Meyer Movements of tiger sharks ( Galeocerdo cuvier ) in coastal

Externally and internally implanted sonic transmitters were used to track the movements of eight tiger sharks (Galeocerdo cuvier) ranging between 200 and 417 cm total length (TL), captured by hook-and-line on the south coast of Oahu, Hawaii, between 1993 and 1997. Attachment of the transmitters was facilitated by the fact that captured sharks exhibited tonic immobility when restrained and inverted at the side of the tagging vessel. Three common themes emerged from the horizontal movements of the tracked sharks: (1) o€shore movements away from the island, (2) extended periods of directed, ``straight-line'' swimming, (3) orientation to the Penguin Banks ± a shallow bank located ' 35 km from the release point. In shallow water (<300 m) the sharks swam predominantly close to the bottom, in open water (>300 m) they swam within the mixed layer at depths of 80 m. One shark dove brie ̄y to 335 m. The average estimated swimming speed of sharks traversing open water was 0.29 body length (BL) s. Two sharks were recaptured after termination of the tracks; one of these sharks was recaptured twice, with a total time at liberty of 377 d. The data suggest that Hawaiian tiger sharks move within large home ranges and that they can eciently navigate between distant parts of their range, even when this requires crossing open ocean waters. Introduction In Hawaii, the long-term rate of shark attacks on humans is low, averaging ' 2:0 con®rmed attacks per year over the past several decades (Wetherbee et al. 1994). Between 1958 and 1997, only three con®rmed fatal shark attacks were reported. However, two of these fatal attacks occurred within a 12 mo period in 1990 and 1991, and several other, non-fatal attacks occurred during the same period (Wetherbee et al. 1994). This series of attacks rekindled widespread public debate about the appropriateness and the feasibility of shark-control programs. Because most shark attacks in Hawaii are probably attributable to tiger sharks (Galeocerdo cuvier; Randall 1992), most of the discussion focused on the behavior of this species. In fact, an improved understanding of the behavior of tiger sharks has pertinence beyond Hawaii, since tiger sharks are circumglobal in distribution and are considered one of the world's most dangerous species (Baldridge 1973; Compagno 1984; Randall 1992). Previous shark-culling programs conducted in Hawaii between 1959 and 1976 included prolonged, statewide ®shing that killed nearly 5000 sharks of several species (Tester 1969; Wetherbee et al. 1994) but, despite killing over 500 tiger sharks in these programs, there is no evidence that this widespread ®shing e€ort had an e€ect on the rate of attacks on humans in Hawaii (Wetherbee et al. 1994). Consequently, much of the recent debate focused on whether there are alternative methods of addressing the perception that unchecked shark populations pose a threat to public safety. For instance, rather than culling large numbers of sharks from throughout the region, could ®shing e€ort focused on a speci®c area result in a lasting, localized depletion of dangerous sharks in that area? A related question is whether ®shing for sharks at the site of an attack soon after it occurred would be likely to capture the shark responsible for that attack. Marine Biology (1999) 134: 665±673 jj: jjj ± jjj Ó Springer-Verlag 1999 Communicated by M.H. Horn, Fullerton K.N. Holland (&) Hawaii Institute of Marine Biology, University of Hawaii at Manoa, P.O. Box 1346, Coconut Island, Kaneohe, Hawaii 96744, USA Fax: 001 (0)808 236-7443 e-mail: [email protected] B.M. Wetherbee NOAA-NMFS-NESC, 28 Tarzwell Drive, Narragansett, Rhode Island 02882, USA C.G. Lowe á C.G. Meyer Department of Zoology and Hawaii Institute of Marine Biology, Edmonson Hall, University of Hawaii at Manoa, Honolulu, Hawaii 98822, USA An understanding of the movement patterns of tiger sharks in Hawaiian waters is critical to evaluation of the potential e€ectiveness of localized ®shing; it would be easier to reduce the numbers of strongly site-attached sharks occupying stable home ranges than wide-ranging individuals with no aliation to a home range. However, there have been no studies of the daily movements of tiger sharks or other potentially dangerous sharks in areas where there are large numbers of humans in the water. Acoustic telemetry has been used to track the movements of white sharks, Carcharodon carcharias (Carey et al. 1982; Strong et al. 1992), mako sharks, Isurus oxyrinchus (Carey et al. 1981; Casey and Kohler 1992; Holts and Bedford 1993), scalloped hammerheads, Sphyrna lewini (Klimley and Nelson 1984; Klimley 1993) and blue sharks, Prionace glauca (Sciarrotta and Nelson 1977; Carey and Scharold 1990), but these tracks occurred far o€shore or away from swimming beaches. One tiger shark was tracked with an acoustic pinger for 48 h around French Frigate Shoals, in the Northwestern Hawaiian Islands (Tricas et al. 1981), but French Frigate Shoals is a low, dry, uninhabited atoll, whereas the main Hawaiian Islands are high, wet, and have large human populations. The topography of the islands is pertinent because freshwater streams and estuaries have great impact on the geography of coral reef ecosystems and therefore on the movements of their inhabitants. Also, human and agricultural refuse is much more prevalent around populated coastlines and is often transported down rivers and estuaries. This anthropogenic material can be a source of food for tiger sharks (Lowe et al. 1996) and could potentially in ̄uence their behavior and movement patterns. The distribution and behavior of other shark species di€ers between the high and low Hawaiian Islands (Wetherbee et al. 1996, 1997). The speci®c focus of the current study was to determine the short-term movement patterns of large tiger sharks captured close to areas of high recreational ocean use in Hawaii and to use the results to evaluate whether localized ®shing e€ort could cause local depletions of tiger-shark populations. Based on the fairly restricted movements of the shark tracked at French Frigate Shoals (Tricas et al. 1981) and what was previously thought to be the behavior of tiger sharks in coastal waters (Compagno 1984), we hypothesized that tiger sharks near Oahu would display predominantly coastal behavior and possibly exhibit repeated diel movements to the same parts of the reef; i.e. they would display mesoscale site-®delity. Materials and methods