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Executive summary
The Great Barrier Reef World Heritage Area is of immense
aesthetic value and great economic importance supporting tourism and
fisheries worth more than $1 billion annually to the Australian
economy. Information on status and natural variability of populations is
essential for informed management of such an extensive area. The AIMS Long-term Monitoring Program is designed to provide information on
key groups of organisms (particularly crown-of-thorns starfish, corals,
algae, and reef fishes) on appropriate spatial scales over the length and
breadth of the Great Barrier Reef Marine Park (GBRMP).
This report represents the synthesis of monitoring data collected over
the last six years which required approximately 3600 person days at sea.
The report provides the first integrated analysis of patterns of change in
numbers of crown-of-thorns starfish, abundances of reef fishes and
cover of benthic organisms over most of the Great Barrier Reef (GBR).
New results are presented from 1996 to 1998 and these build on previously reported information. The results contained within this report
are intended as a primary source of strategic information for the
effective management of the GBR.
Broadscale manta tow surveys have now been carried out, over
the length of the GBR in 11 latitudinal sectors, for a continuous period of
13 years (1985-1998) and have played a significant role in our increased understanding of the crown-of-thoms-starfish (COTS) phenomenon.
The perimeters of 110 reefs were surveyed using manta tows in the 1996
field season, 74 reefs in 1997 and 108 reefs in 1998.
Intensive surveys on reefs in six sectors began in the 1993 field
season. Coral and fish are surveyed annually on fixed sites within one habitat
on each reef. Fifty-one reefs were surveyed in 1996, 45 in 1997 and 48
in 1998. For most reefs, these surveys have been conducted at least
five times in six years and consequently there are now sufficient data
to estimate temporal trends.
Major results are:
Crown-of-thorns-starfish
The percentage of reefs on the GBR with outbreaks of COTS
is increasing. Active or Incipient Outbreaks were observed on
14.8% of the 108 reefs surveyed in 1998. This compares with
12.2% in 1997 and 10% in 1996. The highest recorded percentage of reefs with Active Outbreaks over the 13 years of surveys was17.1% in 1987.
A similar pattern is seen in the overall density of COTS on
the GBR. The overall number of COTS per tow recorded in 1998 was
0.23. This has increased from 1997 and 1996 when the numbers
were 0.14 tow-1 and 0.11 tow-1, respectively. For comparison,
the overall number of COTS tow-1 on the GBR during the last
major COT'S outbreak in 1989 was 0.87. Details of their current distribution are as follows:
- The abundance of COTS is high in the Cooktown /
Lizard Is. sector but did not increase greatly between 1996 and 1998.
- There is evidence of increases in COTS numbers in
the Cairns and Innisfail sectors. This is consistent with a
southward drift and supports the hypothesis that secondary outbreaks
are caused by larvae being transported between reefs by the
East Australian Current.
- COTS numbers increased greatly in the Swain Reefs in1998, largely due to a dense population on Horseshoe Reef.
- There were no significant changes in COTS
abundance within the other seven GBR sectors.
Coral Cover
The salient changes on the Great Barrier Reef over the last
six years reflect the impact of cyclones and COT'S on reef communities and their subsequent recovery from
such disturbances. Key results found were:
- The highest mean values for cover of living coral on
the perimeters of reefs were found in the Cape Grenville sector
(43%) in 1996 and in the Pompey sector in 1997 (46%) and 1998 (43 %).
In recent years both of these sectors have been largely free
from recorded COTS outbreaks or severe cyclone disturbances
(Categories 3-5).
- Lowest values for reef-wide live coral cover were in the
Cape Upstart and the Cairns sectors (18% each) in 1996, the
Cape Upstart sector in 1997 (17%) and in the Cooktown / Lizard
Is. sector (17%) in 1998. These sectors have been affected by
both COTS and cyclones.
- Permanent survey sites on NE faces of reefs showed
that cover of hard coral was highest in the mid-shelf region of
the Townsville sector (47%) in 1996 and in the outer shelf region
of the Cooktown / Lizard Is. sector in 1997 (43.7%) and 1998
(52.4%). These regions have been observed recovering from COTS
and cyclone activity respectively.
- Coral cover on permanent survey sites was lowest in the mid-shelf region of the Cooktown / Lizard Is. sector in 1996 (18%),1997 (14%) and 1998 (15%): an area where there is current
COTS activity.
- Coral cover on the permanent survey sites increased over
six years in inshore and outer shelf regions of the Cooktown /
Lizard Is. sector, the mid-shelf and outer shelf regions of the Cairns
and Townsville sectors and the Capricorn / Bunker sector.
- Coral cover on the permanent survey sites declined over
six years in the inner region of the Swain sector. This was due to
a COTS outbreak at Reef 22-088.
- Currently coral cover on the permanent survey sites
is increasing in the outer shelf region of the Cooktown / Lizard
Is. sector and in the Capricorn / Bunker sector. Both these
regions have been recovering from storm disturbance.
- Currently coral cover on the permanent survey sites
is declining in the inshore region of the Cairns sector, possibly
due to COTS, and the rnid-shelf regions of the Townsville
and Whitsunday sectors, attributed to the influence of Cyclone
Justin in March 1997.
- The recent wide spread coral bleaching occurred after
most sampling in the 1998 field season was complete. The extent
of mortality due to bleaching will be assessed by the 1999 surveys.
Reef fishes
While many groups of fishes showed significant long term
and current trends in various regions, there were only a few instances where a majority of groups showed a consistent trend in a
region:
- The majority of larger, more mobile fish taxa showed
an increase in abundance over the six years of surveys in
the Capricorn / Bunker sector. Several groups, such as surgeonfishes, butterfly fishes and wrasses, continue to do
so. Coral cover has increased greatly in this region from very
low levels in 1989; the fish assemblages may be changing as the
coral communities recover.
- The majority of groups of reef fishes showed an increase
in relative species richness over the time of the surveys in the
outer shelf region of the Cooktown / Lizard Is. sector and the
Capricorn / Bunker sector. These are both regions where coral cover has increased greatly over that time.
- The majority of groups of reef fishes showed a declining
trend in relative species richness in the outer shelf region of
the Whitsunday sector. This was not associated with significant trends in coral cover but may be related to the effects of
Cyclone Justin.
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