Information about the freshwater and estuarine species of fish of Buton Island is very limited and virtually nothing is known about their distribution or ecology. An field guide to the fishes of Western Indonesia and Sulawesi (Kottelat et al., 1993) contains keys and descriptions of more than 1000 species, but specific information about Buton is not included. The only other published work contains a preliminary checklist of 102 species of fish from Sulawesi that includes 13 new species and 6 species endemic to the region (Tjakrawidjaja & Hadiaty, 1996). This is a report on the results of the first fish survey to be carried out on the fish populations on Buton. In view of the prolonged geographical isolation of the Wallacea region and the relatively intact rainforest that still occurs on the island, this survey was both timely and important for the long-term management and protection of this unique environment.

The objectives were to:

• Conduct a survey of freshwater and estuarine fish on Buton Island.
• Develop a database on the numbers of each species at each location.
• To record standard lengths of all individuals and to preserve representative specimens for further taxonomic work at Balitbang Zoologi, Bogor and at the British Museum.
• To investigate possible relationships between the distribution of fish and environmental characteristics including flow rate, pH and salinity.

Fish samples were collected by electrofishing using portable equipment generating a potential of 400 volts between the fishing electrodes. The fish shocker was 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 either 30 or 60 minutes.

Seine nets were used to sample lakes, larger rivers and estuaries where suitable banks were available to land the catch. Two sizes of nets were used. The smaller net was approximately 1200mm deep and 18m long, with a mesh diameter of 15mm (measured as the maximum distance between diagonally opposite corners with the mesh extended). The larger net was  3m deep and 100m long with a mesh diameter of 20mm stretched mesh.

The position of each site was recorded using GPS. Water samples were collected for later analysis in the laboratory for calcium, nitrate and phosphate concentrations. Temperature, pH and water conductivity was measured using portable meters. Current velocity was recorded at a height equivalent to 60% of the water column depth using a digital flow rate meter. A general description of each site was made that included typical depths and widths of the watercourse, the presence of deep pools and riffles and the degree of disturbance/land use of the surroundings. The substrate was characterised using the Wentworth Classification of substrate particle substratum size.

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. Up to 6 individuals of each species were prepared as museum specimens. For this purpose, reference numbers stamped onto Dymo labels were attached to each individual with No. 8 cotton thread, either through the thin tissues that attach the jaws to the rest of the snout, or through the muscle on the ventral surface between the opercula. For Anguilliformes, labels were attached through the body musculature. Before fixation, specimens were pinned to pieces of foam in order to extend the dorsal, anal and caudal fins. After approximately 7 days fixation in 10% formalin (4% formaldehyde), specimens were transferred to 70% alcohol.

I am grateful to Dr Agus Tjakrawidjaja from Bogor Museum who provided invaluable help in the field and advice on species identification.

Total number of species: A total of 46 sites were sampled over a 6-week period between 31 July and 7 September 2001. The sites were selected to cover the widest possible range of freshwater and estuarine habitats. These included the rivers and streams around Labundo-bundo (some located in the Kakanauwe and Lambusanga forest reserve areas), rivers local to Maligano in North Buton and three lakes in the vicinity of Lasalimu. A total of 79 species were tentatively identified, representing 57 genera from 27 families. New fish, not previously encountered, were found in nearly every sample throughout the survey. The cumulative total number of species showed no sign of levelling out and continued to increase with each sampling day. This suggests that a complete species list for Buton will be considerably greater than 79 and is likely to exceed 100.

Many species could not be identified beyond the generic level for a number of reasons. In many cases the taxonomy of the relevant families is not developed sufficiently to allow reliable identification to species and it is clear that many families of fish from this region are in need of taxonomic revision. It is also likely that some of the specimens collected are undescribed species that are new to science. Further taxonomic work on the material deposited at museums in Bogor and London will be necessary before a definitive species list can be compiled. A provisional list is attached (Table 1).

The physical environment: Measurement of the physical properties of the water revealed the demanding environmental conditions to which fish are exposed in the lakes and rivers on Buton. Water temperatures ranged between 24.3 and 35.2 ° C and at these temperatures the amount of dissolved oxygen is very low and consequently, there was a predominance of species adapted to live in habitats subjected to low oxygen levels. Well represented in the collections were species of ‘sleepers’ and ‘gobies’ (Superfamily Gobioidei), ‘climbing perch’ (Superfamily Anabantoidei), ‘walking catfish’ (Family Clariidae) and a number of ‘eels’ (Order Anguilliformes). All these fish have specialised adaptations to survive in low oxygen tensions and some are able to obtain oxygen directly across the body surface. Although oxygen tensions are higher in fast turbulent water, in these environments fish must be adapted to withstand the strong currents. Gobies (Family Godiidae) have modified suctorial pectoral fins that can attach to substratum and members of this family dominated the fish fauna in fast flowing waters.

As the survey was carried out in the dry season, many riverbeds were either completely dry or contained slow flowing water and/or static pools. The average flow rate in the fastest flowing regions was 0.22 m sec ^–1. The highest flows were found in the steeper upland regions where the substrate was composed of boulders, cobbles and pebbles with an infill of gravel and sand. In rivers bordered by cultivation, or that flowed through disturbed areas where logging had occurred, the substrate was often dominated by fine deposits of sand and silt. In these rivers the fish community consisted of common species of ‘sleepers’ (Family Eleotrididae) and introduced species such as tilapia (Oreochromis sp) or catfish (Clarias sp).

In undisturbed rivers, several species of fish were found that are adapted to feed on insects and other invertebrates that fall into the water from the overhanging vegetation. The ‘halfbeaks’ (Family Hemiramphidae) are especially interesting because of their greatly extended lower jaw. One species (Nomorhamphus sp.) that was common in rivers in undisturbed forest could not be identified from the available keys and may be a new species.

The largest lake at Lasalimu was severely eutrophic and contained very large numbers of tilapia. The two other smaller lakes in the same area were also in poor condition.

Additional analysis and outcomes: The data collected on the environmental conditions at each site will be used to explore the different fish assemblages on Buton and help define the conditions that limit species distribution. This data will be analysed using PRIMER software.

Reports

New species will be formally described in the literature and a paper entitled An Initial Checklist of Freshwater and Estuarine Fish Species on Buton Island will be submitted for publication in The Journal of Fish Biology by Dr David Bird (University of West of England) and Agus Tjakrawidjaja (Indonesian Institute of Sciences) by may 2002.

Table 1: Provisional list of fish species collected from the rivers, lakes and estuaries on Buton. Note that species identification has not been checked. Uncertain identifications are marked with an asterisk.