|
The Marine Conservation Society Seychelles, |
||
| incorporating the Shark Research Institute Seychelles. | ||
| HOME |
2. General considerations applying
to the use of 50 x 5 m benthic belt transects
A comprehensive methodological study by Mapstone and Ayling (1998) showed that, for visually assessing the abundance of discrete benthic organisms, such as corals and many mobile invertebrates, benthic belt transects measuring 50 x 5 metres often provide the least biased density estimates, a key concern for field surveys, particularly when considering the operational constraints of most survey programs. In the central Great Barrier Reef region, benthic belt transects of these dimensions have been used successfully since 1994 as part of the most detailed ecological field study undertaken to investigate the complex predator-prey relationship that exists between scleractinian hard coral communities and the crown-of-thorns starfish (A. planci) (Engelhardt et. al. 1997, 1999 & 2001). Similarly, 50 x 5 benthic transects have also been employed as part of an ongoing coral reef assessment program operating in Seychelles (Engelhardt 1998, 2000 & 2001). For the purposes of monitoring corals and other benthic reef organisms, both the selection of survey sites and the placement of individual transects within sites are haphazard at all times. Two replicate 50 x 5 metre (2 x 250 m2) transects are sampled at each site. Transects are placed at an oblique angle down the available site profile from as shallow as possible (typically 1-2 m depth) to a maximum depth of 15 metres. The placement of transects across the available reef profile and depth range ensures the collection of a representative sample that truly reflects the range of environmental conditions that exist at the site. In contrast to rigidly depth-stratified sampling (i.e. where two fixed depths have been pre-selected for transect placement), the abovementioned method avoids possible problems of missing data and, consequently, insufficient replication due to local-scale differences in individual sites’ profiles where exact matching pairs of depth-specific transects can not be obtained. To improve the accuracy of density estimates for all the ecological parameters recorded, the observer searches transects intensively as two 2.5 metre wide lanes. |
|
 |
3. Estimating hard coral abundance
(quantitative % cover estimates)
Reliable estimates of hard coral abundance may be obtained using either one of two widely accepted methods – the Line-Intercept Transect (LIT) methodology and the Visual Estimation Transect (VET) methodology. Both these methods have distinct advantages and disadvantages that should be considered prior to selecting one in favour of the other. The LIT method requires relatively little prior training as only a small range of benthic life form categories need to be identified and recorded. A disadvantage is that the method is quite time consuming and labour intensive. In contrast, the VET method requires generally higher skill levels and ongoing training to provide reliable estimates of the % cover of corals. However, the method is fast and efficient allowing for larger areas of reef to be assessed in the time available. Ideally, when sufficient and suitably qualified personnel are available, the two methods may be used in combination. 3.1 Line-Intercept Transect (LIT) methodology Line-intercept data are recorded within three separate 10-m long segments along each of the full 50 m long belt transects. Specifically, LIT data is recorded in the 0-10, 20-30 and 40-50 m segments along each transect. As a direct result of the transect placement down the reef slope at an oblique angle, the 0-10 m segments always correspond to the shallowest depths sampled while the 40-50 m segments correspond to the deepest section of the transects with the 20-30 m segment sampling benthic cover at intermediate depths. The following 9 benthic categories are recorded for each LIT:
Corals of the genera Acropora and Pocillopora are recorded as separate coral categories reflecting their general importance as key ecological components of Indo-Pacific coral reefs communities. A site-specific estimate of the % cover of respective benthic life forms is calculated using the data obtained from the 6 individual LIT segments sampled at each site (3 segments x 2 transects). For subsequent statistical analyses, individual LIT segments are considered as independent measurements as their respective starting and / or end points do not affect each others physical location. Furthermore, statistical comparisons of depth-specific (shallow vs. deep) estimates of % cover for all the benthic categories recorded may also be conducted. 3.2 Visual Estimation Transect (VET) methodology Visual estimates of live hard coral cover (% LHCC) and soft coral cover (% SOCC) within individual transects are recorded as '10%-range estimates' - that is coral cover is visually estimated as a % cover category with a 10%-range (i.e. 5-15%, 25-35%, 40-50% etc.). The only exception to this rule is the use of a smaller 5%-range estimate where LHCC or SOCC is found to be extremely low and is estimated to be less than 5% cover. Individual range estimates are recorded for each of the five 10-m segments that comprise a full 50 x 5 m benthic transect – namely the 0-10, 10-20, 20-30, 30-40 and 40-50 m segments. The mid-points of the 10 individual range estimates (5 segments x 2 transects) recorded at each site are used to calculate the mean percent cover for individual reef sites, (i.e. a midpoint of 10% is used where the range estimate is recorded as 5-15%). Throughout the analyses, the estimation error is set either at +5% (for all 10%-range estimates) or +2.5% (for 0-5% cover estimates only). This conservative error margin (which is typically higher than the calculated standard error would be) is chosen as it more accurately reflects the inherent spatial variability and patchy distribution of most coral reef organisms including corals (Mapstone and Ayling 1998). 4. Estimation of scleractinian coral diversity (qualitative description) Site-specific levels of scleractinian coral diversity are determined by locating hard corals occurring within the full 50 x 5 m transect and identifying them to both Family and Genus level. An overview of local-scale coral diversity is obtained from the resulting simple absence / presence data. |
|
| Last updated September 21, 2003 |
This paper is copyright and published with consent of the author ; it may not be copied or distributed without written consent. |
|
|
|
Comments or problems on this Web site to the |
|