OBSERVATIONS OF BREACHING BEHAVIOR IN JUVENILE BULL SHARKS, CARCHARHINUS LEUCAS

Transcription

1 OBSERVATIONS OF BREACHING BEHAVIOR IN JUVENILE BULL SHARKS, CARCHARHINUS LEUCAS TOBEY H. CURTIS (1,3) * AND LAURA J. MACESIC (2) Florida Program for Shark Research, Florida Museum of Natural History, University of Florida, Gainesville, FL (2) Department of Biological Sciences, Mount Holyoke College, South Hadley, MA (3) Current Address: NOAA, National Marine Fisheries Service, Northeast Regional Office, 55 Great Republic Drive, Gloucester, MA 01930, USA ( Tobey.Curtis@noaa.gov)

2

3 Biological Sciences OBSERVATIONS OF BREACHING BEHAVIOR IN JUVENILE BULL SHARKS, CARCHARHINUS LEUCAS TOBEY H. CURTIS (1,3) * AND LAURA J. MACESIC (2) Florida Program for Shark Research, Florida Museum of Natural History, University of Florida, Gainesville, FL (2) Department of Biological Sciences, Mount Holyoke College, South Hadley, MA (3) Current Address: NOAA, National Marine Fisheries Service, Northeast Regional Office, 55 Great Republic Drive, Gloucester, MA 01930, USA ( Tobey.Curtis@noaa.gov) ABSTRACT: Breaching is a behavior observed in numerous marine vertebrate species, and is typically associated with feeding, predator avoidance, parasite removal, or signaling. Few shark species have been documented to display natural breaching behavior unassociated with hook and line capture. We describe observations of breaching by juvenile bull sharks, Carcharhinus leucas, in the Indian River Lagoon, Florida. Breaching is not a behavior typically associated with bull sharks, and has led to species misidentifications in the past. The behavior was observed on 18 occasions over a twoyear period in the study site, including a day in which 11 breaches were observed in a small estuarine creek. Conditions that appear to promote breaching behavior in bull sharks are identified, and potential motivations are discussed. Key Words: Carcharhinidae, predatory behavior, signaling THE conspicuous behavior of breaching, or intentionally leaping above the surface of the water, has been documented in many marine vertebrate species. Breaching, and other types of aerial activity, have most often been observed and described in whales and dolphins (e.g., Whitehead, 1985; Wursig et al., 1989; Whitehead, 2002), as well as in a number of bony fishes (e.g., Helfman et al., 1997) and some mobulid rays (e.g., Marshall and Bennett, 2010). Several species of sharks are known for aerial acrobatics when hooked by recreational fishers (Compagno, 1984); however, far fewer species have been observed performing a natural breaching behavior. Such species include the spinner (Carcharhinus brevipinna), blacktip (C. limbatus), thresher (Alopias vulpinus), basking (Cetorhinus maximus), and most famously, the white (Carcharodon carcharias), whose spectacular breaching behavior has been the subject of several nature documentaries (Compagno, 1984; Castro, 1996; Klimley et al., 1996; Martin et al., 2005; Brunnschweiler, 2005). Despite these observations, the motivations for such behavior are poorly understood and rarely discussed (Brunnschweiler, 2005). Due to their cryptic environment and the inherent observational difficulties, natural behavioral observations of sharks remain quite sparse in the scientific literature. * Corresponding author Tobey.Curtis@noaa.gov 253

4 254 FLORIDA SCIENTIST [VOL. 74 Young bull sharks have been reported to occasionally spin out of the water (Compagno, 1984), but there are no reports in the literature on the frequency of this behavior, the environmental conditions that promote breaching, or the biological function associated with this behavior. The bull shark is one of the largest of the carcharhinid sharks. Born at approximately 75 cm total length (TL) (Snelson et al., 1984), it can reach lengths of 400 cm and weigh up to 600 kg (McCord and Lamberth, 2009). The bull shark is considered to be a rather slow moving species, but is capable of fast swimming and burst speeds when pursuing prey (e.g., schooling fishes, catfishes, stingrays) (Compagno, 1984; T. Curtis, unpubl. obs.). Herein, we present a summary of observations of natural breaching behavior by juvenile bull sharks in the Indian River Lagoon (IRL), Florida, which serves as a nursery area for this species (Snelson et al., 1984; Curtis, 2008; Curtis et al., 2011). We describe the environmental conditions where breaches were observed, and discuss the potential motivation for the behavior and its significance. MATERIALS AND METHODS All observations took place in 2004 and 2005 in the IRL, central Atlantic coast of Florida, between Titusville and Melbourne (28u 389 to 28u 029 N). Observations occurred during the course of a broader study on the distribution and movements of bull sharks in the lagoon (Curtis, 2008; Curtis et al., 2011). The date, time, and location of each observed breach was recorded, as well as the estimated TL of the breaching shark. Environmental measurements were collected at the site of each breach, including water depth, temperature, salinity, and secchi depth. The presence of potential prey species was also noted. RESULTS AND DISCUSSION During the study period, a total of 18 breaches by juvenile bull sharks were documented in the IRL. Breaching sharks were approximately cm TL. Most of the observed breaches were clustered in two main areas of the lagoon: adjacent to the outfall of the Florida Power and Light power plant in Frontenac (28u N, 80u W), and in Crane Creek in Melbourne (28u N, 80u W). Bull sharks were observed to breach over water depths of 1 4 m, but the exact depth at which breaches were initiated could not be determined. A typical breach consisted of a shark propelling itself through the surface at a near vertical angle (Figure 1), and then falling back into the water on its side. However, bull sharks also occasionally exhibited head-over-tail flips, or spinning along the long axis of the body upon exiting the water. They often breached to approximately 1 m above the water s surface. On 12 March 2004, one breach was observed, and on 19 March 2004, two breaches were observed at the power plant outfall in Frontenac. The mean water temperature at the outfall was 32 uc, salinity was 28 psu, and secchi depth was 2 m. Water depth at the breaching locations was shallow (,2 m). Several other fish species were also observed in the area, including mullet (Mugilidae), ladyfish (Elopidae), black drum (Sciaenidae), and catfish (Ariidae). Observations of sharks jumping out of the water at this site during winter months were previously reported by Snelson and Williams (1981); however, they hypothesized that the sharks were C. limbatus, a species more

5 No ] CURTIS AND MACESIC BREACHING BULL SHARKS 255 FIG. 1. Photograph of a breaching juvenile bull shark, taken in Tampa Bay, Florida. Copyright John Hartung ArtThatSmiles.com. often associated with breaching (Compagno, 1984; Castro, 1996). Based on our observations, and the absence of blacktip shark captures in northern IRL shark surveys (Snelson and Williams, 1981; Adams and Paperno, 2007; Curtis et al., 2011), it is most likely that the jumping sharks were C. leucas. Bull sharks have also been observed breaching in the warm water refuge canal at Tampa Electric s Big Bend Power Station in Tampa Bay, Florida (Figure 1; W. Anastasiou, Tampa Electric Co., pers. commun.). We also observed 11 breaches over a 27-hour period on June 2004 in Crane Creek. During this observation period, there was considerable salinity (and presumably density) stratification in the water column. Mean surface salinity was 12.5 psu, while mean bottom salinity (at 2 3 m) was 30.9 psu. Water temperature averaged 30.1 uc, and secchi depth averaged 0.8 m. This salt-wedge stratification may have promoted breaching behavior, in that the distinct halocline may have agitated the sharks, or it could have provided advantageous conditions for ambush predatory tactics on surface-oriented prey. Dense shoals of mullet were present in the creek and were occasionally seen breaching themselves, presumably to avoid predation. The limited visibility in the creek would also leave surface-oriented fish vulnerable to attacks from below. However, successful prey capture by the breaching sharks was not visually confirmed. Breaching in marine vertebrates is hypothesized to be associated with feeding strategies (e.g., Compagno, 1984; Martin et al., 2005), predator avoidance (e.g., Helfman et al., 1997), removal of ectoparasites (e.g.,

6 256 FLORIDA SCIENTIST [VOL. 74 Compagno, 1984; Whitehead, 2002), mating behavior (e.g., Marshall and Bennett, 2010), or signaling in a social context (e.g., Wursig et al., 1989; Klimley et al., 1996; Whitehead, 2002). Evidence of breaching as a feeding strategy is demonstrated by the white shark, which is known to propel its entire body out of the water when ambush attacking its pinniped prey at the surface (e.g., Klimley et al., 1996; Martin et al., 2005; T. Curtis and L. Macesic, unpubl. obs.). In False Bay, South Africa, the Polaris breach was the most common initial predatory strategy used by white sharks, and resulted in a high feeding success rate (Martin et al., 2005). Other hypothesized functions for breaching in white sharks, including intraspecific communication (Klimley et al., 1996), is plausible but has proven more difficult to confirm. Given the conditions where bull shark breaches were observed in the IRL, we suggest that the behavior was most likely associated with feeding. Known prey species were visually observed in surface waters during periods of frequent breaching, and appeared to demonstrate anti-predation behaviors. Dislodging of ectoparasites cannot be ruled out based on our observations, as other sharks that were examined in the same location carried copepod parasites on their fins and head. Predator avoidance is an unlikely motivation for breaching in these bull sharks, as potential predators of juvenile bull sharks are scarce in the IRL (Curtis et al., 2011). Signaling is also not likely to be a motivating factor, since such communication is typically reserved for cooperative hunting, competition, or courtship of potential mates (Wursig et al., 1989; Klimley et al., 1996; Whitehead, 2002), all of which might be unlikely behaviors for a juvenile shark. However, at the sites where most breaches were observed (i.e., Crane Creek and power plant outfalls) multiple sharks were aggregated in a relatively confined space. Therefore, breaching frequency may be a function of proximity to conspecifics. Whether this is the result of social interaction or coincidental with predatory activity requires more observations. Because of the potentially high energetic costs of breaching, the behavior should result in some type of tangible survival benefit. If breaching is a predatory tactic in these bull sharks, it could provide high feeding success rates under specific conditions (Martin et al., 2005), thereby increasing the fitness of the sharks. Although it is not known how frequent or widespread the behavior may be in this circumglobal species, it may be restricted to brackish or freshwater environments. Juvenile bull sharks have been reported to breach in other Florida and Texas estuaries (Figure 1; M. Heyde, Miami Seaquarium, pers. commun.), and in Western Australia estuaries (J. Witty, Murdoch University, pers. commun.), and large adult bull sharks have been observed breaching in some South African rivers (M. McCord, Old Harbor Museum, pers. commun.). More observational studies will continue to improve our knowledge of the behavioral repertoire of bull sharks and other species, and help determine the adaptive function of such behaviors. ACKNOWLEDGMENTS For various forms of logistical support during the course of this study we wish to thank F. Snelson, G. Burgess, D. Parkyn, D. Adams, T. Vigliotti, the numerous

7 No ] CURTIS AND MACESIC BREACHING BULL SHARKS 257 volunteers who assisted with field work, Canaveral National Seashore, Merritt Island National Wildlife Refuge, and the Florida Fish and Wildlife Conservation Commission. LITERATURE CITED ADAMS, D. H. AND R. PAPERNO Preliminary assessment of a nearshore nursery ground for the scalloped hammerhead off the Atlantic coast of Florida. Pp In: MCCANDLESS, C. T., N. E. KOHLER, AND H. L. PRATT JR (eds.), Shark nursery grounds of the Gulf of Mexico and the east coast waters of the United States. American Fisheries Society, Symposium 50, Bethesda, MD. BRUNNSCHWEILER, J. M Water-escape velocities in jumping blacktip sharks. J. Roy. Soc. Int. 2: CASTRO, J. I Biology of the blacktip shark, Carcharhinus limbatus, off the southeastern United States. Bull. Mar. Sci. 59(3): COMPAGNO, L. J. V FAO species catalogue. Vol. 4. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Part 2 - Carcharhiniformes. Food and Agricultural Organization, United Nations FAO Fisheries Synopsis 125(4, pt. 2): CURTIS, T. H Distribution, movements, and habitat use of bull sharks (Carcharhinus leucas, Müller and Henle 1839) in the Indian River Lagoon system, Florida. Masters thesis. Univ. Florida, Gainesville, FL., D. H. ADAMS, AND G. H. BURGESS Seasonal distribution and habitat associations of bull sharks in the Indian River Lagoon: A 30-year synthesis. Trans. Am. Fish. Soc. 140: HELFMAN, G. S., B. B. COLLETTE, AND D. E. FACEY The Diversity of Fishes. Blackwell Science, Inc., Malden, MA. KLIMLEY, A. P., P. PYLE, AND S. D. ANDERSON Tail slap and breach: Agonistic displays among white sharks? Pp In:KLIMLEY,A.P.AND D. G. AINLEY (eds.), Great White Sharks: The Biology of Carcharodon carcharias. Academic Press, San Diego, CA. MARSHALL, A. D. AND M. B. BENNETT Reproductive ecology of the reef manta ray Manta alfredi in southern Mozambique. J. Fish Biol. 77: MARTIN, R. A., N. HAMMERSCHLAG, R. S. COLLIER, AND C. FALLOWS Predatory behavior of white sharks (Carcharodon carcharias) at Seal Island, South Africa. J. Mar. Biol. Ass. U.K. 85: MCCORD, M. E. AND S. J. LAMBERTH Catching and tracking the world s largest Zambezi (bull) shark Carcharhinus leucas in the Breede Estuary, South Africa: the first 43 hours. Afr. J. Mar. Sci. 31(1): SNELSON, F. F., T. J. MULLIGAN, AND S. E. WILLIAMS Food habits, occurrence, and population structure of the bull shark, Carcharhinus leucas, in Florida coastal lagoons. Bull. Mar. Sci. 34: AND S. E. WILLIAMS Notes on the occurrence, distribution, and biology of elasmobranch fishes in the Indian River lagoon system, Florida. Estuaries 4: WHITEHEAD, H Humpback whale breaching. Invest. On Cetacea 17: Breaching. Pp In: PERRIN, W. F., B. WURSIG, AND J. G. M. THEWISSEN (eds.), Encyclopedia of Marine Mammals. Academic Press, San Diego, CA. WURSIG, B., E. M. DORSEY, W. J. RICHARDSON, AND R. S. WELLS Feeding, aerial and play behavior of the bowhead whale, Balaena mysticetus, summering in the Beaufort Sea. Aquat. Mamm. 15(1): Florida Scient. 74(4): Accepted: April 27, 2011 Florida Academy of Sciences E