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SUMMARY |
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.
References
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.
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