Coral reefs have key species that control the community structure and composition, and exert influence on the stability and permanence of the reefs. When a key species is lost, there are important effects in the structure and diversity of the coexistent taxa. We analyzed the reef fish community in a period subsequent to the mass mortality event of the sea urchin Diadema mexicanum, reported in May 2009 in La Entrega, Huatulco, Mexico. Visual censuses were carried out monthly from August 2009 to July 2010 in order to record abundance and species richness. The data were used concurrently with data generated previously (February 2006-January 2007) in La Entrega using the same methodology in order to obtain community ecological Indices (Shannon´s diversity, Pielou´s evenness and Simpson´s dominance). Significant differences in fish assemblages regarding the periods before and after the die-off were tested using MDS, PERMANOVA, and SIMPER. The results showed a significant increase of abundance of fishes after the event of mass mortality, compared to the period previous to the event. There were significant differences in fish assemblages between periods.The dissimilarity between the periods was explained by the presence of Haemulon maculicauda (28.96%), Thalssoma lucasanum (22.17%), Selar crumenophthalmus (12.93%) and Stegastes acapulcoensis (11.44%). Our results suggest that the disturbance produced by the disappearance of D. mexicanum had important effects on the icthic community with feeding habits similar to those of the urchin, affecting more importantly to S. acapulcoensis and in less intensity to other herbivores such as Scarus gobban and Prionurus punctatus; meanwhile other fishes which are carnivores (for instance H. maculicauda) also increased importantly in abundance, suggesting that the disappearance of the urchin and in consequence its competitive effect, probably also propitiated a significant increase of other invertebrates which constitute the food source for carnivores. 

Keywords: trophic structure, community structure, competition, Echinodermata, Echinoidea, South Pacific.