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 Leg
one
Leg
two
Acoustic
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Whale
sharks
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Whale
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feeding
Whale
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Biological
oceanography
of the NW Shelf
1999
|
Biological
oceanography
of the North
West Shelf 2002
| The North West
Shelf Biological Oceanography project continued in April
2002, with the AIMS research vessel RV
Cape Ferguson working in Exmouth Gulf, the
Woodside lease area WA-271-P (approximately 35km northwest
of NW Cape), and Ningaloo Reef. Each year from March to
May, whale sharks aggregate on the continental shelf off
the central Western Australian coast. At Ningaloo Reef,
whale sharks are often found swimming close to the reef
front, within a few kilometres of the shore and in water
of less than 50 m deep.
|
Towers at
NW Cape
|
|
A tourist industry based on snorkelling
with the sharks in this area has developed over the last
10 years and is now worth over $12 m annually to the local
economy of the Ningaloo region. Greater understanding of
both the nearshore and offshore biological oceanography
and the associated trophic links is desirable for sound
environmental management.
Spotter
planes were used to guide
tracking vessels to whale sharks
off Ningaloo.
|
|
|
With this is mind, this research cruise
was divided into two parts-
-
Biological
Oceanography and Krill
(Leg one)
– focussed on studies similar to those
performed in the summers of 1997/1998 and 1998/1999,
with the addition of light trapping for krill and
hydroacoustic work.
-
Whale
Sharks (Leg two) – focussed on the local ecology
of whale sharks near Ningaloo Reef, with the RV Cape
Ferguson working in tandem with the CALM vessel
which was able to rapidly intercept whale
sharks using a light plane.
An earlier field trip had deployed
oceanographic instrumentation to record seasonal and
event-based fluctuations in thermocline depth in response
to the regional wind field, and hence to infer the extent
of upwelling.
|
|
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Outcomes
and Future Directions
| Biological
Oceanography – The study of trophic linkages
between oceanic production and Exmouth Gulf is approaching
finality, with the field work undertaken in the summers of
1997/98 and 1998/99 having described the processes forcing
pelagic production in the area, and highlighting the
excess of production over consumption in the area. In 2002
measurements were conducted to quantify the proportion of
production potentially available to benthic food chains.
In 2003 the aim is to finalise this work in the area by
conducting experiments on benthic metabolism in Exmouth
Gulf.
|
|
| Krill
- The significance of krill in the ecosystem of
the North West Cape area and Ningaloo remains ill-defined.
Light trap catches in 97/98 and 98/99 were greatest during
December and January, with subsequent work in April 2001
demonstrating the occurrence of surface swarms of krill
off Ningaloo. Only very small krill catches were taken by
light trap during this present trip, and it was only at
the end of the trip that a single surface swarm of kill
during daylight hours was located. The factors regulating
the abundance of krill in the area are still not fully
understood, but the data collected so far suggests that
the role of krill as intermediaries between ocean
processes and the seasonal aggregation of whale sharks may
not be as significant as previously thought.
Woodside
lease area - Indications are that the physics
of the Woodside lease area is complex, with this area
appearing to be typical of oligotrophic tropical oceans
and so not intrinsically highly productive. However, this
area is certainly implicated in internal tide and internal
wave activity which previous work has shown to introduce
new nutrients into shelf-break areas resulting in episodic
high production. Occupation of this area for only three
days does not allow for longer term generalisations to be
made.
|
Swarm of
krill
|
|
Whale
Sharks - Hydro-acoustic surveys in the path of
acoustically tagged sharks enabled interpretation of their
daytime patterns of movement. It is suspected that the
erratic diving behaviour of sharks reflects a search for
chemical cues as to the whereabouts of zooplankton
aggregations. Future work will repeat these surveys in the
tracks of sharks with a hydrophone system
operating from the Cape Ferguson. The Critter-cam system
will also be used again in order to confirm feeding
behaviour. At the time of writing satellite tag
results are still being obtained from one whale shark. The
passage of this shark northward, through the Woodside
lease area and then out WNW to sea is an intriguing result
and one worthy of further investigation.
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