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Reef monitoring
Sampling design and methods
Program summary
 Program
design and sampling
methods
Broadscale surveys
Benthic video surveys
on fixed transects
Fish counts on fixed
transects
SCUBA searches on
fixed transects
References
Program design and sampling
methods
The AIMS Long-term Monitoring Program is designed
to detect changes in reef communities over time at a regional scale. In this
context, reefs in a region; are those that lie in one of three of
three positions across the continental shelf (inshore, mid-shelf, outer shelf) within one
band of latitude (a sector).
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Surveys by the Long-term Monitoring Program involve three tasks; manta tow surveys for crown-of-thorns starfish
(COTS) and reef-wide coral cover (broadscale surveys), surveys of sessile benthic organisms using video and visual counts of reef fishes. Broadscale surveys
cover reefs in 11 sectors. Reefs in six of the sectors are surveyed intensively.
 A map of Queensland and
the GBR showing
sectors or regions where data is collected. Sectors labelled in blue
have permanent survey sites. Those labelled in black are sampled using
broadscale surveys only.
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Sampling methods
The core survey reefs are sampled in two stages: The perimeter of each reef
is surveyed using manta tows. Fishes and benthic organisms are then surveyed
intensively at three sites in a habitat that is standardised across reefs. The
intensive survey sites are located in the first stretch of continuous reef (excluding vertical
drop-offs) to be encountered when following the perimeter from the back reef
zone towards the front reef in a clockwise direction. The sites are usually
situated on the north east flank of the reef. Sites are separated by at least
250 m where possible. There are five
50 m transects within each site. These transects were initially laid
haphazardly, roughly following depth contours with 10 - 40 m between them.
Transects are permanently marked with a star picket at each end and with lengths
of reinforcing rod at 10 m intervals. Transects run parallel to the reef
crest at about 6-9 m depth.
 A schematic diagram
of the sampling design on a single reef.
Broadscale surveys
AIMS began broadscale surveys of the Great Barrier Reef in the mid-1980s. These surveys were incorporated into the LTMP in 1992. The primary objective of the broadscale surveys is to detect and monitor populations of COTS on the Great Barrier Reef. Manta tow surveys also include estimates of percent cover of living hard corals, living soft coral and recently dead hard coral, allowing assessment of the impact of COTS outbreaks and other large-scale disturbances.
Percent cover of living hard and soft coral and dead hard coral is calculated from the manta tow results by representing each cover category by the mid-point of its range. Coral cover, the number of COTS per reef and the average number of COTS per tow are used to assess the outbreak status of each reef. There are four categories: Active Outbreak (AO), Incipient Outbreak (IO), Recovering (RE), or No recent Outbreak (NO).
An examination of manta tow data from reefs of all categories found that 90% of
reefs with active outbreaks supported >1500 COTS km-2.
This is approximately 0.22 COTS per two-minute tow. After consideration of the
relative costs of Type I and Type II errors, the criterion for an Active
Outbreak was revised upwards to 1.0 COTS per tow. This represents a starfish
density that is highly likely to cause net decline in corals. In this report the criterion of 0.22 COTS per tow is referred to as "Incipient outbreak" level.
Recent methodology studies
A complementary program undertaken by the Great Barrier Reef Marine Park Authority allowed for
comparison of LTMP manta tow estimates of COTS with those obtained from intensive scuba surveys. The results showed that broadscale surveys were effective in detecting the movement and speed of the current outbreak of COTS in the central GBR. Six reefs were defined as
out-breaking (AO and IO) as compared to 12 reefs using scuba searches. The study determined that an outbreak threshold of 0.1 COTS per tow would give an 82% agreement between the two methods (1). A second study undertaken as part of quality control by the LTMP compared live coral estimates between observers and with estimates obtained from video footage. Observations were within one AIMS category (see table 1) in 89% of cases between observers and 86% of cases for video data (2). These results suggest that broadscale surveys methods are effective in monitoring crown-of-thorns populations and for estimating coral cover with enough
accuracy to detect moderate to large scale disturbance and recovery.
Table
1 Percent cover categories for estimates of coral cover
collected during manta tow.
|
Category
|
% Cover
estimate |
|
0
|
0 |
|
1
|
0
- 10 |
|
2
|
11 - 30 |
|
3
|
31 - 50 |
|
4
|
51 - 75 |
|
5
|
76 - 100 |
For
more information about broadscale surveys
Standard
operational procedures number 1
Benthic video surveys
on fixed transects
Sampling technique
Benthic organisms are surveyed on the five marked transects within each site
on the core reefs. A 50 cm wide swathe is recorded along each 50 m
transect using a digital video camera held 15-20 cm above the substrate. Percent
cover of corals and other benthic categories are estimated using a point sample:
approximately 200 systematically-dispersed points are sampled from each video transect.
Percent cover estimates did
not differ significantly when the face-centered cube (5 points per screen) was compared to single
random points(3). Therefore that this still represents an efficient
sampling strategy for those using basic equipment configurations. A
methodological study established that 750 random points per transect represents
the "best" possible sampling intensity for transects filmed at this
focal distance. A comparison of mean and precision estimates for lesser number
of points indicated that at 200-250 points there was no significant difference
for any benthic group or lifeform(3). An example of power calculations for hard
coral at a sampling intensity of 200 points per transect follows; In 2000
Agincourt Reef had moderate coral cover at 37%. The minimum detectable
rate of change for this reef is ±10%. The power to detect a trend
occurring over the six years of survey is greater ±5%
(4).
Accuracy and observer variability\
Sample points on the video footage are typically identified to genus level
for hard and soft coral. Algae and other organisms are placed into functional
groups. A comparison of field identification and identical points on the video
footage found that 'Benthic groups’ (mean accuracy of 90% (±
8)) and ‘families of hard coral’ (91% (± 7))
were identified most accurately and least variably from video images. Life-forms
of hard corals proved to be the least accurate and most variable level of
classification with a mean accuracy rating of 74% (±
16) (4). Observer error was small relative to estimates of cover. For example,
if hard coral cover is 30%, the mean confidence interval due to both between-
and within-observer variability was 27.2% to 32.8% (5).
For
more detailed information about Benthic video surveys
on fixed transects
Standard
operational procedure number 2
Video
transects - frequently asked questions
Fish counts on fixed transects
Sampling technique
Fishes from a list of 191 species, representing 10 families, are counted on the
five 50 m transects at three sites on each reef. All species are
largely non-cryptic, easily identified underwater and include both
commercial and non-commercial taxa. Because the surveys span the annual
recruitment season, 0+ individuals are excluded from counts. These are
distinguished from adults by their small size and often distinctive colouration.
Large mobile fishes and damselfishes are counted separately on transects 5m and
1m wide, respectively. The general survey procedure at each site involves
an experienced observer swimming along the central transect line counting large
mobile fishes to 2.5m either side of the mid line. Absolute numbers
of target fish species are recorded using a pencil and underwater paper
(attached to a slate). An assistant swims approximately 10m behind the
observer laying a tape measure over the substratum along the centre line
of the transect. On completion of the five transects in which large mobile
fishes have been counted, the observer returns along the same transects (which
are now marked with a tape along the centre line) and undertakes counts of
smaller damselfishes (Pomacentridae). Observer swimming speeds average 10 metres per minute. In order to reduce observer bias,
at the end of every transect the observer identifies an object estimated to lie
at the outer edge of the transect. The perpendicular distance between this
object and the transect centre line is then measured, thus providing the
observer with a frequent reference to the desired transect boundaries. To
further limit observer bias, throughout the sampling season regular pair wise
comparisons are made between all observers used in this program.
Once field sampling is complete all data is entered onto a customised database.
This data is then checked against the original data sheets to ensure no
transcription errors have occurred. Abundance data for each fish species is
subsequently summed over the five transects at each site on each reef to provide
reasonable sample sizes for analysis and interpretation.
For
more detailed information about fish counts on fixed
transects
Standard
operational procedure number 3
SCUBA searches on fixed transects
SCUBA searches provide information on sources of coral mortality
to assist in interpreting trends in benthic cover on permanent sites. SCUBA searches are designed to provide a more detailed
picture of the causes and relative scale of mortality than is possible with either the manta tow technique or the video technique for sessile benthos. SCUBA searches are carried out on the
fixed transects used by the AIMS LTMP for visual fish surveys and video surveys of sessile benthos. A 2m belt (1m either side of the central tape measure) is visually searched along each 50m
transect and data recorded for the following categories (Table
2).
Table
2.
|
Category |
|
|
Crown of Thorns starfish |
Total count in 3 size classes |
|
Crown of Thorns starfish scars |
Total count |
|
Drupella sp |
Total count |
|
Drupella scars |
Total count |
|
White syndrome disease scars |
Total count |
|
Blackband disease scars |
Total count |
|
Unknown scars |
Total count |
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Coral bleaching |
Estimate of bleaching as a percentage of live coral cover on
the transect. |
For
more detailed information about SCUBA searches on fixed transects
Standard
operational procedure number 1
References
-
Miller IR and Muller
R (1999) Validity and reproducibility of benthic cover estimates
made during broadscale surveys of coral reefs by manta tow.
Coral Reefs. 18: 353-356.
-
De'ath Glen (2000)
Analyses of Crown-of-thorns Starfish Data from the Fine Scale
Surveys and the Long Term Monitoring Program Manta Tow Surveys.
CRC for the Great Barrier Reef World Heritage Area Technical
Report.
-
Christie, C and
Mapstone, B (unpub study) Investigations of Sampling Strategies
for Estimating Coverage and Diversity from Underwater Video Film
-
R.Ninio, S.Delean,
K.Osborne and H.Sweatman (in press) Estimating cover of benthic
organisms from underwater video images: variability associated
with multiple observers.
Standard operational procedures index
Standard
operational procedures
December 18, 2008
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