The capture fishery and aquaculture sectors play an important role in the Indonesian economy through increased income, diversification of livelihoods, supply of animal protein, and foreign exchange earnings. In 2015, the total fishery production of Indonesia was 16,954,344 metric tons, of which 455,270 metric tons was obtained from inland capture fisheries, 6,065,060 metric tons was obtained from marine fisheries, and 10,074,014 metric tons was obtained from aquaculture fisheries production ¹ . A total of 1,300 fish, including 40 endemic species are known to inhabit in the freshwaters of Indonesia, with 16 exotic species recorded in Indonesia ² . The production from the inland capture fisheries of Indonesia comes from wetlands (rivers, lakes, swamps, oxbow lakes, floodplains, etc). In Riau Province, one of the rivers that produce freshwater fish from capture fishery is the Kampar Kanan river. At present, major Cyprinidae species such as Leptobarbus hoevenii, Osteochilus haselti and Rasbora argyrotaenia, along with exotic species, such as common carp ( Cyprinus carpio) and Nile tilapia ( Oreochromis niloticus), are the available species of fish at local food markets.

According to Mulyadi ³ , Kampar Kanan river is one of the largest rivers in Riau Province. It is approximately 213.5 km long and between 125 to 143 m wide, with significant capture fishery potential. Since 1996, this river has been dammed into a reservoir (Koto Panjang Reservoir) for the operation of a 114 MW hydroelectric power plant. The dam height is 96 m and located at altitude of 85 m above sea level ³ , and at the geographical position 0 ^o17'23.76˝N and 100 ^o52'53.39˝E. However, at the location of the dam, there is no fishway. The abiotic and biotic characteristics of river ecosystems can be affected by the construction of dams. These conditions have an impact on mortality and failure of fish migration ^{4– 7} . The hydrologic regime of streams changing from lotic to lentic can influence the water retention in the reservoir. In general, the lentic condition causes a decrease in native species and then an increase in exotic species ^{8, 9} . Furthermore, the degradation of aquatic fauna habitats can be caused by an increase in homogeneity of water channels, which has an effect on the seasonal flow variability of river ¹⁰ . The reduction in river runoff also affects the habitat and distribution of fish fauna.

On the other hand, there are serious threats to the original fish biodiversity in the downstream and upstream regions of the reservoir due to the dam of the hydroelectric power plant, such as sand mining in river, land use change and aquaculture activity with cages, which can affect the depth of river water, food availability, and breeding sites. Amadi et al. ¹¹ state that original biodiversity can be eroded by habitat degradation and alien species impact. Meanwhile, aquaculture heavily impacts the structure and diversity of local fish communities ^{12, 13} . Hence, it was essential to study fish diversity continuously in different ecosystem areas, including upstream and downstream areas at Koto Panjang Reservoir, Riau Province, Indonesia.

Our results showed that the highest abundance and diversity of fish species collected in June, July and August, which may be due to a lesser water current in the study area during these dry season months. According to Kriaučiūnienė et al. ²¹ the abundance of fish species in rivers can be affected by river discharge. However, future alterations in river water temperature will have a significantly larger influence on the abundance of fish than river discharge. Overexploitation and illegal sand mining have affected the abundance of fish in the Kampar Kanan river ²² . Furthermore, the fundamental measures of aquatic ecosystems, including species richness and diversity indices, are influenced by the alterations in abiotic factors such as river water temperature and discharge ^{23– 26} .

Our study also revealed significantly lower species diversity in KM and BB (upstream reservoir) compared to that of RB and KK (downstream reservoir). The overall richness in KM was much lower than that found in RB (18 vs. 37). During the research period, we also recorded a slight increase in exotic species in the upstream reservoir and a decrease in native species. In contrast, in a Portuguese reservoir, it was found that there was a slight increase in exotic species in the downstream reservoir ⁴ . This result might be a consequence of cumulative impacts of cultivation of fish in floating net cages, such as Nile tilapia and common carp. According to Russel et al. ²⁷ , Nile tilapia cause the extinction of native fish species by preying on eggs, fry and small fish of other species. On the other hand, the decrease in species richness can be influenced by aquaculture activity, such as water quality degradation, intensified competition, invasive species, and habitat fragmentation ^{11, 28– 30} . Koto Panjang Reservoir was categorized as eutrophic, with level index values ranging from 4.6-5.2 ³¹ . According to Edwards ³² , species with high environmental tolerance would be survive in poor environmental conditions, such as high pollution caused by food waste at the reservoir, while the sensitive species will disappear. Furthermore, fish populations in a reservoir can be affected by hydropower dams ^{3, 4} . In addition, the intensifying competition for food and space between wild species and the large number of cultured species will lead to a decrease in wild fish numbers ^{33, 34} .

Our study confirmed the existence of three species, Pangasius pangasius, Wallago leerii and Chitala hypselonotus, at Koto Panjang Reservoir that were not found previously by Warsa et al. ³⁵ and Krismono et al. ³⁶ . The study area also represents the area of the Kampar Kanan river with the largest fish species, including W. leerii, H. wyckii and P. pangasius. The construction of the dam for power generation purposes is posing serious threats to the biodiversity of Kampar Kanan river. After the construction of the Koto Panjang dam on Kampar Kanan river, the movement of fish upstream has been restricted. Osteochilus kelabau has not been reported at RB in our study although this species has previously been found in this area ³⁷ . Similarly, the population of H. wyckii has also been dramatically reduced and restricted between Koto Panjang Reservoir barrages and Kampar Kanan river after the construction of these barrages. H. wyckii in the Kampar Kanan river is categorized as ‘vulnerable to endangered’ ³⁸ . According to Piria et al. ³⁹ , the disturbances to the fish assemblage pattern have coincided with the presence of multiple stressors of human origin, such as pollution, flood protection and dam construction. Meanwhile, dams can of change the hydrological dynamics, patterns of biological production, loss of native species in the downstream regions and distribution of organisms in space and time ^{3, 5, 10, 40, 41} .

There are a number of inadvertently introduced fish species in the upstream and downstream Reservoir, such as O. niloticus, Cyprinus carpio, Leptobarbus hoevenii and Pangasius hypophthalmus, while rest of the 40 species belong to the native fish fauna of the Kampar Kanan river ³⁷ . The unique feature of the abundance data was that the exotic family Cichlidae is well established and its population is increasing day by day, especially in the upstream areas at Koto Panjang Reservoir. In recent years, the Koto Panjang Reservoir has had very important roles, such as housing power plants with a capacity of 114 MW and serving as a fishery capture and aquaculture area with floating net cage farming, especially for the cultivation of O. niloticus. Cichlidae was the third most abundant family. These alien fish species represent a significant risk for the local fish community and other aquatic animals. Gu et al. ³⁰ state that the invasion of O. niloticus negatively affected the fishery economy and native fish species in the Pearl River of Guangdong Province, China.

In addition to the species richness (S) analysis, Shannon diversity index (H’), Simpson diversity index (D) and evenness (H/S), we also analyzed the Sorenson’s coefficient (CC) between the upstream and downstream sites. The Sorenson’s coefficient for each site varied between 0.54 and 0.92. The highest value of Sorenson’s coefficient was recorded between KM and RB, at 0.92, and the lowest value was recorded between BB and RB, at 0.54 ( Table 5). According to Sorenson’s coefficient, these communities have quite a bit of overlap or similarity.

The high diversity of native species in the water body will decrease their tolerance in poor aquatic environments. In contrast, the invasive fish species have a high tolerance to poor water quality ⁴² . The interaction between different species, combined with the limnological and physical properties of the aquatic ecosystem, may influence the diversity and distribution of fish fauna ⁴³ . In this study, the diversity of fish to be found smaller than Fhitra and Siregar ⁴⁴ , who reported 58 fish species belonging to 23 families and 40 genera from Kampar Kanan river. Simanjuntak et al. ⁴⁵ described 86 species and 21 families in the Kampar Kiri river of the Kampar District. On the other hand, Nurdawati et al. ⁴⁶ reported 96 species in the Batanghari river, Indonesia. Additionally, Bahri ⁴⁷ described 86 species in the Musi river and Kottelat and Whitten ⁴⁸ reported 1300 species of freshwater fish across Indonesia that live in wetlands (rivers, lakes, bogs, oxbow lakes, floodplains, etc.).

The results indicate that the diversity and distribution of fish fauna in upstream and downstream areas of the Koto Panjang Reservoir were varied, and the evenness was low. The abundance and composition of fish in each site was dominated by Cyprinidae families, although exotic species were more dominant in the upstream reservoir compared to the downstream reservoir areas. Therefore, the management of the river and reservoir in a more holistic manner is important, for example, the management of land use, sand mining and aquaculture activity, as well as possible habitat restorations. All the factors above are a prerequisite for the environmental sustainability and conservation of fish diversity in the upstream and downstream areas at Koto Panjang reservoir and other regions.