Pascale Chabanet

Disturbance impact on reef fish communities in Reunion Island (Indian Ocean)

Abstract Coral reefs in Reunion Island are subjected to eutrophication due mainly to anthropogenic activities following urbanisation of the watershed. This study assesses the impact of consequent coral reef degradation on fish communities. A quantitative study on fish communities was conducted on two sectors (undisturbed and disturbed) of a fringing reef. Data were collected using visual census techniques. The ichthyofauna is structured into three main communities (back reef, inner reef and outer reef). No significant difference was found between the undisturbed and disturbed sectors for the outer reef flat communities. Reef perturbation was found to affect fish abundance mainly on the back reef zone. In the undisturbed back reef zone, the overall fish abundance was almost double that in the disturbed one. The fish communities of the former showed higher homogeneity than the latter. Perturbation of the behaviour of the most abundant fish populations of the back reef zone is discussed with respect to their ecological preferences. Species may also be affected by disturbance, owing to their changing habitat and food availability.

Mass Settlement and Post-Settlement Mortality of Epinephelus Merra (Pisces: Serranidae) On Réunion Coral Reefs

An unusual mass settlement event of the grouper Epinephelus merra occurred on the fringing coral reefs of Reunion Island (south-western Indian Ocean) in April 1994. This major event coincided with stormy conditions associated with the new moon period. The abundance of new settlers was surveyed for 45 days following settlement on both the reef flat and the back reef area of the coral reefs of St Gilles, La Saline and St Leu. Mean densities of settlers were higher on reef flats (maximum 474 ind/20 m 2 at La Saline) than on back reef areas (maximum 156 ind/20 m 2 at La Saline). Seven weeks later, densities of settled E. merra had drastically decreased, and did not differ with site and zone (18.5 ind/20 m 2 ). Percentages of post-settlement mortality at that period ranged from 85 to 88% in back reef areas and from 93 to 95% on reef flats. A strong correlation between density of recruits and per capita mortality suggests density-dependent early post-settlement mortality for E. merra . The limitation of food resources and shelters, perhaps combined with an increase of diseases due to high population densities of settlers, was probably the main cause of mortality just after the mass settlement. Predation by reef fish, including cannibalism between settlers, is probably responsible for most subsequent mortality.

Spatial pattern of size distribution of four fish species on Réunion coral reef flats

A comparative study of the size distribution of four fish species was performed on three geomorphological zones (back reef, inner reef flat and outer reef flat) on two comparable fringing reef areas (disturbed and non-disturbed) of Reunion Island, S.W. Indian Ocean. The species chosen Chaetodon trifasciatus, Dascyllus aruanus, Stethojulis albovittata and Ctenochaetus striatus were selected both for their abundance and their diverse diets. The method used is based on the estimation of fish size directly underwater by comparison to a graduated table, i.e. photographs of the species for which different size-classes were previously determined. For the two areas, the size of fishes increases from the back reef zone to the outer reef flat, except for D. aruanus which is not present in sufficient numbers on the outer reef flat to allow a statistical analysis. This suggests the existence of a nursery zone in the back reef from which these species might colonize the reef flat. On the back reef zone and the inner reef flat, there is a significant difference in the size distribution of fishes between the disturbed and non-disturbed areas, except for S. albovittata. The number of juveniles is significantly lower in the disturbed area, probably due to environmental perturbations.