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U Engelhardt,Reefwatch Australia
M Hartcher, Reefwatch Australia
N Taylor, Reefwatch Australia
J Cruise, Reefwatch Australia
D Engelhardt, Reefwatch Australia
M Russell, Reefwatch Australia
I Stevens, Reefwatch Australia
G Thomas, Reefwatch Australia
D Williamson Reefwatch Australia
D Wiseman, Reefwatch Australia
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EXECUTIVE SUMMARY
Intensive transect-based surveys of crown-of-thorns starfish (Acanthaster
planci) and associated live hard coral cover were conducted on 19
mid-shelf reefs located in the Cairns and Central Sections of the Great
Barrier Reef Marine Park.
The 1999-00 surveys recorded a total of 3,996 A. planci on 19
reefs. In 700 benthic transects, there were 939 (23.5% of total) juvenile
starfish, 1,659 (41.5%) sub-adults and 1,398 (35.0%) adult starfish. The
dominance of sub-adults indicates that populations of crown-of-thorns
starfish (COTS) are growing on many survey reefs. This is likely to lead
to renewed active outbreaks of adult starfish throughout much of the survey
area.
Juvenile starfish (esimated age 1): The average density of juvenile
A. planci across all reefs surveyed in 1999-00 was 1.34 individuals
per 250 m2. This is the second highest density of juvenile starfish recorded
in the six years since the inception of the fine-scale surveys in 1994-95.
The highest average density of 3.30 starfish per transect across all reefs
was recorded in 1998-99. The 1998-99 estimate provided an early indication
of the renewed outbreaks currently developing on reefs in both the Cairns
and Central Sections of the Great Barrier Reef Marine Park.
Juvenile A. planci were recorded throughout the study area.
Rudder Reef (offshore Port Douglas) and John Brewer Reef (offshore Townsville)
recorded the highest numbers with 125 and 100 starfish, respectively.
Potentially unsustainable juvenile densities were found on reefs which
were both affected and unaffected by recent A. planci outbreaks. In areas
that had recently suffered significant starfish-induced coral mortality
(i.e. remnant live hard coral cover of <10%), we noted a preference
of juvenile and sub-adult starfish to feed on small recently recruited,
hard corals. These starfish cohorts could significantly impact the onset
and progress of coral recovery on starfish-affected reefs.
Sub-adult starfish (estimated age 2): As predicted after completing
the 1998-99 fine-scale surveys, densities of sub-adult starfish on many
survey reefs have risen significantly. Currently, the combined densities
of sub-adult and adult starfish are exceeding threshold levels on 11 (57.9%)
of 19 reefs surveyed. This has led to the re-classification of these reefs
as incipient reef-wide (IO) or incipient spot outbreaks (ISO). It is of
particular concern that incipient outbreaks were recorded on 10 mid-shelf
reefs that have experienced active COTS outbreaks in recent years. Renewed
outbreaks appear to be developing on these reefs, three to five years
after previous outbreaks. This casts significant doubt over the long-term
sustainability of the outbreak phenomenon in parts of the central Great
Barrier Reef (GBR) region. A lack of live hard coral cover on many of
these reefs may prevent the full maturation of the large 1997-98 cohort
of starfish in some areas. However, their current feeding activity is
already impacting coral recovery on reefs which have been previously affected.
Our observations during the 1999-00 surveys suggest that sub-adult and
juvenile starfish fed preferentially on small, recently settled, hard
coral recruits. This pattern is likely to impact significantly on the
recovery process, at least in the short term.
Adult starfish (estimated age 3 or older) exceeded sustainable densities
on seven (36.8%) of the 19 reefs surveyed in 1999-00, with two reefs (17-034,
17-047) recording actively outbreaking populations in both front- and
back-reef zones. On five reefs (16-068, 17-051, 17-064, 18-030, 18-031),
outbreaking populations of adult starfish were restricted to either the
back- or the front-reef zone.
The cumulative total of mid-shelf reefs and zones within these reefs
that have been (post-outbreak), are currently (active outbreak) or soon
will be (incipient outbreak) affected by COTS outbreaks has reached 100%.
Since the inception of the fine-scale surveys in 1994-95, none of the
core reefs or zones within these reefs that have been surveyed regularly
(ie. 39 reefs) have escaped the latest COTS outbreak episode. Fine-scale
survey reefs were initially chosen at random and are deemed to be representative
of approximately 100 mid-shelf reefs within the survey area. Therefore,
we suggest that, since early-mid 1990, live hard coral cover on virtually
all mid-shelf reefs between Lizard Island and Townsville is being affected
by the current outbreak phenomenon. Observations and reports from reef-users
also suggest that a significant number of outer- and inner-shelf reefs
adjacent to our survey area are being affected by increasing numbers of
A. planci.
Our most recent surveys have confirmed the widespread nature and effects
of the large 1997-98 cohort of A. planci on mid-shelf reefs in the central
GBR region. This cohort, identified by fine-scale surveys in 1998-99,
is the largest and geographically most widespread age class of A. planci
recorded on the GBR. As this cohort approaches maturity (age of first
reproduction is estimated to be reached by November / December 2000),
there is potential for further significant impacts on coral communities
throughout the central GBR region.
The 1999-00 survey results suggest that some mid-shelf reefs in this region
will soon experience renewed outbreak activity only three to five years
after the previous outbreak episode. This pattern suggests a possible
shift from the previously recorded 15 to 17 year gap of starfish outbreaks
in this region. A significant shortening of periodicity is likely to be
unsustainable in the long-term because hard coral communities on affected
reefs would not have sufficient time to completely recover and regenerate.
Increasingly chronic outbreaks of A. planci could result in many
permanently degraded reefs that are unable to recover from high-frequency
ecological disturbances. Apart from the obvious and serious ecological
implications, our findings also suggest there would be significant economic
impacts of such a scenario. The identified trends apply to reefs located
offshore Port Douglas and Cairns; the scenario outlined above would have
serious implications for the future operations and sustainability of the
regional reef tourism industry.
Consequently, efforts to assess any detrimental trends and conditions
should be given the highest priority. A long-term strategic response to
further studies of this possibly increasing outbreak frequency and/or
intensity is urgently needed. An increasingly chronic outbreak situation
could indicate an 'unnatural' (i.e. human-induced) process. Therefore,
there should be a renewed focus on evaluating the role of activities such
as fishing of the natural predators of A. planci and increased nutrients
in coastal waters as a result of regional land use patterns. We suggest
that scientific risk-assessment analyses should be carried out as a matter
of the highest priority. This scientific approach is the only feasible
way to determine the probabilities that the factors mentioned above are
contributing to the apparently changing characteristics of this important
phenomenon.
Intensive fine-scale monitoring of A. planci and associated live
hard coral cover should be continued and extended to maximise the chance
of identifying further reef degradation and reduced coral recovery rates
in the study area in a timely manner.
For a pdf file of the report contact CRC Reef on info@crcreef.com.
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