Richard Fitzpatrick - Digital Dimensions, Townsville QLD 4810
Introduction
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 allows the remote tracking of sharks using satellite technology to learn more about the home range and migration
patterns of these animals. Last year the CSIRO Marine Research successfully satellite tagged 'Neal' the Great White shark. Neal showed
a northern migration during the winter months.
The satellite tagging of the 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 least 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 this 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 some tigers
stay in the one location, while others have travelled up to 1,850 n. miles (Randall, 1992). It is however well known that tigers migrate into
higher latitudes from tropic seas during the warm months.
Occurring in a wide variety of oceanographic conditions tiger sharks 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 north-western
Hawaiian Islands. The shark ranged over an area of 100 km2 , travelling faster by day then night, 68% of the day activities 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 with readily coming inshore at night, however being rarely are seen in the shallows during the day.
Tigers do not school, have been observed to be associated in groups of two to six or more, but assembles as such maybe transitory or due to the
presence of food (Randall, 1992).
Food habits vary greatly with the availability of food for the tiger
sharks, and they consume a huge variety of animals than any other shark.
Rays, sea turtles, sea snakes, bony fishes, sharks, seals, sea birds,
dolphins, cephalopods, crustaceans, jellyfishes, all maintain part of
the tiger sharks food source. Even terrestrial mammals with the addition
of man made products, including indigestible plastics and metals. Due to
immense size and the course serrated of their sharp teeth, functioning
like a saw the tiger sharks have an incredible ability to consume large
prey and even the seemingly indestructible carapace of a giant sea
turtle (Randall, 1992).
Aims
In a joint program Digital Dimensions, CSIRO Marine Research
(CMR) and Undersea Explorer scientists are studying the movement patterns of tiger sharks in
far northern area of the Great Barrier Reef.
The scientists hope to discover:
ˇ
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whether tiger white sharks mix 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
ˇ how long they stay in and how frequently they visit particular places.
From 12-25 of February 2002 an expedition was mounted to the waters
surrounding Raine Island to satellite tag Tiger Sharks.
Project Funding and Sponsorship
Funding was
gained through the production of a television documentary being produced
by Tele-images (France). Film Projects (Australia) and Digital
Dimensions (Australia)
Sponsorship by Undersea Explorer (Australia)
and Mares (Australia).
Materials & Methods
On the 21/2/02 a 3.25m a female tiger shark was attracted to the
research vessel MV Floreat using floating bait of reef fish. Due to the
rough conditions it was impossible to attract the tiger close to the
boat. A barbless hook was used to pull the shark to the back of the
vessel where a tail rope was attached and the hook removed. Ropes were
then looped around her pectoral fins to raise the shark to expose the
dorsal fin. The tiger was measured and a tissue sample taken for DNA
analysis by JCU PhD Student Will Robbins.
A satellite tag was secured to
the dorsal fin by two bolts. The shark named 'Nicole' was released
within 10 minutes of capture.
The Satellite tag consists of an ARGOS satellite transmitter, long life
batteries, aerial and a saltwater switch, all in a waterproof plastic
pod. The pod was designed and made by engineers at CMR. It is
streamlined in design and weighs very little in water. This is so it
will not interfere with Nicole fin when she swims.
The transmitter sends a unique identifying code via radio waves to polar
orbiting satellites that are fitted with ARGOS receivers. The signal
from the tag is then sent to a receiving station in France where the
tag's position on the earth's surface is calculated. CMR scientists
receive the positions and plot Nicole's track. Radio waves do not travel
very well through seawater so the tag will only transmit to the
satellites when the aerial is out of the water. This is why the
scientists placed the tag on Nicole's dorsal fin, and why there is not a
position for Nicole every day.
On average, the tag transmits Nicole's position every two to three days.
The accuracy of the position depends on how long the aerial is out of
the water. The best positions can place Nicole to within 150 m of where
ever he goes in the world. The saltwater switch on the tag ensures that
it does not transmit when underwater, thereby saving battery power.
Results
The tiger shark tagged was 325 cm in length (to tip of caudal) and a
mature female (females mature from approximately 250 cm males mature
from 226 cm (Hennemann, 2001)). She had no distinct scars, but she still
had very dark obvious flank bars. Estimates of age and growth have been
made for this species of shark using the von Bertalanffy growth curve, a
tiger that is 300 cm long is about 15 years old (Randal, 1992) therefore
'Nicole' maybe close to this age. Yet without actual verification on
this particular specimen that this is only an estimation.
Nicole was tagged on the northern tip of the Raine Island in the far
northern section of the Great Barrier Reef (110 35.971 S, 140 03.428 E)
on the 22/2/02. When the shark was not on the surface long enough for a
location fix and signal is still sent indicating that the shark has
surfaced. Analysis of this surfacing data correlating to time and tides
is still to be carried out.
No transmissions where received from Nicole between 18th of April and
the 22nd of May indicating that she didn't surface during this period.
The area of the gulf in which she was last reported is very shallow.
Nicole may have changed her feeding preferences during this period to
focus on prey found on the seafloor such as rays and other bottom
dwelling species.
However it is interesting that being in such shallow
waters she didn't surface once.
Nicole was accidentally caught in a Barramundi fishing net on 22nd May
2002. The tag was reported to be free of fouling and still firmly
attached to the dorsal fin which showed no ill effects.
Future Research
Like the satellite tagging projects on the Great White Shark ('Heather'
and 'Neale') in Victoria 2000-2001, this research is looking to provide
a further insight into the activity patterns and migrations of the
elusive Tiger Shark.
Together with the future research via satellite
tagging more Tiger Sharks hopefully we will accomplish a strong
beginning towards answering the piles of unknown questions. It is well
known through game fishing records, tagging and telemetry efforts that
these sharks do migrate in the warmer months into higher latitudes and
hopefully a better understanding of their seasonal variation can be
achieved.
Additionally how far do they travel? How far if a all from
Australia? Do they migrate to certain feeding grounds, like the area
where 'Nicole' was tagged and released Raine Island or breeding grounds,
nursery areas. Do they follow patterns or visit certain areas
frequently?
Inevitably all research from different studies combined from
all aspects of ecology and biology can lead towards erasing
misconceptions but lead towards a better understanding and conservation
of the Tiger Shark.
A further expedition is planned for this coming summer to deploy a
further 4 satellite tags and a number of standard ultrasonic pingers.
References
Hennenmann, R.M (2001). Sharks & Rays, Elasmobranchs Guide Of The
World.IKAN - Unterwasserarchiv, Frankfurt Germany.
Last, P.R & Stevens, J.D (1994). Sharks and Rays of Australia. Fisheries
Research & Development Corporation.
Pepperell, J, West, J & Woon, P (1991). Shark Conservation. Proceedings
of an International Workshop on the Conservation of Elasmobranchs held
at Taronga Zoo, Sydney, Australia 24 February 1991
Randall, J.E (1992). Review of the Biology of the Tiger Shark
(Galeocerdo cuvier). Aust. J. Mar. Freshwater Res. 43: 33-43
the team
