Tracking of intermittent transmitters:
Permanent receivers, model Vemco VR1, were deployed underwater to receive
the signals from tagged animals. Receivers were sited at North Horn and sites
varying distances away to gauge residence time and movement of animals from
this site. Data is downloaded from the receivers via VR1PC interface software.
Results
Identification
Thirty individual whitetip sharks have been identified at North Horn.
Twenty-three have been tagged with AVID microchip identification tags,
measured and sexed. Six mature females and one male whitetip shark have been
implanted with internal ultrasonic transmitters.
Growth measurements
Repeated measurements taken over a period of 2 years show no increase in
growth for 13 individuals. Median length of the North Horn whitetip shark
population is 1.4 metres.
A particularly interesting aspect is the capture and measurement of two
very recently born individuals (approximately 0.7metres length) which if
recaptured will provide valuable growth data.
Tracking and movement patterns
The data logger receivers are able to detect the transmitters to a range of
approximately 300 metres. Tracking records show all five sharks to be present
within the small area of North Horn, for a high percentage of the daylight
periods between 0600 to 1800 hours
Presence at North Horn during night periods, between 1800 to 0600 hours, is
relatively low. During these periods tagged individuals are recorded passing
datalogger stations on the reef wall 1 km away and to each side of North Horn.
Figures 5 & 6 show representive 8-day periods during the 8 month recording
period to date for two different sharks.
Records from the west wall-2km site were for only one animal and extremely
infrequent. No records were observed for the west wall-5km or lagoon logger.
Sharks were rarely recorded at any sites other than North Horn during the
daylight hours. Individual records show consistent daytime presence at North
Horn, but with occasional periods of absence for days at a time. These periods
of absence do not correlate between the different sharks and no pattern is
obvious at this stage.
Summary:
The tracking patterns show that female whitetips are extremely
site-attached to North Horn during daylight hours. This corresponds to
reported nocturnal feeding behaviour of T.obesus and observations in the wild.
The detection of individuals at sites 1km distant from North Horn is
consistent with night activity to feed in the reef shallows.
The lack of detection at sites further than 1km and within the closest
lagoon point show a small home range for these females. Periods of daytime
absence from North Horn do not correlate between sharks, so no inference is
made to seasonal movement patterns. These absences could, however, be related
to breeding or birthing events
The tracking records must be considered only a starting point in predicting
home range and behaviour. With more data logger stations and a longer time
span it will be possible to accurately map the daily and seasonal movement
patterns of individuals.
The identification of birthing and nursery areas will be important in
conservation management. At present two recently born individuals have been
microchip tagged and further individuals will be valuable for wild growth
studies from birth.
It is widely believed that most shark fisheries operating today are
unsustainable. It is vital to have solid biological information on shark
species to instigate effective management policies.
