Large-scale associations between macroalgal cover and grazer biomass on mid-depth reefs in the Caribbean

Since the 1970s, macroalgae have become considerably more abundant on many Caribbean reefs and overfishing of grazing fishes has been implicated as a contributory factor. We explored relationships between algal cover and grazers (biomass of herbivorous fishes and abundance of the sea-urchin Diadema antillarum) on mid-depth reefs (12-15 m) in 19 areas at seven locations in Jamaica, Barbados, Belize, Grand Cayman and Cuba, between April 1997 and April 1998. Diadema antillarum density was never > 0.01 m(-2), while herbivorous fish biomass (acanthurids and scarids greater than or equal to 12 cm total length) varied from 2-5 g m(-2) in Jamaica to 17.1 g m(-2) in Barbados. and was strongly correlated. negatively with macroalgal cover and positively with 'cropped' sub-stratum (sum of 'bare', turf and crustose-coralline substrata) cover. However. overfishing of herbivorous fishes alone cannot explain the widespread abundance of macroalgae. as even on lightly fished reefs, macroalgal cover was mostly > 20%. Herbivorous fish populations on those reefs were apparently only able to maintain approximately 40-60% of reef substratum in cropped states, but due to low space-occupation by coral and other invertebrates. 70-90% of substratum was available to algae. The abundance of macroalgae on lightly fished reefs may therefore be a symptom of low coral cover in combination with the continuing absence of Diadema antillarum.