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HERB I VO RO U S FI SH

ORITY ON

F1 1

RI

ABUNDANCE

RE

TH

EF

S FOR HEAL

What

What Is It?

BENCHMARK

Herbivorous fish are fish that eat plant material. Their

Maintain

Maintain current levels of herbivorous

key functional role on reefs, through their grazing

fish abundance (regional average of 3

activity, is to keep non-encrusting algae in check so the

to 6 large parrotfish/m2).

to

).

algae do not outcompete corals for space. Relatively

TA R G E T

high levels of grazing are necessary for corals to retain

a competitive advantage.

To be developed.

This indicator measures the abundance of herbivorous

RED FL AG

fishes and focuses on two key families: surgeonfish

To be developed.

(Acanthuridae) and parrotfish (Scaridae).

Why

Why Do We Measure It?

Herbivory is probably the single most important factor

also result in incidental coral loss, as they scrape and

influencing interspecific interactions on Caribbean

chew at the reef surface.) Large-bodied parrotfish are

reefs. Changes in herbivory rates can produce

very effective in cropping algae. Reducing or even

significant, rapid changes in reef community structure.

eliminating fishing of parrotfish and surgeonfish, in

However, directly measuring herbivory rates is difficult,

order to sustain high abundances, is essential for

so we often use herbivorous fish abundance as a

sustaining reef function and ecosystem integrity.

proxy indicator instead.

Production of algal biomass is estimated to be about 8

Herbivorous fishes, through their grazing and scraping

g (wet weight) per m2 per day on shallow patch reefs40.

activities, help constrain the growth of non-encrusting

If herbivores eat 10 to 20% of their body weight per

algae and affect algal distribution, abundance and

day in algae, then an herbivore biomass of 40 to 80

species richness. Their ability to keep algae cropped

g/m2 would be needed to maintain the algal biomass

provides corals better habitat to recruit and grow.

at equilibrium. This would be equivalent to three to

six moderately large (500-1000 g) parrotfish every

In the wake of the 1980s regional die-off of the

square meter (if they were the only herbivores).

herbivorous long-spined urchin (Diadema), herbivorous

fishes play an even more important role in grazing

How Do We Measure It?

How

macroalgae and reducing overgrowth on coral reefs.

Divers measure fish density and size along belt

In particular, parrotfishes have become critically

transects. For example, the AGRRA and MBRS

important herbivores, although they may not be able

methodologies recommend ten replicate transects, 2

to fully "replace" the functional role of the long-spined

m wide by 50 m long3,8.

urchin Diadema. Reefs where both herbivorous fish

and Diadema are prevalent tend to have much lower

A stationary visual census technique, where fish are

macroalgal overgrowth and provide a more conducive

counted for a specified time within a visual cylinder,

habitat for coral growth.

has also been used in the region, though not as

commonly41.

Herbivorous fishes may be able to keep macroalgae

in check on reefs dominated by hard corals (> 40%

Density (fish per unit area) and size together are used

coverage) and other sessile organisms. But on the

to estimate biomass.

low-coral-cover reefs that are common today (<

20% coral cover), the overwhelming abundance of

Herbivorous fish guilds are categorized according to

macroalgae exceeds the ability of herbivorous fishes

how they graze. Scaridae (parrotfish) are scrapers,

alone to graze down the algae. Thus many MAR reefs

Acanthuridae (surgeonfish) and Microspathodon

today are dominated by macroalgae.

chrysurus (yellowtail damselfish) are browsers, and

other Pomacentridae (damselfish) are denuders.

Grazers like parrotfishes and surgeonfishes are

especially effective herbivores because they occur in

large numbers. (At the same time, their feeding can

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