the independence of stomach 

 temperature with ambient water for these endothermic 

 sharks as reported by Carey et al. (1981). It is also 

 interesting to note that although precipitous tempera- 

 ture fluctuations were generally absent, a rapid drop in 

 temperature from 24° to 20°C was observed with tag 

 WM1 on 8 September at 32.3 m depth— a fluctuation 

 that could have resulted from another feeding event 

 that brought cool food matter into the stomach. Simi- 

 lar reductions in stomach temperatures due to feeding 

 have been noted for white sharks (McCosker, 1987). The 

 range of temperatures recorded by each of the two white 

 marlin tags appears rather broad for an endothermic 

 shark, however, and although the temperature at depth 

 was not measured, the delay in stomach temperature 

 closely resembles the pattern of blue shark internal 

 temperatures {Prionace glauca) measured in the Mid- 

 Atlantic (Carey and Scharold, 1990). 



The diving behavior recorded by the three tags also 

 corroborates ingestion of the tags by sharks. Carey et al. 

 (1982) reported that a tagged white shark off Long Is- 

 land, New York, made frequent dives to the bottom dur- 

 ing a 3.5-day acoustic tracking period. White sharks are 

 known to dive to depth while scavenging whale carcasses 

 (Dudley et al., 2000; Carey et al., 1982). A juvenile white 

 shark also tracked by Klimley et al. (2002) spent far 

 more extended times at depth than either white marlin 

 tag. Although the programming of the tag on the opah 

 precludes such fine-scale analyses of diving behavior, the 

 available data are not inconsistent with the mako tracks 

 in the study of Klimley et al. (2002). However, the short 

 duration dives with frequent returns to the surface seen 

 with the two white marlin tags most closely resemble 

 those of blue sharks (Carey and Scharold, 1990) and 

 were notably missing from the tracks of three shortfin 

 makos observed by Klimley et al. (2002). 



If sharks were indeed the scavenging animals, it 

 is likely that the tags were regurgitated, rather than 

 egested through the alimentary canal, whereupon the 

 PSAT floated to the surface and was able to transmit 

 the archived data. The narrow diameter of the spiral 

 valve in the elasmobranch gastrointestinal tract would 

 likely be too narrow to allow the undamaged passage 

 of an object the size of a PSAT, even for a large shark. 

 Although the available literature describing regurgita- 

 tion abilities of pelagic sharks is rather limited, Hazin 

 et al. (1994) reported that 35% of blue sharks brought 

 aboard for scientific study had everted and protruding 



stomachs. Economakis and Lobel (1998) also stated 

 their belief that regurgitation of ingested ultrasonic 

 tags was the primary cause of lost tracks for grey reef 

 sharks iCarcharhinus amblyrhynchos) on Johnston Atoll 

 in the central Pacific Ocean. 



Conclusions 



The temperatures and dive depths recorded by the opah 

 tag and both white marlin tags after apparent ingestion 

 share similarities, yet also contain sufficient information 

 to indicate the different identities of the ingesting organ- 

 isms. The dive depths in all cases ranged from the surface 

 to over 500 m, whereas the temperatures remained rela- 

 tively constant at several degrees above the background 

 SST, even during deep dive events. Temperature ranges 

 alone strongly indicate sharks rather than odontocete 

 whales were the ingesting organisms. However, limited 

 literature on the internal stomach temperatures of the 

 various pelagic sharks forces us to rely on telemetered 

 diving behavior data for further species identification, 

 which we used in the present study to suggest that blue 

 sharks ingested the two white marlin tags (on account 

 of the broad range of recorded temperatures) and that 

 an endothermic shark ingested the opah tag. 



It is not possible to account for all of the factors that 

 may result in the failure of satellite tags to transmit 

 data, but the results from these three PSATs indicated 

 that biological activities such as predation and scaveng- 

 ing may play an important role. We believe that the 

 most consistent explanation for the data transmitted 

 by these three tags is that they were ingested by large 

 sharks. One cannot calculate the probability that a 

 tag could be engulfed whole without physical damage 

 to the tag, survive for several days in the caustic en- 

 vironment of a digestive system, and be regurgitated 

 with sufficient battery power to transmit data to the 

 Argos satellites, but we suspect that the probability is 

 not very great. We expect that a far greater number of 

 tags may have had similar fates, that is to say, they 

 were damaged by predation or scavenging and digestion 

 processes or were regurgitated later in the transmis- 

 sion cycle, when the PSAT batteries had insufficient 

 remaining power for successful data transmission. The 

 failure of satellite tag to transmit data is frequently 

 considered to be the result of internal tag malfunction 

 or user error. However, these three data sets clearly 

 indicate that the failure of PSATs to function may also 

 be due to predation or scavenging events. 



Acknowledgments 



The authors would like to thank the Captain of the FV 

 Sea Pearl and Captain Greg O'Neill of the FV Carol Ann, 

 Don Hawn (University of Hawaii), who deployed the tag 

 on the opah, Evan Howell (PIFSC) for analyses of the 

 opah data, Andrij Horodysky (VIMS), who provided a 

 critical review of the manuscript. Melinda Braun (Wild- 



