Western Indian Ocean 

Coral Reef Fishes

Disentangling Drivers of Abundance of Coral Reef Fishes in the Western Indian Ocean

A new study published 21 March 2019 in Ecology and Evolution journal shows that spatial and geomorphological histories remain a major influence on the structure of reef fish assemblages in the WIO. The above insights are pivotal for managers seeking to balance the long‐term sustainability of artisanal reef fisheries with conservation of coral reef systems.

This paper has been published under the creative commons attribution and is available at https://doi.org/10.1002/ece3.5044 for download. Similarly, data analyzed in this paper are archived in open format on Dryad.

This study was led by Melita Samoilys with co-authors Andrew Halford and Kennedy Osuka of Coastal Oceans Research &  Development in the India Ocean (CORDIO) with funding from Western Indian Ocean Marine Science Association (WIOMSA) through the Marine Science for Management Programme (MASMA). 

The study was conducted in four countries of the Western Indian Ocean (WIO) i.e. Comoros, Madagascar, Mozambique and Tanzania and aimed at understanding the drivers of the structure of coral reef fish assemblages for their future conservation. This was based on analyzing the patterns in density and biomass of an assemblage of reef‐associated fish species from 11 families, and their association with 16 biophysical variables



The analysis revealed strong country affiliations of reef fish assemblages with geographic location and reef geomorphology explaining 32% of the observed variation in fish assemblage structure. Productivity (chl, a) and reef exposure or slope were also important explaining 6%–8% of the variation.

Where spatial effects were not significant between mainland continental locations, fishing effects became evident explaining 6% of the variation in data. No correlation with live coral was detected. Only 37 species, predominantly lower trophic level taxa, were significant in explaining differences in assemblages between sites.

Heat map illustrating the spatial distribution and abundance (square‐root‐transformed) of the 37 species/taxa found to be significant indicators of the five cluster groupings from a total species list of 123 species. Significance derived from IndVal analysis (Dufrene & Legendre 1997). The 3rd and 4th groups illustrate the most diverse assemblages, while species that are ubiquitous spanning most sites are illustrated in horizontal bands
CAP ordination of (a) fish density and (b) fish biomass with (i) vectors indicating the relationship between significant* spatial and environmental variables and the fish assemblages from all 4 countries; and (ii) vectors indicating the fish species most influential in delineating differences between fish assemblages from all four countries (Pearson correlation >0.45). * Significance tested via IndSpecies package in R
  • Reef geomorphology is closely linked to standing biomass, with “ocean‐exposed” fringing reefs supporting high average biomass of ~1,000 kg/ha, while “lagoon‐exposed fringing” reefs and “inner seas patch complex” reefs yielded substantially less at ~500kg/ha.
  • Further, the results indicate the influence of benthic communities on fish assemblages is scale dependent.
  • The study also pointed out that 60% of the variation in fish assemblage remain unexplained and recommended the need for more spatially structured and controlled monitoring programs with better measures of fishing pressure to properly understand the influence of anthropogenic effects on coral reef systems.

Melita Samoilys acknowledges invaluable diving support from Bemafaly Randriamanantsoa, Saleh Yayha, Humphrey Mahudi, and Denis Macharia. We are grateful to David Obura for collecting the benthic data and to James Mbugua for mapping analyses and production of maps. MS was funded through the Perivoli Trust.