New results have just been published from our work on fish biodiversity and community structure in the Western Indian Ocean. Published in the Journal of the Marine Biological Association of the United Kingdom, we found significant associations between fish functional groups and certain reef habitats. We identified five reef habitat types, distributed unevenly among the 4 countries studied.
Habitat A, dominated by hard coral, was distributed across all countries – Tanzania, Mozambique, Comoros and Madagascar. Habitat E, in which algal turf was dominant, indicative of high levels of degradation, was found in the Comoros, Tanzania and Mozambique. Habitat D, with high cover of fleshy algae, was absent from the Comoros. Habitat B, with high benthic diversity encompassing relatively high hard coral, turf algae and rubble cover was only present in Comoros and Madagascar. Habitat C, in which the cover of soft corals was highest, was restricted to Madagascar. We interpret habitat A to be the healthiest reef state while habitats D and E are degraded.
The hard- and soft-coral dominated habitats (A and C) had a high abundance of herbivorous small excavators. Detritivores and herbivorous grazers were particularly high in habitat B while corallivores and invertivores were high in habitats B and C. The highly degraded habitat D, likely reflective of a phase shift from coral to macro-algae, showed no linkage to any fish functional group. The semi-degraded habitat E was associated with a high abundance of planktivores and herbivorous grazers. Habitat B, with a diverse benthic composition but dominated by hard corals, had diverse fish functional groups.
Interestingly, degraded habitats D and E had the highest mean biomass of all functional groups combined. Biomass of omnivorous fish was high in habitat E while that of browsers peaked in habitat D. It is possible that the high biomass of omnivorous fish in semi-degraded habitats is due to their diverse diet, while fleshy algae may provide shelter to browsers. However, the role browsers play in fleshy-algae dominated habitats is still poorly understood.
These linkages can be useful in conservation planning to prioritize reefs that support diverse habitat types and diverse fish communities, or for specific targets for particular habitats and/or functional groups. The specificity of these linkages can help identify specific management interventions based on trophic relationships. For instance, reduction of land-based nutrient inputs to the sea coupled with area closures in habitats D and E may not only improve the water quality but also lead to an increase in functional diversity of fish, and phase shift reversal from algae- to coral-dominated habitats.