Reef monitoring

Program design and sampling methods
Broadscale surveys
Benthic video surveys on fixed transects
Fish counts on fixed transects
SCUBA searches on fixed transects
References

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).

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 6-9m depth.

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.

Sampling technique

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.

Manta tow estimates of COTS compared with intensive SCUBA search surveys

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. In Miller and Muller (1999) 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 outbreaking (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

Intensive survey sites

Benthic organisms and fishes are surveyed on the five marked transects within each site on the LTMP core reefs every second year and reefs where the effect of rezoning the GBRMP is investigated in the alternate years ( Sweatman et al , 2008)

Benthic video surveys on fixed transects

Sampling technique

Up until the 2006 field season a video camera was used to film a 50cm wide swathe for benthic transects. Since the beginning of the field season in 2007 the LTMP has used digital still cameras. Fifty photographs are taken at 1m intervals along each 50 m transect using a digital still camera held approximately 40 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 photo transect.

Christie and Mapstone (unpublished) used different sampling strategies to investigate whether or not percent cover estimates of benthos differed significantly. They found that percent cover estimates did not differ significantly when the face-centered cube (5 points per screen) was compared to single random points. Therefore using random points or fixed points to estimate the percent cover still represents an efficient sampling strategy for those using basic equipment configurations. This study also 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 fewer than 750 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 for the benthic transects 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% (4).

For more detailed information about benthic surveys on fixed transects

Standard operational procedure number 7

Standard operational procedure number 10
Photo 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 and the length of predatory fish has been estimated, 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
Drupellascars	Total count
White syndrome disease scars	Total count
Blackband disease scars	Total count
Unknown scars	Total count
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

Counts of juvenile hard corals

Juvenile hard coral counts provide information on the abundance and composition of genera on reefs and within subregions. This information may be helpful for predicting what changes in coral demography may occur in the following years.

Juvenile hard corals that are less than 5cm diameter are counted along a belt transect (length of dive slate (34cm)) for the first 5m of the transect. These counts have been conducted since the beginning of the field season in 2007. Within the

water quality program, small soft (octocorals) and hard (scleractinian) corals have been recorded for three categories forinshore reef monitoringsince 2004.

References

1. Miller, I.R. 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.
2. De'ath, G. (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.
3. Christie, C and Mapstone, B (unpublished study) Investigations of Sampling Strategies for Estimating Coverage and Diversity from Underwater Video Film
4. Ninio, R. Delean, S. Osborne K. and Sweatman H. (2003) Estimating cover of benthic organisms from underwater video images: variability associated with multiple observers. Marine Ecology Progress Series 265:107-116
5. Sweatman, H. Cheal, A. Coleman, G. Emslie, M. Johns, K. Jonker, M. Miller, I. and Osborne, K. (2008) Long-term Monitoring of the Great Barrier Reef. Status Report Number 8. Australian Institute of Marine Science, Townsville, Queensland, Australia. pp 369