Project Title: Bahamas Tiger Shark Research Project: Using steroid hormone analysis, ultrasonography and telemetry as non-invasive tools for examining movement patterns and reproduction in tiger sharks
The tiger shark (Galeocerdo cuvier) is the largest predatory fish in tropical seas. While the behavioral ecology of tiger sharks has been well-studied in the Pacific and Indian Ocean, little is known about the movement patters of tiger sharks in the Atlantic Ocean. By satellite tracking tiger sharks tagged off of Florida and the Bahamas, Hammerschlag et al. (2012) found that female tiger shark displayed a pelagic phase lasting several months, with movements following the Gulf Stream, warm currents that support productivity at all trophic levels. Sharks also exhibited an inshore, high residency phase at a popular dive tourism site in the Bahamas, nicknamed Tiger Beach (TB), located on the Northwest edge of little Bahama Bank. Tiger sharks at TB are provisioned by divers year-round by boats to lure tiger sharks within close proximity of dive tourists. While the long-term, large-scale movements of tiger sharks are not impacted by dive tourism at TB, it remains unknown if and how high residency, daily movements, of female sharks at TB are impacted by the provisioning tourism. This would be of particular conservation significance if female sharks were aggregating at TB for reproductive purposes, such as mating or gestating, as hypothesized by Hammerschlag et al. (2012).
To fill this gap in knowledge, the overall goal of this project is to evaluate the relationship among reproductive state, diet, body condition and residency patterns of female tiger sharks at TB to determine the following:
- Are tiger sharks using TB as a mating ground, feeding area or gestation ground? If not, where are these critical areas?
- What are the overall residency patterns of sharks within TB?
- Does dive tourism affect tiger shark behavior and movements at TB? If so, how?
- How much movement is there in and out of TB? What are the migration patterns of tiger sharks in the subtropical Atlantic?
- How much time are tiger sharks remaining in the Bahamas Shark Sanctuary? If they move out of the Sanctuary, where and when are the sharks most vulnerable to fishing exploitation?
- How are shark movement patterns influenced by their body condition and health?
Reproduction and Movement Patterns
Determining the reproductive strategies and mating behaviors of animals is crucial for management and conservation. Historically, the reproductive biology of sharks has been studied by sacrificing the animals; however, in this study, we will be performing ultrasounds on tiger sharks as well as taking blood samples for hormone analysis to determine reproductive status of these large predators. Coupling these techniques (ultrasonography imaging and blood hormone analysis) will allow us to examine the reproductive state (e.g. pregnancy), cycle and length of gestation of sharks during our ongoing research. After their “pregnancy check-ups,” the sharks are tagged with satellite transmitters to track their movements and evaluate behavioral patterns that may reveal locations where mating, gestation, or giving birth is occurring.
To examine the large scale movement patterns of tiger sharks in the subtropical Atlantic, we are tracking sharks using satellite tags. The satellite tags are affixed to the dorsal fins of the sharks and transmit to orbiting satellites to provide the position of the sharks. To examine the fine scale residency patterns of tiger sharks at TB, we are using acoustic tagging and tracking. Acoustic tags are small transmitters (AA battery size) that are surgically implanted in the abdomen of the sharks. The transmitters send off an ultrasonic signal that is picked up by a series of hydrophones strategically positioned throughout TB. These hydrophones record the date and time when an acoustically tagged tiger shark swims within about 500 m of a hydrophone.
Hammerschlag N, Gutowsky LFG, Gallagher AJ, Matich P, Cooke SJ. (2017). Diel habitat use patterns of a marine apex predator (tiger shark, Galeocerdo cuvier) at a high use area exposed to dive tourism. Journal of Experimental Marine Biology and Ecology, 495: 24-34.
Sulikowski J, Wheeler CR, Gallagher AJ, Prohaska BK, Langan JA, Hammerschlag N. (2016). Seasonal and life-stage variation in the reproductive ecology of a marine apex predator, the tiger shark Galeocerdo cuvier, at a protected female dominated site. Aquatic Biology, 24: 175-184
Hammerschlag N, Broderick AC, Coker JW, Coyne MS, Dodd M, Frick MG, Godfrey MH, Godley BJ, Griffin DB, Hartog K, Murphy SR, Murphy TM, Nelson ER, Williams KL, Witt MJ, Hawkes LA (2015). Evaluating the landscape of fear between apex predatory sharks and mobile sea turtles across a large dynamic seascape. Ecology, 96(8): 2117-2126.
Irschick, DJ, Hammerschlag N, (2014). Morphological scaling of body form in four shark species differing in ecology and life-history. Biological Journal of the Linnean Society
Irschick DI., Hammerschlag N. (2014). A new metric for measuring condition in large predatory sharks. Journal of Fish Biology; 85(3), 917-926.
Gallagher, AJ, Wagner, DN, Irschick, DJ, Hammerschlag N. (2014).Body condition predicts energy stores in apex predatory sharks. Conservation Physiology:2 DOI: 10.1093/conphys/cou022
Hammerschlag N, Gallagher AJ, Carlson JK. (2013)A revised estimate of daily ration in the tiger shark (Galeocerdo cuvier) with implications for assessing ecosystem impacts of apex predators. Functional Ecology, 27 (5): 1273-1274
Hammerschlag N, Gallagher AJ, Wester J, Luo J, Ault JS. (2012)(Cover) Don’t bite the hand that feeds: assessing ecological impacts of provisioning ecotourism on an apex marine predator. Functional Ecology, 26(3): 567-576
Hammerschlag N, Sulikowski J. 2011. Killing for Conservation: The Need for Alternatives to Lethal Sampling of Apex Predatory Sharks. Endangered Species Research 14: 135–140