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Thesis - NSU Access Only
M.S. Marine Biology
David S. Gilliam
Kevin E. Kohler
Stony coral coverage data was compared from three different methodologies: diver colony length x width measurements (DLW), image colony length x width measurements (ILW), and image colony tracing (ITA). An archival data set consisting of diver measurements (DLW) and 30m2 belt transect photographs from 28 sites was used to compare image measurement (ILW) and traced area (ITA) coral coverage data. All image measurement and tracing were done using Coral Point Count with Excel extensions V3.3 (CPCe). Using diver measurements (DLW) as the standard for comparison, hypothesis testing, mathematical quantification, and error analysis was used to compare the data obtained from each methodology.
Diver visual acuity is superior to image resolution, but diver measurement proved to be highly variable. Conversely, image-based techniques proved to be very precise monitoring tools, but lacked the sensitivity of the human eye. On average, image measurements (ILW) reported 85.1±10.6% and traced areas (ITA) reported 55.6±7.7% of diver measurement (DLW) reported coral coverage (p. 32). Tracing precision (ITA) was greatest, followed by image measurements (ILW), reporting ≈14.7x and ≈4.6x more precision than diver measurements (DLW), respectively (p. 29). Additionally, an in depth-study was done on image colony tracing (ITA) to explore its potential as a monitoring tool. The detailed nature of tracing was highly susceptible to the effects of wide-angle photography. Using in situ images, traced area precision was 1.0-4.6% at the image center where lens distortion is minimized. Accuracy was greatly affected at the quadrat periphery where lens distortion caused a negative bias of 26.7%, however, objects centered in the quadrat resulted in a slight positive bias of 3%. Effects of image scaling and calibration associated with image tracing proved to affect estimated coral coverage less than 0.01%.
Initially this study was conducted under the premise that diver measurements were water-intensive and not necessarily efficient. Provided either image-based technique was able to approximate diver reported coral coverage, they could be efficient and economical alternatives for resource managers. The results of this study suggest that each of the three methodologies were not exact replacements for the other. Each could be substituted, but not without consequence. Since no single methodology will suit every monitoring or financial situation, selection should always involve the technique yielding the most statistically sensitive data. More importantly, the implementation of complementary, rather than exclusive monitoring methodologies, may prove to be the most reasonable solution.
Shaun M. Gill. 2006. Comparison of Stony Coral Coverate Data Obtained by In Situ Measurements and Image Analysis. Master's thesis. Nova Southeastern University. Retrieved from NSUWorks, Oceanographic Center. (120)