| Summary: | Herbivores affect the species composition and quantity of algae on reefs and this in turn affects the overall benthic community structure as algae compete for space among various taxa including corals and sponges. The main groups of herbivores on reefs are sea urchins, such as Diadema antillarum (Phylum Echinodermata: Order Echinoidea) and fishes such as acanthurids and scarids (Phylum Chordata: Order Pisces). Populations of the sea urchin, Diadema antillarum suffered mass-mortality beginning in 1983 throughout the tropical northwest Atlantic and in many areas have still not returned to their pre-mortality abundances. The Florida Keys reef system supports herbivorous fishes that are largely unharvested and provides a unique system in which to examine the almost exclusive influence of fishes as reef herbivores. Diversity, density, size (length and biomass), and grazing intensity of 14 species of herbivorous fishes (Fam. Acanthuridae (3 species) and Scaridae (11 species)) were determined at three reef community types that varied in depth and physical relief and at one reef community type among seasons to better understand herbivory as a structuring force on reef communities. The greatest number of herbivorous fish species was recorded at shallower reef communities with greater physical relief. Density, biomass, and grazing intensity of herbivorous fishes were greatest at shallow, relict reef flat communities with moderate to low physical relief. Grazing rate varied seasonally with fish grazing more actively during the summer than winter while overall herbivore density remained constant. Medium-sized to small (average length of 5-14 cm) adult, and juvenile phase fishes had higher grazing rates than larger, terminal phase (average length of 14-31 cm) fishes. Experimental exclusion of herbivores resulted in a unique species composition and biomass of macroalgae among seasons, but not among reef community types. Macroalgal biomass was greater where herbivores were experimentally excluded, especially during the summer. These data provide ecological information for managers to assist in the development of management plans to protect populations of herbivorous fishes and the overall balance between corals and algae on reefs especially in systems where other major herbivores, such as sea urchins, are missing.
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