[Coral-List] Best practice for LIT surveys
allison.billiam at gmail.com
Mon Jan 5 09:54:25 EST 2009
Vera and I seem to be talking about two different aspects of LIT surveys.
If I understand her methods correctly, Vera's results reflect variation
among contiguous but *spatially distinct samples* (ten sections of a 50 m
In contrast, my results reflect variation among *repeat surveys of the
"same" spatial location* (i.e., transects re-set as closely as possible in
the same location, using the same stakes etc each time). My results
demonstrate that repeat LIT surveys results can be both accurate and
precise, with very high statistical power, if transect lines are re-deployed
with care and surveyed using a plumb-line (1999 International Conference on
Scientific Aspects of Coral Reef Assessment, Monitoring, and Restoration,
NOVA, Fort Lauderdale, Florida, poster and oral presentations). Such repeat
LIT surveys can be useful for detecting change over time, require about 30
minutes per 10 m, have low capital costs and do not depend on expensive gear
that can cease to function (or worse) in the field or lab.
Vera, I am interested to know what roughly was the average slope of the reef
with respect to the horizontal and if your quadrats were set horizontal to
or in conformity to that slope.
On Sun, Jan 4, 2009 at 12:41 PM, Wera Leujak <wera.leujak at web.de> wrote:
> Dear Lindsay,
> we have tested six commonly used coral survey methods (line intercept
> transect, line point transect, mapping of quadrats, photoquadrats analysed
> in 2 different ways and video belt transects) on a Red Sea reef and found
> LIT to perform poorly in terms of precision and accuracy (Leujak & Ormond
> 2007, Journal of Experimental Marine Biology and Ecology 351, pp 168-187)..
> LIT overestimated hard and soft coral cover by 3 to 6% (live coral cover
> was 34%, predominantly large colonies) and we believe that this is due to
> what we termed the contour effect . Although our transect line was
> stretched taught and attached every 5m along the transect the tape still
> tended to follow the contours of larger colonies, thereby estimating the
> circumferential area of the coral rather than the smaller planar area (as is
> the case for quadrats that served as our baseline). Also, using LIT in the
> field was extremely time consuming (10m recorded during 1 dive of approx.
> 60min) and we had difficulties determining what substrate lay beneath the
> transect line, even when using a plumb line. We were often tempted to record
> a coral which might be directly below the line in case an error is made by
> omitting to record it.
> Power analysis indicated that the sample size required to yield an 80%
> chance of detecting even a 20% relative difference in total hard coral cover
> was 135m for LIT, with much greater sample sizes required to detect similar
> differences in the cover of individual coral growth forms. The poor
> repeatability of LIT was responsible for these high sampling sizes. We
> therefore did not recommend the use of LIT in a monitoring programme for Red
> Sea reefs but suggested photoquadrats instead.
> Best regards
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