SCIAKROTTA and NELSON: DIKL BEHAVIOR OK BLUE SHARK 



gian waters from depths as great as 370 m. Davies 

 and Bradley (1972) observed individuals at depths 

 between 100 and 275 m during a descent in the 

 submersible Deepstar 4000. A large school of 

 northern anchovy was also observed in this depth 

 range and a predator-prey relationship was sug- 

 gested, although the possibility of the sharks 

 following the descending submersible could not be 

 eliminated. 



In regard to temperature, Strasburg (1958) re- 

 corded 99^ of his catches over the range of 7° to 

 20°C, with 67% between 10° and 15°C. Thus, 

 temperature alone may not be reason for the ap- 

 parent absence of blue sharks from the offshore 

 study area during January and February 1972 

 when the surface temperature was about 13 °C. 



The navigational mechanism employed by the 

 sharks during their island-oriented migration is 

 unknown. Traditional explanations for such fish 

 movements include sun-compass orientation, vi- 

 sual landmark recognition, and orientations to 

 chemical or thermal gradients. None of these 

 mechanisms seem plausible in view of the con- 

 stancy of the pelagic environment, depths usually 

 occupied during the movement, and the relative 

 darkness in which the movements often occurred. 

 Orientation to magnetic or electric fields is one 

 possibility that must be considered in view of the 

 recent findings of Kalmijn (1971, 1973) dem- 

 onstrating magnetic/electric responses in sharks 

 of adequate sensitivity for such a mechanism. 

 Another possibility is orientation by passive 

 acoustic means to the sounds of the island 

 shoreline, in a manner similar to that suggested 

 by Evans (1971) for dolphins. 



The diel inshore-offshore migration shown by 

 this study must also be considered in view of the 

 much longer range movements exhibited by blue 

 sharks. Individuals off California are known to 

 segregate by sex, and seasonal changes in sex 

 ratios imply seasonal north-south migration, 

 perhaps in response to water temperature (John- 

 son 1974; Bane 1968; Tricas 1977). Tagged indi- 

 viduals have exhibited some very long-range 

 movements. One blue shark tagged by Bane off 

 Newport Beach, Calif, in July 1967 was recovered 

 in December of the same year about 1 ,300 km west 

 of Nicaragua. Another tagged by D. R. Nelson 

 (unpubl. data) off San Diego, Calif, in October 

 1966 was recovered in October 1969 about 1,800 

 km west of the Galapagos Islands, a distance of 

 4,000 km from its tagging site. This shark was 

 captured only 8 days short of a full 3 yr at liberty 



and, therefore, did not appear to be participating 

 in any seasonal north-south migration. Both of 

 the above sharks were recovered by Japanese 

 fishing vessels, presumably longlining in rela- 

 tively deep, cool water. 



ACKNOWLEDGMENTS 



We sincerely thank the many persons who con- 

 tributed to this study, especially E. Standora (ini- 

 tial development and testing of telemetry system), 

 H. Carter and D. Ferrel (circuit design), and J. 

 Hall (assistance during trackings at sea). We also 

 acknowledge the Office of Naval Research for 

 financial support, through contract N00014-68- 

 C-0318, under project NR-104-062, for the pro- 

 gram of shark research of which this study is a 

 part. 



LITERATURE CITED 



aasen, o. 



1966. Blahaien, Prionace glauca (Linnaeus), 1758. Fis- 

 ken Havet 1966(1): 1-15. 



Bane, G. W. 



1968. The great blue shark. Calif. Curr. 1:3-4. 

 BIGELOW, H. B., AND W. C. SCHROEDER 



1948. Sharks. In J.Tee-Van.C.Breder.S. F. Hildebrand, 

 A. E. Parr, and W. C. Schroeder (editors), Fishes of the 

 western North Atlantic, Part one, p. 59-546. Mem. Sears 

 Found. Mar. Res., Yale Univ. 1. 

 CAREY, F. G., AND K. D. LAWSON. 



1973. Temperature regulation in free-swimming bluefin 

 tuna. Comp. Biochem. Physiol. 44A:375-392. 



CASEY, J. G. 



1976. Migrations and abundance of sharks along the At- 

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CLARKE, M. R., AND J. D. STEVENS. 



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COUSTEAU, J. Y., AND P. COUSTEAU. 



1970. The shark: splendid savage of the sea. Doubleday 

 and Co., Garden City, N. Y., 277 p. 



DAVIES, I. E., AND R. P. BRADLEY. 



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1971. Orientation behavior of delphinids: Radio telemetric 

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Ferrel, d. w., d. r. Nelson, T. C. Sciarrotta, e. a. Stan- 

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1974. A multichannel ultrasonic biotelemetry system for 

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Finstad, w. O., and d. r. Nelson. 



1975. Circadian activity rhythm in the horn shark, 

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