Similar impacts of fishing and environmental stress on calcifying organisms in Indian Ocean coral reefs

Calcification and reef growth processes dominated by corals and calcifying algae are threatened by climate and fishing disturbances. Twenty-seven environmental, habitat, and species interaction variables were tested for their influence on coral and calcifier cover in 201 western Indian Ocean coral reefs distributed across similar to 20 degrees of latitude and longitude and up to 20 m deep. These variables predicted more of the total between-site variance of calcifying organism cover (similar to 50%) than coral cover (similar to 20%). Satellite-derived environmental variables of temperature, light, and water quality predicted more of the coral and calcifier cover than feeding interactions when groups of related variables were analyzed separately. Nevertheless, when simultaneously evaluating all variables, the environmental variables better predicted coral cover, but proxies of feeding interactions better predicted calcifier cover. Coral and calcifier cover were most consistently negatively influenced by sea surface temperature distributions (right skewness), but the orange-lined triggerfish Balistapus undulatus consistently had a strong positive association with coral and calcifier cover. Herbivorous fish and Diadematidae sea urchins were not positively associated with coral and calcifier cover. A primary prey of B. undulatus, the rock-boring sea urchin Echinometra mathaei, had a strong negative association with coral cover and particularly calcifier cover. Island reefs had higher calcifier abundance than fringing reefs, which probably results from high Acropora and B. undulatus but low E. mathaei abundance. When comparing all variables and models, these taxonomic associations had more influence than environmental stress variables on calcifiers. Given the important predatory role of B. undulatus in controlling E. mathaei populations, fishing restrictions on this species could help attenuate calcification losses predicted by climate change.