Definition and Importance:
Herbivorous fish biomass refers to the mass of key herbivorous fish families, parrotfish and surgeonfish, in each reef area. These fish play a crucial role in maintaining coral health by grazing on algae that compete with coral. Healthy populations of herbivorous fish are essential for preventing algae from overgrowing and smothering corals. 

A shift in herbivory rates—due to a decline in herbivore populations, for instance—can lead to rapid and significant changes in reef appearance and function. When algae-eaters become scarce, reefs that were once vibrant with colorful corals can quickly become overrun by fast-growing algae.

The abundance of herbivorous fish is also influenced by the presence and effectiveness of their predators. For species like the rainbow parrotfish, their numbers also depend on the availability of mangroves, which serve as essential nursery habitats.

One of the strengths of this indicator is its responsiveness to management efforts. For example, reducing fishing pressure (a form of predation) can lead to an increase in herbivorous fish populations, as we have seen in the different MAR countries several years after these fish were protected in different areas on the reef. 

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How Do We Measure it?
Under the Atlantic and Gulf Rapid Reef Assessment AGRRA protocol, herbivorous fish biomass is measured through fish surveys conducted by divers along belt transects. Divers swim deploying a transect line, recording the species, size, and number of fish observed within a set area (e.g., 30 meters long by 2 meters wide with 10 transects per site). The collected data is then used to calculate biomass using established length-weight relationships for each species. This method provides consistent and reliable data on the abundance and biomass of fish populations.