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THE INFLUENCE OF FOREST DISTURBANCE ON FISH DISTRIBUTION
Objectives
1)
To sample 15
sites and to rate them according to levels of disturbance.
2)
To compile a
checklist of all species within these sites and to establish a link between the
species found and the disturbance rating.
3)
To study
individuals from 5 key species from these sites and attempt to establish a link
between morphology and the disturbance rating.
A
field guide to the freshwater fishes of Western Indonesia and Sulawesi (Kottelat
et al., 1993) is the main source of
information for identifying freshwater fishes on Buton.
In 2001 a survey was carried out on Buton Island with a view to
establishing a checklist of the freshwater and estuarine fish and has been
continued this year.
Further
research is required on the factors affecting species distribution.
Fish are greatly affected by the land that surrounds the rivers in which
they live, and the vegetation can vary dramatically from primary forest to
agricultural land. Some species are
affected by such variations more than others.
The ‘halfbeaks’ (Family Hemiramphidae) for instance, rely on insects
that fall from overhanging vegetation and are therefore unlikely to be found in
a disturbed forest with little canopy cover.
The canopy will also influence the amount of sunlight reaching the river,
raising or lowering water temperature and this can be the major factor in
determining oxygen tensions in the water. Some
species such as eels (Family Anguillidae) are able to deal with low oxygen
levels far better than others. Furthermore,
agricultural land such as paddy fields or cocoa plantations may influence water
quality through chemical runoff of fertilisers and pesticides.
Increased disturbance around rivers will also have subtle influences on
the food chain, particularly the invertebrates on which fish feed.
The distribution of some species may therefore be restricted by the
availability of suitable food.
It is
conceivable that if disturbance can determine the type and number of species
found in a stretch of river, it may also influence the morphology of those
species. For example, a stretch that has been cleared for agricultural
purposes may suffer from increased runoff, particularly in the wet season.
Flood hydrographs for deforested basins exhibit a higher peak early on in
the rain event causing higher flow rates. Fish
inhabiting faster flowing streams may possess fins with higher aspect ratios
than those in slow flowing regions.
Method
Fish
samples were collected by electrofishing using portable equipment that generates
a potential of 400 volts between the fishing electrodes.
The fish shocker is powered by a 24 volt battery and set to generate
pulsed DC to the electrodes. To
standardise fishing effort between each site, three operators fished
continuously for 60 minutes. At a
few sites, where access was difficult or where battery life was a limiting
factor, samples were collected over 30 minutes.
Fish
were anaesthetised in benzocaine and the Standard Length of all the fish in each
sample recorded. Fish were
identified to species where possible (Kottelat et
al., 1993) and representative individuals photographed alive.
50 individuals
of 5 key species were chosen for morphological study;
Rhyacichthys
asporo
Eleotris
melanosoma
Ophieleotris
aporos
Glossogobius
spp.
Sicyopterus
spp.
A total of
26 measurements and observations were taken from each individual including total
length, caudal fin length, pectoral fin length etc. using a pair of digital
vernier callipers.
The
position of each site was recorded using GPS.
Temperature, pH and water conductivity was measured using portable
meters. 3 water samples were taken from each site to measure
turbidity levels. A general
description of each site was made that included typical depths and widths of the
watercourse and the presence of deep pools and riffles.
The substrate was characterised using the Wentworth Classification of
substrate particle substratum size. This
system also serves to indicate flow rate, i.e. the smaller the substrate the
faster the flow.
Under
normal conditions electrofishing would usually cover around 100m of river.
The forest survey was therefore carried out along a 120m section, 0m
being the point at which fishing began. At
30m intervals along the stretch a 10m by 10m transect was marked out,
alternating between left and right banks. In
each transect a total of 11 observations were made in order to assess the
structure and condition of the forest.
Such observations included the presence or absence of; fallen trees,
stumps, fire, lianes, bole climbers, ferns, epiphylls and the height of the moss
line and canopy. All trees within
the transect with a girth over 31.4cm in diameter were measured and recorded.
The coverage of ground, scrub, understorey and overstorey was rated
according to the Braun-Blanquet scale. Percentage
canopy cover was also measured using a transect at 30m intervals from the centre
of the river. Any other relevant
observations were noted such as the presence of agriculture or settlement. This data was then used to rate each site according to the
level of disturbance. Attempts were
made to sample sites with a range of disturbance, from virtually primary forest
to intensely cultivated land.
Nearly 600 fish have been caught and recorded. Morphological data has been collected for 200 of these individuals. General observations at this stage are;
- Lower diversity of species in primary forests compared to more disturbed land
- Eels (Family Anguillidae) found in almost all sites
- Halfbeaks (Family Hemiramphidae) found mainly in undisturbed forest
Final
report title
A
dissertation entitled The effects of forest disturbance on freshwater fish
distribution and morphology, Buton Island, Indonesia
will be produced by Matthew O'Brien, University of Leeds by May 2003.
References
A J. Whitten, M. Kottelat, S. Wirjoatmodjo, S. Nurani Kartikasari. (1993). Freshwater fishes of Western Indonesia and Sulawesi. Periplus Editions (HK) LTD.