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EFFECTS OF DISTURBANCE ON FISH ABUNDANCE AND DIVERSITY ON ACROPORA TABLES AT THREE REEF LOCATIONS IN THE WAKATOBI
Introduction
Coral
reefs are diverse ecosystems influenced by abiotic and biotic interactions,
disturbances in this ecosystem can have both positive and negative influences on
these interactions and consequently on the diversity and abundance of life in
this environment.
Corals
of the genus Acropora make up a large
proportion of the biotic factors in a reef system, they can be found in a
variety of morphologies however, this study is focused only on tabulate forms of
Acropora. These corals provide a
sheltered environment for many juvenile and adult fish alike, many fish such as
butterfly fish feed on Acropora polyps, other species such as wrasse will feed
directly by biting off parts of the branches.
The
impacts of disturbances on Acropora morphology and consequently on the abundance and diversity
of fish associated with Acropora is
relatively understudied, this study therefore intends to investigate the effects
of disturbance on the morphology, fish abundance and diversity, fish location
within the tables and diurnal influences on fish species associated with
Acropora.
This
will be achieved by identifying the degrees and types of disturbances
experienced at three sites, monitoring the abundance and diversity of fish
species associated with Acropora, comparing morphologies of the Acropora at each
site, and finally taking diurnal readings.
The
sites studied were that of Kaledupa, a pristine coral reef site experiencing
minimal human impacts, Sampela, a degraded reef site heavily influenced by human
activities and Hoga, an intermediate site all located in the Wakatobi Marine
National Park, S.E Sulawesi, Indonesia.
Qualitative
observations and personal interviews with university researchers working in the
Wakatobi Marine National Park were used to estimate the types and degrees of
disturbance found at each site.
Acropora
tables located in two distinct areas of each of the three sites were chosen
haphazardly from depths ranging from 0.3m – 18m; fish abundance and diversity
were recorded using visual fish counts, fish sizes were also estimated and the
location of the fish (within, above or below) relative to the Acropora
tables was also noted.
Morphological
data was collected following fish abundance and diversity measurements to
prevent influencing fish behaviours or movements, this included branching order,
nearest neighbour, diameter, table height, table thickness and also depth
measurements. Salinity was measured using a hand held refractometer, eutrophic
depth was measured using a secchi disc, these measurements along with
temperature were taken at the beginning of each data collection.
Sedimentation rates were determined using standard sedimentation traps
placed at each site for a seven-day period.
Fish
census’s were carried out for each site using a standard visual fish census
technique, 50m transects were laid (monitored 2.5m either side of the transect
line) on the reef crest, flat and slope, species diversity and abundance was
monitored. Coral communities are measured along similar transects to the
fish census however, the data collected is done so using line intercept transect
techniques. 20 Acropora tables
were tagged at c.5m depth; fish abundance, size and diversity was monitored on
these tables over a.m. and p.m. periods to account for temporal variability.
Statistical analysis will include multivariate analysis, PCA analysis, cluster
analysis and ANOVA.
Disturbances
differ in their type and degree throughout the sites, Kaledupa is influenced by
physical disturbances of wave action coming from the Northern side of Hoga
Island, it is also influenced by bomb fishing, Sampela however, is more heavily
influenced by re-suspension of sediments caused by the shallow loose sediment
reef floor, bioturbation and the removal of the mangrove areas surrounding
Sampela. Hoga is an intermediate of
these two extremes it has currents arriving from all compass points which are
gentle, it is a no fishing zone therefore the main human influence is that of
diver impacts.
Numerical
data shows that the numbers of species does not vary between each of the three
sites however, the numbers of individuals at Kaledupa is much higher than those
of Hoga and Sampela.
The
branching orders of the three sites are similar; the most obvious morphological
difference as yet is the much smaller nearest neighbour data obtained for Hoga
in comparison to Sampela and Kaledupa.
Mean
table height is largest at Sampela, the total height of the Acropora is also
greatest at Sampela, Kaledupa and Hoga are similar in their measurements.
Eutrophic
depth is shallowest at Sampela, Hoga and Kaledupa show similar results, salinity
ranges from 30ppt at Hoga to 32ppt at Sampela, temperature shows little
fluctuations between sites and is averaged to 28oC.
It
is clearly evident from the tagged Acropora measurements that there is a diurnal difference in species
abundance and diversity, which is greatly reduced, in the evening measurements.
In
summary, it is the types of disturbance and the degree to which each reef is
exposed to these disturbances that influences the morphology of the Acropora
tables. Differences in the fish
size and their localities with regards to position on Acropora
may also be influenced by these disturbances, further statistical analysis will
clarify this data and provide a better understanding of the importance of Acropora
at each of the reef sites.
A final year
dissertation report will be completed by May 2003 by Suzanne Wilson, University
of Plymouth.