Satellite tagging of tiger sharks at Raine Island

There is very little known about the basic biology and ecology of tropical sharks. Over recent years there has been an increased concern over the status of shark stocks around the world. The reproductive strategies and life history of sharks make them extremely vulnerable to overfishing.

Recent advances in technology allow for remote tracking of sharks using satellite technology to learn more about the home range and migration patterns of these animals.
Raine Island (11o 35" S., 144o 02" E.) is an elongate sand cay approximately 830m long and 430m wide at its widest point, and is the primary site for the largest cohort of nesting green turtles in the world. Tiger sharks are also known to frequent this area, particularly during peak nesting season, from October to February. Satellite tagging of tiger sharks at Raine Island offers a unique opportunity to study the movement patterns of this tropical apex predator. Elsewhere in the world migration patterns of tigers have been found to coincide with their food sources; e.g. albatross chicks fledging in the French Frigate Shoals northwest of Hawaii, where these animals are known to migrate vast distances.

Observations at Raine Island over the last number of years indicate that during the turtle nesting season there is a large population of tiger sharks during this period. Such a population is not present in the winter months, suggesting that these sharks migrate to the region. It is hoped that through this study we will be able to demonstrate the geographic linkages between turtles and tiger sharks, and the role of Raine Island in the ecological interconnectivity of the Great Barrier Reef Province. Findings from standard tagging of tiger sharks in different parts of the world have been difficult to interpret. It has been found that some tigers stay in the one location, while others have travelled up to 1,850 n. miles (Randall, 1992). It is well known, however, that tigers migrate into higher latitudes from tropic seas during the warm months.

Occurring in a wide variety of oceanographic conditions, tiger sharks range from close inshore to the open ocean. Their diel movements have been tracked over a 24 hour period by Tricas et al. (1981)(Randall, 1992) where a 400 cm female was tagged with a telemetric device in the north-western Hawaiian Islands. The shark ranged over an area of 100 km2, traveling faster by day then night. 68% of the day activity was spent deeper than the reef drop-off at 40 m, whereas 83% of the nocturnal activity was spent in depths shallower than the drop-off. Numerous authors have described their diel movements as readily coming inshore at night, however rarely being seen in the shallows during the day.

Tigers do not school, but have been observed in groups of two to six or more. These assemblies may be transitory or due to the presence of food (Randall, 1992). Eating habits vary greatly with the availability of prey items, and tiger sharks consume a larger variety of animals than other sharks. Rays, sea turtles, sea snakes, bony fishes, sharks, seals, sea birds, dolphins, cephalopods, crustaceans, and jellyfishes all form part of the tiger sharks' food source. Even terrestrial mammals are consumed, as well as man-made products, including indigestible plastics and metals. Due to their large size and their coarse serrated teeth which function like a saw, tiger sharks have an incredible ability to consume large prey and even the seemingly indestructible carapace of giant sea turtles (Randall, 1992).

Aims
In a joint program with the Australasian Natural History Unit, CSIRO Marine Research (CMR) and Undersea Explorer, researchers are studying the movement patterns of tiger sharks in the far northern area of the Great Barrier Reef.
The scientists hope to discover:

• whether tiger white sharks travel between different areas within Australia, and whether they leave Australian waters
• where they go in different seasons and whether that changes between years
• what areas are important to them (such as feeding, breeding or nursery grounds)
• what pathways they follow, and the length of stay and frequency of visitation of particular places

Project Funding and Sponsorship
Funding for the shark research has been gained through the production of a number of documentaries and through private sponsorship.

Documentaries

• 'Shark Tracker' - Tele images and Digital Dimensions
• 'Natures Warzone' - National Geographic / TVNZ
• 60 minutes Australia

Sponsorship

• Undersea Explorer (Australia)
• TUSA (Australia)
• MAKO Sunglasses
• Australasian Natural History unit

Private Sponsors

• Bruce Paterson
• Jim Oates
• Diversion Travel