Scientific research topic 2006

INFLUENCE OF STRUCTURAL COMPLEXITY AND LIVE CORAL COVERAGE ON SELECTED FUNCTIONAL GROUPS OF REEF ORGANSIMS
Farfeleder Ingrid, Gith Rainer, Haberleitner Elisabeth, Kompatscher Sarah, Konz Astrid, Loretto Matthias-Claudio, Macherhammer Franz, Mausz Michaela, Mekis Matthias, Milosevic Gonzalez Marko, Peter Ilse, Schöpf Verena, Sonvilla Christine, Wascher Claudia, Wiesbauer Verena

ABSTRACT

This year marks the 12th annual coral reef ecology field course held by the University of Vienna in the Red Sea. The course – which took place between 23 October and 3 November 2006, consisted largely of a marine part, held for the 5th time in Dahab, Egypt, and two days of terrestrial excursions in the surrounding desert and mangrove ecosystems. One focus was on surveying two environmentally similar reefs differing mainly in scuba diving pressure. The goal was to examine the abundance and occurrence of dominant reef-associated fishes – herbivores (Scaridae, Acanthuridae), corallivores (Chaetodontidae) and omnivores (Pomacentridae, selected Labridae) – and to determine whether ecological factors such as habitat-substrate characteristics and structural complexity play a role in structuring the different functional groups.
A second focus was on the corallivorous snail Drupella cornus. This study was designed to determine potential correlations between the abundance, biomass and size of D. cornus and the infection rates and degree of destruction of coral colonies, specifically of the genus Acropora.
For this study, a series of 20 m x 5 m belt transects were laid at two depths (3 and 8 m) in four sites – three at the so-called “Islands” diving site (Sites 1,2,4) and one at “Rick’s Reef” (Site 3, less frequented by scuba divers). Special photographic ID cards were made to help identify the selected fish and coral species. The structural complexity and the substrate composition were recorded using point sampling along the transect lines (every 50 cm using a plumb).
We counted a total of 2645 fish from 5 different families. Pomacentrids were by far the most abundant group. Several fish families showed significant differences between certain reef sites, although the pattern was not consistent over all fish groups. Differences were also recorded between depths: the Acanthuridae and Pomacentridae, for example, were significantly different between zone 1 (3 m) and 2 (8 m). Finally, four coral-associated pomacentrids were most abundant at the more pristine site 3, which also had the highest coral coverage and exhibited the most Acropora colonies. In all four study sites, the shallow zone had a higher density of Acropora spp. than the deep zone. Study site 3 (Rick’s Reef) showed by far the highest density for both the shallow and the deep zone. A total of 470 D. cornus were collected. Most Drupella found (321; 68%) were in the smallest size class 1 (10-19mm). Length-weight ratios were calculated for 81 individuals and showed an exponential relationship. The larger size clases apparently preferred different coral species than the smallest size class. The number of Acropora colonies invaded by D. cornus was low throughout the different Acropora species, mostly under 10%. There was a significant correlation between invasion of colonies by D. cornus and colony damage.