Chapter 6

Case Study: White Shark Movements in the North Pacifi c Pelagic Ecosystem

Andre M. Boustany, Kevin C. M. Weng, Scot D. Anderson, Peter Pyle and Barbara A. Block

Abstract

The advent of novel electronic tracking technologies has allowed researchers to study the movements of ocean animals in their natural environment. Pop-up tagging of adult white sharks off the coast of central California provided insight into the movement patterns and environmental preferences of these sharks in the North Pacifi c Ocean. Tagged white sharks spent long periods, up to 8 months, in the pelagic ecosystem and far from land. While offshore, sharks experienced a wide range of depths and temperatures. The purpose of these offshore excursions remains unknown, but further tagging over wider size ranges and geographic areas should help to address these questions.

Key words: white shark, Carcharodon carcharias, satellite tagging, Lamnidae, pelagic shark movement, North Pacifi c.

Introduction

White sharks (Carcharodon carcharias, Lamnidae) are large apex predators with the capability to conserve metabolic heat and maintain body temperatures warmer than ambi-ent (Carey et al., 1971, 1982; Goldman et al., 1996; Goldman, 1997). They are found in all major ocean basins, from temperate to tropical seas, although most visual identifi -cations have been from temperate waters (Compagno, 2001). Traditionally, white sharks were considered primarily coastal in nature, aggregating around pinniped colonies to feed (Klimley, 1985; Long et al., 1996). Yet occasional sightings of white sharks far from land (Compagno, 2001), their occurrence at oceanic islands (Taylor, 1985), and genetic stud-ies suggesting movement between continents (Pardini et al., 2001) indicate that they are capable of making long-distance, open ocean migrations. However, it has been diffi cult to study white sharks within and across the entire pelagic ecosystem. Recent advances in satellite telemetry have overcome this limitation by allowing researchers to track pelagic fi shes far from land, independent of research vessels (Block et al., 1998).

Sharks of the Open Ocean: Biology, Fisheries and Conservation. Edited by M. D. Camhi, E. K. Pikitch and E. A. Babcock

© 2008 Blackwell Publishing Ltd. ISBN: 978-0632-05995-9

White Shark Movements in the North Pacifi c 83

One of the newest electronic tagging technologies is the pop-up satellite archival tag (PSAT; Block et al., 1998; Marcinek et al., 2001). These tags are attached externally to marine animals, where they collect pressure, ambient temperature, and light-level data at user-defi ned intervals. At a preprogrammed date, the tags release from the fi sh, fl oat to the surface, and transmit summaries of stored data to researchers through the Argos satellite system. Light-level data in conjunction with tag-recorded sea surface temperatures (SST) can be used to estimate longitude and latitude (Wilson et al., 1992; Teo et al., 2004).

Methods and Results

Between 1999 and 2003, 25 adult (3.7–5.3 m) white sharks were tagged with PSATs at Southeast Farallon Island (SEFI), off the central coast of California (Boustany et al., 2002). White sharks are present at SEFI in the fall and early winter but are absent at other times of the year (Ainley et al., 1985). Males return to the island yearly, but females are believed to return on alternate years (Anderson and Pyle, 2003), suggesting a biennial breeding period. It is currently unknown where breeding or pupping takes place in the North Pacifi c. To examine the horizontal and vertical movement patterns of white sharks when not at SEFI, PSATs were applied to the dorsal musculature of the sharks for periods ranging from 14 to 274 days. To date, 12 tags have successfully reported data through the satellites.

Tagged sharks remained in the vicinity of SEFI for periods ranging from 0.5 to 4 months after tagging. Previous acoustic studies have indicated that white sharks are actively search-ing for prey at this time (Goldman et al., 1996; Goldman and Anderson, 1999; Klimley et al., 2001). While in this area, temperature ranges were narrow (8–14ºC) and diving was shallow as sharks were restricted by the continental shelf. Sharks spent the major-ity of time (�95%) between the surface and 50 m depth. In the late fall and early winter, sharks moved offshore toward the southwest, taking them into the warmer temperatures of the eastern Pacifi c’s subtropical gyre (Fig. 6.1). The earliest movements off the con-tinental shelf took place in late November, whereas other sharks remained near the tag-ging location until early March. As the white sharks moved offshore, diving increased with maximum dive depths extending below 700 m. Deeper diving, coupled with move-ment into subtropical waters, expanded the temperature range that sharks experienced (4.6–26.2ºC). One male shark showed movement to the Hawaiian Islands in 2 consecu-tive years, remaining in the area for at least 4 months before returning to SEFI (Boustany et al., 2002). Reports of white shark predation on cetaceans near the Hawaiian Islands and the timing of the shark’s arrival with the humpback whale calving season suggest that movement to that area may be for feeding (J. Naughton, National Marine Fisheries Service, personal communication).

All other white sharks that were tracked for longer than 1 month moved toward a region of the subtropical eastern Pacifi c midway between Baja California and the Hawaiian Islands (Fig. 6.1). This region is characterized by a lack of seamounts, strong temperature gradients, or prominent currents, raising the question of why white sharks are attracted to this area. These sharks remained in this area for up to 8 months with no observed movement back to California coastal areas, suggesting that the period of

84 Sharks of the Open Ocean

pelagic residency may be a signifi cant portion of their yearly cycle. Depths recorded by the tags indicate that sharks were routinely making deep dives (�300 m) while in this region. The fact that subadult white sharks tracked off the coasts of California, Australia, and South Africa have not shown extensive periods of pelagic residency (K. C. M. Weng et al., unpublished data; Bonfi l et al., 2005; Bruce, 2008) suggests that these movements may be related to breeding in adult sharks. Although young-of-the-year white sharks are found predominantly in southern California waters (Klimley, 1985; Dewar et al., 2004), it remains possible that these small sharks may have traveled to coastal areas from else-where. However, the possibility of an offshore pupping ground does not explain the movement of males into pelagic waters. Because current hypotheses regarding duration of gestation suggest an 18-month time period, it seems unlikely that mating and pupping would occur over the same period (Mollet et al., 2000), and it is thus possible that sharks travel to this offshore area to mate (Anderson and Pyle, 2003).

Electronic tags have revealed new insights into the pelagic behaviors of white sharks. It remains uncertain whether white sharks of differing size classes and from other geo-graphic areas exhibit similar movement patterns as these adult white sharks tracked in the eastern North Pacifi c. Ongoing tagging of both adolescent sharks in the eastern Pacifi c as well as adult sharks off Australia and South Africa should help to discern the infl uences of tagging location and ontogenetic shifts in behavior on the pelagic residency of white sharks. Such data will improve our understanding of white shark movements and will be critical to establishing protective measures to conserve these populations worldwide.

160ºW 150ºW 140ºW 130ºW 120ºW 110ºW

20ºN

30ºN

40ºN

�

Fig. 6.1 Release location (black triangle) and pop-up locations of male (white circles) and female (gray circles) white sharks tagged between 1999 and 2003.

White Shark Movements in the North Pacifi c 85

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