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 Integrated Florida Reef Tract

Project Title

1.4. Role of Herbivorous Fishes on the Trophodynamics of the Florida Reef Tract

Key Investigators Robert K. Cowen, Su Sponaugle, Michelle J. Paddack
Project Duration December 15, 1999 – December 14, 2003
OBJECTIVE(S) There is evidence that herbivory is a major force moderating algal community dynamics on coral reefs. This project is designed to investigate “top-down” processes in the energy flow of the coral reef system of the Upper Florida Keys.  “Top-down” here refers to the impact of herbivorous fishes on the benthic algae they consume. Little is understood about these processes in reefs that have experienced a switch from coral-dominated to macroalgal-dominated reef. This study will help reef managers predict possible trajectories of reef-wide changes, the strength and variability of the forces acting on the reefs

Calculation of consumption rate requires four components of data collection on herbivorous fishes: size-specific density, length-to-weight relationships, bite rates, and bite yields.  Years 1 and 2 were dedicated to the first two components.  The focus of Year 3 was collection of bite rate data for each of the three major herbivorous fish families (parrotfish, surgeonfish, and damselfish).  Data were collected on feeding rate, foray size, and food targets for each hour of the day.  Patterns of daily feeding rates were established for each of five species and from this, average number of bites per day was calculated.  Data were also partitioned by size and life history phase of fish in order to determine how feeding rate varies within each species.  Feeding rate data were collected from three reefs, one of each of the three different reef types examined in Years 1 and 2 (high relief barrier reef: Molasses Reef, low relief relict reef: Pickles Reef, and patch reef: White Banks).  These data provide crucial information in the analysis of consumption rates, indicating variation among and within species and among different sites.

Hourly bite rate observations were conducted for five of the major reef herbivores: Stoplight parrotfish (Sparisoma viride), Redband parrotfish (Sparisoma aurofrenatum), Striped parrotfish (Scarus iserti), Queen parrotfish (Scarus vetula), surgeonfish (Acanthurus bahianus).  Diurnal grazing patterns were consistent over all species, sizes, and sites.  Fish began to move from sleeping areas at first light of day and slowly began grazing.  Grazing rates increased until 60-90 minutes after sunrise when bite rates reached a steady rate until 60 minutes before sunset when rates began to decline.  During the last hour of sunlight fish would graze intermittently as they searched for sleeping areas.  By sunset all grazing had ceased and did not resume again until sunrise.  Although overall daily grazing patterns were similar among species, average bite rate differed among species.  Bite rates were similar among species within genera: Sparisoma species had the lowest average bite rate (average 10 bites per minute) and Scarus species and Acanthurus had similar, higher rates (average 22 bites per minute).  The differences among species were maintained throughout the life history phases.   Bite rates did change over the life of a fish; almost all species had higher bite rates as juveniles and decreasingly lower rates as fish grew.  Terminal phase scarids were observed to feed less as they spent more time in territory defense and mating.

A comparison of our bite rate data for S. viride from Key Largo with published data from Bonaire (Bruggemann et al. 1994) shows a trend of lower bite rates in Key Largo.  Causes of these differences are being investigated.  One principle factor being investigated is algal cover.  Experimental work (McClanahan et al. 2000) suggests that grazing rates are higher in areas free of macroalgae.  Macroalgae on Bonaire reefs is sparse relative to Key Largo reefs (Steneck, unpublished).  Other factors such as topography and competitor density are being explored.

Significant differences in bite rates among sites in Key Largo were found for 4 of the 5 species.  Three patterns were noted: Higher grazing rates at Pickles reef relative to Molasses and White Banks (A. bahianus, Sc. iserti, Sp. aurofrenatum), lower grazing rates at Pickles relative to the other sites (Sp. viride), and lower grazing rates at White Banks relative to the other sites (Sc. vetula). In order to discern the cause of these differences (i.e. whether they are due to reef type), further study will be conducted in Year 4 comparing replicates of each reef type.

In our 2001 report, we reported consumption rates of S. viride.  These values were calculated with a combination of our own and published data.  The data collected in Year 3 completes a substantial step toward refining these calculations by providing us with bite rate data from Upper Florida Keys reefs.  The importance of collecting each component of the data from the same reefs is illustrated by the differences seen in bite rates between Bonaire and Key Largo.  Using our bite rate regressions in the calculation of consumption yields consumption rates that are 23-30% lower than calculations based on Bonaire grazing rates.  A final refinement of the dataset will be made this summer when bite yield measures will be obtained for herbivorous fishes from the Florida Keys. 

In summary, herbivorous fishes show similar patterns of grazing activity, however the actual bite rates differ among species.  We showed that grazing rate is influenced by body size and possibly by site.  Grazing rate may be a function of several possible factors: topographic complexity, percent cover/biomass of macroalgae, conspecific density, competitor density, predator density, and temperature.  Future efforts will be directed to exploring these factors.  Because grazing rate is a key component to the calculation of consumption rate, patterns of differing grazing rates may have important implications for the impact of grazing fishes upon the reef and may lead to differences in macroalgal cover among reefs.

Separate progress was made on work related to the recruitment dynamics of reef fishes. We deployed a series of nightly larval light traps in the upper Florida Keys to measure the onshore flux of late-stage fish larvae. Phototaxic late-stage larvae are attracted to and collected by light traps as larvae are settling to the reef. Coupling these data with current meter data collected by Tom Lee’s NCORE physical oceanographic group enabled us to examine in detail the nightly, monthly and seasonal variation in ichthyoplankton assemblages immediately over a shallow coral reef (Sponaugle et al. in press). We are currently preparing a second manuscript that demonstrates that the settlement of a large variety of reef fishes occurred in conjunction with the nearshore presence of a Florida Current spin-off eddy. This is the first direct correlation between a mesoscale physical feature (long proposed to play a role in the recruitment or reef organisms) and a distinct recruitment pulse (Sponaugle et al. in prep). These results will contribute to the further development of NCORE’s ReCONNECT program.


Bruggemann, JH, J Begeman, et al. (1994) Foraging by the stoplight parrotfish Sparisoma viride.  II. Intake and assimilation of food, protein and energy.  Marine Ecological Progress Series 106:57-71.

McClanahan, TR, K Bergman, et al. (2000) Response of fishes to algae reduction on Glovers Reef, Belize.  Marine Ecology Progress Series 206:273-282.


Sponaugle, S., Fortuna, J., Grorud, K., and Lee, T. (in press) Dynamics of larval fish assemblages over a shallow coral reef in the Florida Keys. Marine Biology.


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