Diel movement patterns and habitat preferences of the common thresher shark (Alopias vulpinus) in the Southern California Bight
D. Cartamil A D , N. C. Wegner A , S. Aalbers B , C. A. Sepulveda B , A. Baquero C and J. B. Graham AA Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0204, USA.
B Pfleger Institute of Environmental Research, 315 N. Clementine St., Oceanside, CA 92054, USA.
C Fundacion Equilibrio Azul, PO Box 17116025, Quito, Ecuador.
D Corresponding author. Email: dcartamil@ucsd.edu
Marine and Freshwater Research 61(5) 596-604 https://doi.org/10.1071/MF09153
Submitted: 23 June 2009 Accepted: 12 November 2009 Published: 28 May 2010
Abstract
The common thresher shark, Alopias vulpinus, is the basis of the largest commercial shark fishery in California waters. We used acoustic telemetry to determine the diel movement patterns and habitat preferences of this species in the Southern California Bight (SCB), where commercial fishing for the common thresher shark is concentrated. Eight common threshers (fork length: 122–203 cm) were tagged with temperature and depth-sensing acoustic transmitters and tracked for periods ranging from 22 to 49 h. Tracked sharks preferentially utilized deep offshore waters, and avoided shallower waters over the continental shelf. Mean rate of movement (ROM ± s.d.) was 2.15 ± 0.46 km h−1. ROM and angular concentration (r, a measure of relative linearity) both showed a strong daytime pattern, with highest values at dawn that decreased throughout the day, whereas nocturnal ROM and r were less variable. Daytime vertical movements consisted of either vertical excursions below the thermocline or relatively level swimming within the upper portion of the thermocline. Nocturnally, all sharks remained within the mixed layer. These findings suggest that the common thresher shark is primarily a daytime predator, and have relevance for estimating how the alteration of the set depth of fishing-gear could affect catch rates of this species in the SCB.
Additional keywords: acoustic telemetry, drift gill-net, fishery management, pelagic.
Acknowledgements
This work was supported by California Sea Grant (Grant #R/F-193), the William H. and Mattie Wattis Harris Foundation, the Tinker Foundation, the Moore Family Foundation, UC Mexus-CONACYT, J. Steinitz, and a gift from J. Bohn. D. Cartamil was supported by a fellowship from the Alliance for Graduate Education and the Professoriate, and the Tuna Industry Endowment Fund. We thank T. Thomas, L. Brammer, and K. Duff at the Scripps Institution of Oceanography Machine Science Development Center for custom construction of the acoustic tracking system, and J. Dufour and B. Jones for use of the tracking vessel ‘Saikhon’. D. Holts and the National Marine Fisheries Service provided the tracking vessel R/V ‘Phoenix’ and essential tracking equipment. Thanks to E. Kisfaludy and the many volunteers who assisted in fishing and tracking operations. We also thank D. Checkley, P. Hastings, C. Lennert, R. Rosenblatt, D. Woodruff, and two anonymous referees for commenting on this manuscript. All procedures were performed under the guidelines of the Institutional Animal Care and Use Committee of the University of California, San Diego (protocol S00080).
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