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Video Image Analysis of Coral Reefs in Saudi Arabia: a Comparison of Methods

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Video Image Analysis of Coral Reefs in Saudi Arabia: a Comparison of Methods ©Verein zur Förderung der Paläontologie am Institut für Paläontologie, Geozentrum Wien Beitr. Paläont., 20:99-105, Wien 1995 Video image analysis of coral reefs in Saudi Arabia: a comparison of methods Video-Bildanalyse an Korallenriffen in Saudi Arabien: ein Methodenvergleich by Helge VOGT* VOGT, H., 1995. Video image analysis of coral reefs in Saudi Arabia: acomparison of methods.— Beitr. Paläont. 20:99- 105,1 Figure, 4 Tables, Wien. Abstract lines, indicated large differences in the absolute figures A video system for quantitative analysis of coral reef but little variation of the percentages. communities was employed for counts, length measure­ The main difference, however, is the time factor. The use ments and area calculations of corals and other reef species. of video images saves considerable amounts of expensive At Karan Island three transect lines of 50 m were placed underwater working time. Nevertheless a substantial on the reef at 2,4, and 6 m water depth and recorded by a amount of time is needed during image analysis. Further SCUBA diver with a S-VHS-C camera in an underwater advantages and disadvantages of the video system, as housing. For comparison reasons all transects were also well as its future use are discussed in detail. recorded in situ with the line intercept transect (LIT) - This study is a result of a larger programme which used method. In Manifa Bay the video system was employed video recordings to assess the health situation of coral for counts and measurements of the diameters of the sea reefs after the Gulf War. The more comprehensive findings urchin Echinometra mathaei. of this programme are reported elsewhere (VOGT, 1994). In order to assess the accuracy of area calculations, the areas of coral skeletons were measured on a digitizing Zusammenfassung board. The corals were then placed on an even sea floor and recorded with the video camera and a stereophoto Ein Videosystem zur quantitativen Analyse von Korallen­ apparatus. Area calculations were also performed on the riffgemeinschaften wurde für Zählungen, Längen­ same set of images used for the counts and length meas­ messungen und Flächenberechnungen von Korallen und urements. anderen Riffarten eingesetzt. In einem Riff nahe der Insel For image processing a Macintosh Hsi computer with an Karan wurden drei 50 m lange Transektleinen in 2,4 und installed frame grabber card was employed. Image analy­ 6 m Wassertiefe ausgelegt und von einem Taucher mittels sis was conducted by the use of the NIH Image programme. einer S-VHS-C Videokamera in einem Unterwasser­ In order to scale the video images quadrats, marks on the gehäuse gefilmt. Zu Vergleichszwecken wurden alle lines and laser beamers were applied. Transekte ebenfalls in situ mit der Transektmethode (LIT) A chi-square test for homogeneity of the pooled data aufgenommen. In Manifa Bay wurde das Videosystem zu accepts the null hypothesis that no difference exists bet­ Zählungen und zur Berechnung von Durchmessern des ween the transect method and the video methods in the Seeigels Echinometra mathaei eingesetzt. number of species counted. A comparison of the length Um die Genauigkeit von Flächenmessungen abschätzen measurements of the reef species indicated little difference zu können, wurden zunächst die Flächen von Korallen­ between the two methods. The area calculations of coral skeletten auf einem Digitalisierbrett ermittelt. Die Koral­ skeletons based on video images proved to be acceptable len wurden dann auf ebenen Meeresgrund ausgelegt und in comparison to both the results of the outlining on the sowohl mit der Videokamera, als auch mit Stereophoto­ digitizing board as well as to photogrammetry. A kameras aufgenommen. Darüberhinaus wurden Flächen­ comparison between the results of area calculations based berechnungen von Korallen an dem gleichen Bildmate­ on video images scaled by laser beamers and by marked rial durchgeführt, das auch für die Zählungen und Längen­ messungen verwendet wurde. * Fb. 9, Institut für Ökologie, Abt. Hydrobiologie, Universität Die Verarbeitung der Videobilder wurde mittels eines Essen, Postfach 103 764, 45117 Essen, Germany Macintosh Ilsi Computers mit installierter Digitalisierkarte Mailing address: CEMRINO, P.O. Box 187, 109 San Jose und dem Bildanalyseprogramm NIH Image durchgeführt. Extension, 6200 Dumaguete, Negros Oriental, Philippines, T.: + 63 35 225 3961, Fax: + 63 35 225 5563 Zur Skalierung der Videobilder dienten Quadrate, Mar­ ©Verein zur Förderung der Paläontologie am Institut für Paläontologie, Geozentrum Wien 100 Beitr. Paläont., 20, Wien 1995 kierungen auf den Transektleinen, sowie Laserstrahler. ‘Establishment of a Marine Habitat and Wildlife Sanctuary Ein Chi-Quadrat Test zur Homogenität akzeptierte die for the Gulf Region ’ -project. The major findings, including Nullhypothese, daß kein Unterschied besteht zwischen the analysis of more than 1500 video images, are reported den Zählungen mittels Transektmethode und Video­ elsewhere (VOGT, in prep.). This presentation focuses bildanalyse. Der Vergleich der Längenmessungen von on the methodological aspects of the video method and Riffarten erbrachte ebenfalls nur geringe Unterschiede provides comparisons to currently used recording tech­ zwischen den beiden Methoden. Die Ergebnisse der Flä­ niques. chenberechnungen von Korallenskeletten ergaben im Vergleich zu den Ergebnissen des Digitalisierbrett-Ver- fahrens und der photogrammetrischen Auswertung gute 2. Material and Methods Übereinstimmungen. Ein Vergleich der zur Skalierung Areas and times of investigations verwendeten markierten Leinen bzw. Laserstrahlen er­ gab drastische Unterschiede in den absoluten Werten, Most fieldwork was conducted at three parallel transect aber sehr geringe Unterschiede in den prozentualen An­ lines of 50 m at Karan Island in Saudi Arabia during June teilen. 1993 (Fig. 1). However, the data of the sea urchin counts Trotz geringer Unterschiede in den Ergebnissen bestehen derive from fieldwork conducted in Manifa Bay, Saudi gravierende Unterschiede in den Zeitaufwendungen. Das Arabia, during August 1991 (Fig. 1). The comparison Videosystem ermöglicht erhebliche Einsparung von teu­ study between the areas of corals obtained with a digitizing rer Unterwasserarbeitszeit, erfordert allerdings viel Zeit board, the video method and stereophotography was für die Bildanalyse. Weitere Vor- und Nachteile des Video­ conducted at the Australian Institute of Marine Science systems sowie dessen zukünftige Einsatzmöglichkeiten during August 1990. werden diskutiert. Diese Untersuchungen wurden im Rahmen eines um­ fangreicheren Programmes zur Evaluierung der Auswir­ kungen des Golfkrieges auf die Korallenriffe durchge­ führt. Über die umfassenderen Ergebnisse dieses Pro­ grammes wird an anderer Stelle berichtet (VOGT, 1994). 1. Introduction In order to study changes in coral reefs accurate meas­ urements of important biological factors like e.g. coral cover are essential. For this purpose various labour and time intensive methods using transect lines (e.g. LOYA, 1978; ROSS & HODGSON, 1981; SY et al., 1981) and quadrats (e.g. SCHEER, 1974, 1978; GOMEZ et al., 1981;MERGNER&SCHUHMACHER, 1974;SCHUH- MACHER & MERGNER, 1985) are currently in use. All these methods require that the measuring of corals is conducted in situ. DONE et al. (unpublished 1989) were the first to use video recordings for a survey in a coral reef area. In their study the video tapes were stopped and the species in the middle of the screen were scored. Since then video recordings have contributed to the monitoring f igure 1: Important Saudi Arabian islands with surrounding of threatened reefs on Hawaii (HUNTER & STEPHEN­ coral reefs. SON, 1991). Information about video technology, includ­ ing the use of video for quantitative surveys, image analysis The video recording and image analysis system using computers and time lapse video were provided by MANEY et al. (1990). Estimations on coral cover based Most of the video recordings were obtained by using the on video images were compared with traditional methods Super-VHS-sy stem which provides visibly superior image by UYCHIAOCO et al. (1992). quality to the ordinary VHS-system and allows copies to So far the use of video in coral reef studies has been be made without noticeable quality loss. A Panasonic S- limited to the counting of specimens and estimation of VHS-C camera in an underwater housing (mb sub, Inge­ coral cover. This study demonstrates and introduces the nieurgesellschaft für Unterwassertechnik mbH, Xante- calculation of coral areas and the measuring of distances nerstr. 105, 5000 Köln 60, Germany) was employed for using digital image processing. Most of the fieldwork for all fieldwork in Saudi Arabia; in Australia a VHS-system this investigation was conducted in the framework of the was used. ©Verein zur Förderung der Paläontologie am Institut für Paläontologie, Geozentrum Wien VOGT, H., Video image analysis 101 The recording distance was approximatly 0.5 m for all of 10 cm were placed on the 2,4 and 6 m depth contour fieldwork in Saudia Arabia and 1 m in Australia. line. The coral cover underneath the lines were analysed The analysis of the video images was conducted by the for a comparison between the results of the traditional use of a Macintosh Ilsi computer with an installed ‘Screen LIT-method, the video analyses using marked lines and Machine™’ frame grabber card. The public domain soft­ video recordings with laser beamers. The LIT-method ware ‘NIH Image’ (LENNARD, 1990) was employed for (LOYA,
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