Green2009-herbivore monitoring

Monitoring Functional Groups of Herbivorous Reef Fishes
Methods Details Modifications (if required)
Data
Analysis
Assign each species to a functional group
(see Table 1).
Calculate the area of each sampling unit:
For timed or long swims the area equals the
distance covered by each timed/long swim
multiplied by the transect width. The width
of each transect is 5m for small and medium
sized individuals, and 20m for large
individuals. The length of each timed swim
should be measured by marking the
beginning and end of each swim with a
differential GPS, and measuring the
distance between them using the GPS (or
attaching the GPS to a floating buoy that is
towed by the divers to track the distance
travelled).
For belt transects: the area of each transect
is 250m 2 for small and medium sized
individuals (50x5m transects), and 1000m 2
for large individuals (50x20m transects).
Data
Analysis
cont.
Interpreting
Results
Calculate the total density (per ha) and total
biomass (kg per ha) for timed or long
swims, and the mean density (per ha) and
mean biomass (kg per ha) for belt transects,
for all herbivorous reef fishes and each
functional group.
Analyse the size structure of all herbivorous
reef fishes and each functional group,
particularly the biomass of small (1kg) functional groups of
parrotfishes (scrapers/small excavators and
large excavators/bioeroders).
Analyse coral reef resilience using principal
component analysis based on three key
characteristics of the herbivorous reef fish
populations: total biomass of herbivores;
number of functional groups represented;
and biomass of large parrotfishes.
Results should be interpreted in terms of
what is “natural” for an area, and what the
results mean in the broader context of coral
reef resilience.
For timed or long swims: The average
distance covered by both observers in a
20 min period can also be estimated by
measuring the distance they swim in
5mins in a range of situations (current
speeds, high and low density of reef
fishes) and converting this to an
average distance covered in 20mins.
This method is less precise than using
a GPS, and the GPS should be used if
possible.
If it is not possible to use principal
component analysis, a visual
assessment of results may be useful
based on the same three key
characteristics of the herbivorous reef
fish populations.
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