Tiger Shark Pregnancy

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

Overview

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 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 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 migrations patterns of tiger sharks in the subtropical Atlantic?
  • How much time are tiger sharks remaining in the Bahamas Shark Sanctuary? If they move out 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?

Methodology

Fishing aggregations of gravid females can significantly impact the health of local and regional populations. TB is within the boundaries of the Bahamas Exclusive Economic Zone (EEZ) where sharks are protected from all harvest due to the establishment of a Shark Sanctuary in 2011. If gravid females spend their entire gestation period and potentially give birth within Bahamian waters, then the sanctuary would indeed serve as an effective conservation tool for this species. However, if gravid females spend little time within the sanctuary and/or give birth outside the sanctuary, then they may be vulnerable to exploitation when they move out, reducing the efficacy of the protected area. TB is also heavily exploited by the dive tourism industry where provisioning occurs on a daily basis. If the area is important for reproduction, consumption of poor food quality and/or changes in shark behavior or movement resulting from provisioning could have disproportional negative consequences for reproduction, fecundity and/or offspring fitness.

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 of 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.

Our Research on the Big Screen

Discovery’s Shark Week – Violent Shark Sex

Discovery’s Shark Week – First Ever Tiger Shark Sonogram
Reveals Pups






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, DJ, Hammerschlag N, (2014). A new metric for measuring condition in large predatory sharks. Journal of Fish Biology; DOI: 10.1111/jfb.12484

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