Are the fish of the upper and lower Mekong ... - Springer Link

Chinese Journal of Oceanology and Limnology Vol. 27 No. 2, P. 400-407, 2009 DOI: 10.1007/s00343-009-9132-0

Are the fish of the upper and lower Mekong interconnected?* KANG Bin (康斌)**, PERRETT Lisa, LI Yungang (李运刚), HE Daming (何大明) Asian International Rivers Center, Yunnan University, Kunming 650091, China

Received June 8, 2008; revision accepted July 14, 2008 Abstract The Mekong supports one of the richest inland fisheries in the world, with many of the fish migrating long distance to spawn. Little is known about the fisheries and migration strategies of the Upper Mekong whilst it is supposed that many fish species move between the Lower and Upper Mekong. Most likely, natural fish migration in the river has been altered by dam construction across the mainstream of the Upper Mekong. In this paper, the interconnectivity of fish species between different sections of the Mekong and negative impacts of dams on migratory fish are studied. Of the 162 fish species in the Upper Mekong and the 869 species in the Lower Mekong, 61 species are common. Results show that there is no significant difference at order level between the UM and LM. Similarity coefficients are used to evaluate interconnectivity at species, genus and family levels among four different sections of the Upper Mekong with each other and with the Lower Mekong as a whole. The highest similarity is found between the middle and lower reach of the Upper Mekong at species and genus levels and the middle and upper reach at family level. Of the eight cascade dams, Mengsong Dam in planning is considered as the biggest threat to migratory fish from the Lower Mekong and should be particularly concerned. Keyword: dam; biodiversity; migration; Mekong; interconnection

1 INTRODUCTION The Mekong originates from the Tanggula Range on the Qinghai-Tibetan Plateau. The Upper Mekong (UM) or Lancang River runs within China, and the Lower Mekong (LM) outside of China; the river traverses over 4 880 km from the plateau to the South China Sea (He and Tang, 2000). The Mekong is home to one of the largest inland fisheries in the world, boasting the third richest biodiversity of riverine fish species (Rainboth, 1996). The Mekong watershed cradle diversified flora and fauna in unique geographic conditions; many are endemic. The fish species of the region vary greatly with difference in altitude, landscape, sediment, water, and climate (Kang and He, 2007). Geographically, in the Mekong basin, separation of major fish habitats forces many fish to migrate (Poulsen et al., 2002). Migratory behaviors can be either longitudinal or lateral, with longitudinal migration typically involving a long-distance movement, whilst lateral migrations cover the river channel and floodplain (Baran 2006). Both lateral and longitudinal migration behaviors are largely driven by floods by monsoon, triggering fish to migrate or spawn. Poulsen et al. (2002) classified the

Lower Mekong into three major migration systems: the Lower Mekong Migration System (LMMS, altitude 0–150 m asl (above sea level)), the Middle Mekong Migration System (MMMS, altitude 150–200 m asl) and the Upper Mekong Migration System (UMMS, altitude 200–500 m asl). These systems are naturally interconnected, and many species are known to migrate from one system to another. Compared to the LM, much less is known about the migration systems of the UM. Having known that fish species in UM and LM closely interconnect, as species of Pangasius migrate to and fro (Yang and Chen, 2007); however, the homogeneity and interconnectivity of fish species in the UM and LM remain unclear. Evidence shows that dam construction across the mainstream of the Mekong posed an obstacle to the natural distribution and survival of the fish, causing habitat fragmentation, migratory pathways blocking, and flow regime alteration (Nilsson et al., 2005). At present, dams planned or constructed over the Mekong are in UM; new dams are expect to occur in the LM demanded from regional economic development. *

Supported by National Natural Science Foundation of China (No. 40601096) ** Corresponding author: [email protected]

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Fig.1 Sketch map of the Mekong The national border divides Mekong into the Upper Mekong and the Lower Mekong. The Upper Mekong includes: headwater (source to Changdu), up-reach (Chamdo to Gongguoqiao), middle reach (Gongguoqiao to Jinglinqiao), lower-reach (Jinglinqiao to the borderline). The small frame shows the Buyuan River and the pending Mensong Dam.

In the UM eight cascade dams have been planned, among which two dams, the Manwan and Dachashan have commenced operation. The Xiaowan Dam, which is currently under construction, will be the highest dam in the world upon completion in 2010, standing at over 300 m high. The Mengsong Dam, located near the boundary of China and Laos is still in the planning phase (Dudgeon, 2005). At

present, the influence of dam construction on migratory fishes attracts a great deal of attention worldwide. Despite this however, there are few academic studies on fishes in the UM and the affect of dams on fish production and biodiversity. In this paper, we 1) review the fish composition in the Upper and Lower Mekong; 2) analyze the interconnectivity of habitats at different parts of the

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Mekong based on fish species; and 3) evaluate the possible impacts of dams on migratory fishes.

2 MATERIALS AND METHODS 2.1 Classification and description of river sections in UM and LM In topography, geology and successive hydrodynamic regime, the UM can be divided into the following four sections (Ding et al., 1993): Headwater: from the source at an elevation of about 5 100 m to Changdu at 3 000 m; this section is 564.4 km long boasting the steepest gradient of the whole watershed. Upper-reach: from Chamdo to Gongguoqiao; this V-type valley is the narrowest section of the watershed, with small tributaries crossing the mainstream. Middle-reach: from Gongguoqiao to Jinglinqiao, this is the transition area from the Qinghai-Tibetan plateau to the Yunnan-Guizhou Plateau, showing different kinds of strong incision and fragmentary landforms. Lower-reach: from Jinglinqiao to the borderline, this section is 355 km long at an average altitude of 1 500–2 000 m. It is wide with only a slight gradient, very different from the upper-reach. The Lower Mekong in contrast, can be divided into four parts (Bailey, 1988; Platts, 1988, Hill and Hill, 1995): Part 1: from borderline between China and Burma to Luang Prabang by Chaing Saen, river flows through further inaccessible gorges and is characterized as a transition from cold, swift flows of UM, to flatter, warmer, slower flows of downstream reaches. Part 2: from Luang Prabang to Kratie, river is navigable in reaches separated by major waterfalls, with the most famous one as Khone Fall and surrounded by extensive sloughs, wetlands and small ponds in the floodplain. Part 3: from Kratie to Phonm Penh, the capital of Cambodian, this portion includes Tonle Sap and the Tonle Sap River that interconnects lakes by the mainstream Mekong, with considerably different hydrological, ecological and social contexts. Part 4: from Phonm Penh to the estuary, the river divides into two main channels and many smaller ones flowing through the delta to the South China Sea (Fig.1). 2.2 Data source Investigation into the fish species found in the UM began in the 1950s (Cheng, 1958), with further studies compiled in the monographs “Fishes in

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Yunnan” (in the UM in Yunnan) (Chu and Chen, 1989, 1990), and “The Fishes of the Qinghai-Xizang Plateau” (in the UM in Qinghai and Tibet) (Wu and Wu, 1991). More recent researches have resulted in the discovery of new fish species in the UM (Chen and He, 1992; Chen et al., 1994; He and Chen, 1994; Zhou and Cui, 1996; Chen, 1999; Chen et al., 2003; Kong et al., 2007). After revising all the available materials in combination with local fishers’ experience, Kang and He (2007) analyzed the fish composition and the general ecology in the Upper Mekong basin (including mainstream and tributaries); the findings are the main data source for this research. Fish data from the LM is taken from the Mekong Fish Database (MRC, 2003). 2.3 Calculating fish interconnection To measure the interconnection, similarities between fish fauna of the UM and LM were analyzed. Meanwhile, to clarify the interconnection, similarity among five different regions of the Mekong (one in the LM and four in the UM), between the total number of fish species, between the fish species in the UM and LM, and among the composition of common fish at family, genus and species levels in the UM and LM, were analyzed. Similarity in fish fauna in different areas can be somewhat indicative of habitat homogeneity and connectivity, and can be calculated using the similarity coefficient R: R=c/(a+b−c) where c is the number of fish species found in both two areas, a is that in one area and b is in another (Chu and Chen, 1990). We designed a fish distribution data with ‘river section’ as case and ‘species’ as column. We fill the blanks by 1 (fish appearance) or 0 (no fish). The significant differences among different parts of Mekong, at family, genus and species levels were determined with a Paired-Samples T-test.

3 RESULTS 3.1 Fish composition of the Upper Mekong According to Kang and He (2007), the fish of the Upper Mekong include 6 orders, 21 families, 86 genera and 162 species. In the headwater 11 species were recorded, in the upper reach there were 22 species, whilst 44 species were recorded in the middle reach, and 142 species were recorded in the lower reach. Cypriniformes was the dominant fish order, comprising 72.22% of the total species number, followed by Siluriformes at 16.67%, Cyprinnodontiformes at

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Fig.3 Fish composition of the Lower Mekong at order level

Fig.2 Fish composition of the Upper Mekong at order level. The numeral means the species number of each order

1.85%, Synbranchiformes 0.62%, Perciformes 8.02%, and Tetraodontiformes 0.62% (Fig.2). 3.2 Fish composition of the Lower Mekong According to the fish database published by the MRC (2003), 869 species have been recorded in the Lower Mekong, belonging to 27 orders, 88 families and 347 genera. Cypriniformes, Perciformes and Siluriformes are the most prevalent orders. Cypriniformes consist of 4 families, 99 genera and 341 species, occupying 39.24% of the total. Siluriformes include 12 families, 34 genus and 119 species, occupying 13.69%. The predominantly estuarine species, Perciformes have 26 families, 99 genus and 194 species, occupying 22.32% of the total. Clupeiformes, Beloniformes, Tetraodontiformes, Pleronrctiformes, Anguilliformes, Gasterosteiformes, Synbranchiformes, Myliobatiformes, Mugiliformes comprise of 3.80%, 2.65%, 2.42%, 2.19%, 1.50%, 1.50%, 1.50%, 1.38% and 1.27% of the total, respectively, whilst the remainder made up no more than 1% each (Fig.3). 3.3 Fishes in both the Upper and the Lower Mekong Based on the MRC (2003) and Kang and He (2007), of the 162 fish species in the UM and the 869 species in the LM, 61 fish species are found in both the Upper and the Lower Mekong. This is a relatively high proportion of the total fish species of the Upper Mekong, particularly given the diverse landscape and climate that the Mekong traverses on its journey from the Tibetan Plateau to the South China Sea. Unique

Totally 869 species belonging to 27 orders, 88 families and 347 genera have been recorded. Cypriniformes, Perciformes and Siluriformes were the most prevalent orders, occupying 39.24%, 22.32% and 13.69%, respectively. 1. Cypriniformes 2. Perciformes 3. Siluriformes 4. Clupeiformes 5. Beloniformes 6. Tetraodontiformes 7. Pleuronectiformes 8. Anguilliformes 9. Gasterosteiformes 10. Synbranchiformes 11. Myliobatiformes 12. Mugiliformes 13. Carcharhiniformes 14. Cyprinodontiformes 15. Aulopiformes 16. Atheriniformes 17. Scorpaeniformes 18. Osteoglossiformes 19. Orectolobiformes 20. Pristiformes 21. Elopiformes 22. Batrachoidiformes 23. Lophiiformes 24. Rajiformes 25. Gonorhynchiformes 26. Characiformes 27. Gadiformes

and specialized fish species are found particularly in the headwater and upper reach of the UM as a result of the cold water, rapid flow and the high altitude of the region. The specialization of fish species in the UM is illustrated by the 101 fish species that are found only in this section of the river. These fish belong to 51 genera, 9 families and 3 orders, comprising 63% of the total species in the Upper Mekong. Among the 101 fishes found only in The Upper Mekong, there are 84 fishes in Cypriniformes, representing 83% of the total. 3.4 Fish fauna connectivity between the Upper and Lower Mekong Compared to the Lower Mekong, there are comparatively few fish species found in the Upper Mekong. The similarity coefficient R between the species of the UM and the LM is 0.239 at family level, 0.107 at genus level and 0.063 at species level (Table 1). If only the species of the 22 families seen in both the UM and LM are considered, the similarity coefficient R would increase to 0.290 and 0.145 on genus and species levels, respectively. If only considering the most common orders, Cypriniformes and Siluriformes, the R values would be 0.364 for genus and 0.150 for species.


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Table 1 Similarity coefficients of fish assemblage among different parts of Mekong at family, genus and species levels The Upper Mekong Headwater Upper-reach Species

The Upper Mekong

The Upper Mekong

Middle-reach

0.037

0.241

Lower-reach

0.013

0.093

0.281

0

0.005

0.022

Upper-reach

0.294

Middle-reach

0.121

0.306

Lower-reach

0.047

0.149

0.369

0.003

0.025

0.062

The Lower Mekong

Family

The Upper Mekong

Middle-reach

Lower-reach

0.222

The Lower Mekong

Genera

Upper-reach

Upper-reach

0.600

Middle-reach

0.429

0.714

Lower-reach

0.143

0.238

0.333

0.036

0.060

0.084

The Lower Mekong

0.076

0.181

0.238

The similarity coefficients among sections steadily decreased as the distance increased.

Comparing the fish species in four parts of the UM the highest similarity coefficient R is between the middle and lower reach of the UM at species (0.281) and genus (0.369) levels, while at family level the highest R value is between the middle and upper reaches (0.714). The similarity coefficient between river sections steadily decreases as the distance between the sections increases, at family, genus and species levels (Table 1). Result of a paired-sample T-Test shows no significant difference in fish composition in percent at family (p=1.00), genus (p=0.87) and species (p=0.80) levels (Table 2), but significant difference in the numbers of family (p=0.00), genus (p=0.01)

with each other as well as with the LM, we found that and species (p=0.02) levels between the LM and the UM, indicating that the LM contains many more fish species than the UM, including many estuarine species. Furthermore, when excluding the orders from the analysis that do not appear in both the UM and LM, a higher similarity was observed in both percent and total numbers at family, genus and species levels. A paired-sample T-Test shows there is no significance in total number (family level: p=0.19, genus level: p=0.10; species level: p=0.07) and in percent (family level: p=0.05; genus level: p=0.53; species level: p=0.52) (Table 3).

Table 2 The Paired-Samples T-Test results of fish assemblage in number and in percent between the Upper Mekong (UM) and the Lower Mekong (LM) (Fish composition between the LM and the UM showed significant differences in number but in percent at all family, genus and species level) Mean a

S.D.

s.e.

UM

0.81

1.98

0.38

LM

3.26

5.04

0.97

UM

3.26

12.11

2.33

LM

12.85

25.71

4.95

UM

6.04

23.08

4.44

LM

32.19

74.33

14.30

UM

3.70

9.01

1.73

LM

3.70

5.73

1.10

UM

4.02

14.95

2.88

LM

3.70

7.41

1.43

UM

4.14

15.81

3.04

LM

3.70

8.55

1.65

Level

Family In number

Genus Species Family

In percent

Genus Species

a

95% confidence interval of the difference

t

df

Sig. (2-tailed)

lower

upper

1.05

3.84

3.61

26

0.00

2.54

16.65

2.79

26

0.01

4.77

47.52

2.52

26

0.02

-2.03

2.03

0.00

26

1.00

-4.36

3.72

-0.16

26

0.87

-3.87

3.01

-0.26

26

0.80

The numeral was the mean value of each order (totally 27 orders) containing family, genus and species number

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Table 3 The Paired-Samples T Test results of fish assemblage between the Upper Mekong (UM) and the Lower Mekong (LM) when only fishes of orders both appearing in the two river sections were considered (There were no significances both in number and in percent at all family, genus and species levels, showing high similarity.) 95% confidence interval of the difference Mean a

S.D.

s.e.

UM

3.67

2.81

1.15

LM

8.17

9.54

3.89

UM

14.67

23.70

9.68

LM

41.83

45.59

18.61

Level

Family In number

Genus Species Family

In percent

Genus Species

a

UM

27.17

45.62

18.62

LM

116.00

132.68

54.17

UM

16.67

12.75

5.20

LM

9.28

10.84

4.42

UM

18.11

29.26

11.95

LM

12.06

13.14

5.36

UM

18.61

31.24

12.76

LM

13.35

15.27

6.23

t

df

Sig. (2-tailed)

lower

upper

-3.20

12.20

1.50

5

0.19

-7.88

62.21

1.99

5

0.10

-9.81

187.47

2.32

5

0.07

-14.65

-0.12

-2.61

5

0.05

-29.20

17.09

-0.67

5

0.53

-25.01

14.49

-0.68

5

0.52

The numeral was the mean value of each order (totally 6 orders) containing family, genus and species number

4 DISCUSSION 4.1Fish biodiversity The Mekong is home to one of the most diverse inland fisheries in the world (Rainboth, 1996). In general terms the diversity of fish species generally increase from the headwater to the lower sections of a river (Schlosser 1987; Hankin and Reeves, 1988), due to the lower sections usually being more hospitable to flora and fauna in terms temperature, altitude, flow rate, and nutrient loads. As a longitudinal river, the Mekong is influenced by altitude and latitude and hence there is a far greater biodiversity of fish species found in the LM than in the cold and high altitude regions of the UM. Harsh environment and scarcity of food in the headwater for example, makes it suitable only for Schizothorax and Triplophysa, whilst in the lower reach, steady climate with high temperature, high humidity and abundant rainfall create different niches, resulting in a higher biodiversity of fish, especially of estuarine species. The former studies suggest that the Mekong has a fish fauna estimated at approximately 1 200 species (Sverdrup-Jensen, 2002, Sterling et al., 2006) possibly up to 1 700 (Coates et al., 2003). In the present study, however, it is found that the Mekong as a whole is home to 983 fish species (869 LM + 162 UM-61 LM&UM). This is the most accurate count of the fish species in the whole Mekong River at present; it is possible that the figure underestimates the number of species found in the Upper Mekong. Until

recently, the UM fishery remained largely undiscovered and unrecorded, and with further research of the area comes the discovery of many new and previously unrecorded species, which could raise this count in the future. 4.2 Interconnectivity of the fishes of the Mekong There is some interconnectivity between the UM and LM in the order compositions of fish with no significant difference (Table 2). Given higher biodiversity of fish species in the LM, there is significant difference between the UM and the LM in total numbers of fish per order at species, genus and family levels. Interestingly, using the most common species of orders Cyprinformes and Siluriformes, or using percentage species composition produced significantly similar results between the UM and LM. Although Perciformes is also a key order in the LM, however, it was excluded from this analysis as it is an estuarine species and therefore not suitable for the comparison. Previous researches have revealed distinction in geomorphology of riverbeds and plants between the lower and middle reach of the UM (Li et al., 2002; Yang, 2005; Li et al., 2007). This distinction is greater than that caused by the administrative borderline dividing the Lower Mekong from the Upper Mekong, which shows that it is the region where prominent change in fish species occurs in this region. If this phenomenon is the case, the upper migratory systems of the LM, ending at the Laos (Poulsen et al., 2002)

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should be extended into the modern UM to better describe the fisheries of the LM. However, this is just a hypothesis that needs more data at each section of the LM in the future. 4.3 Influences of dams on fish migration At present eight cascade dams cross the lower reach of the UM are in different phases of planning, construction, or operation. The construction and completion of the dams bring about a negative impact on Mekong fish species, especially migratory ones, and may cause serious environmental consequences, such as blocking migration paths, fragmenting habitats, and their reproductive cycle, as well as changes in flow, depth, dissolved oxygen, light, temperature, and food sources (Baxter, 1977; Zhong and Power, 1996; Nilsson et al., 2005). For example, natural alternation in water level of the Mekong between wet and dry seasons drive fish to migrate from the LM upstream to spawn. Since the dams regulate the natural flowage, flooding will no longer occur. Therefore, stimulation on fish migration and reproduction loses, and the fish migration patterns are forced to alter. The fish species of high breeding, short reproduction cycle, and strong adaptability (r strategy) would survive better than those that opposite (K strategy). For another example, as dams simplify the physical structure of natural waterways with the loss of shallow banks, the heterogeneous habitats would cause endangerment of fish species and decline of fish fingerlings that dependent on free flow. Of the eight cascade dams, Mengsong Dam attracts particular concern as it will block the passage of migratory fish from the LM to important tributaries of the UM, the Buyuan River and the Nanla River (Fig.1). Fortunately, the project is now in consideration of cancellation (Kang et al., 2009). The two rivers are characterized with highly unsorted riverbeds, steep gradient, swift flow, and mild water temperature, forming up a complicated series of microhabitat; the Buyuan River is also a main place for three migratory Pangasius catfish species forage and spawn (Yang et al., 2007). It is very possible that the tributaries are also used by other migratory species and yet their migration paths have not been mapped; and the number of species the Mengsong Dam would affect remains determined. To reduce the artificial ecological impact on fish species, education and awareness on environmental protection must be raised, and decision of dam construction be carefully reconsidered in the balance of benefit between human beings and the environment.

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5 CONCLUSION The Mekong possesses one of the most fish diversity in the world. Totally 983 fish species are found in the whole Mekong, and this number will possibly increase under further research. Fish biodiversity in the LM is greater than that in the UM, which can be attributed to longitudinal habitats change corresponding to altitude and latitude Harsh environment and scarcity of food in the headwater makes it suitable only for Schizothorax and Triplophysa. There is interconnectivity between the UM and LM in the fish compositions at order level, but shows significant differences at species, genus and family levels. The place of primary distinction of fish composition is between the middle reach and the lower reach of the UM. Cascade dam are in different phases of planning, construction, or operation in the UM mainstream, and this brings about a negative impact on Mekong fish species. Education and awareness on environmental protection must be raised to reduce the impacts on fisheries. Decision of dam construction should also be carefully reconsidered in the balance of benefit between human beings and the environment. References Bailey, R. G., 1988. Ecographic Analysis, A Guide to the Ecological Division of Land for Resource Management. U.S.D.A. Forest Service, Misc. Pub. 1465, Washington D. C. Baran, E., 2006. Fish Migration Triggers in the Lower Mekong Basin and Other Tropical Freshwater Systems. MRC Technical Paper No. 14, Mekong River Commission, Vientiane. Baxter, R. M., 1977. Environmental Effects of Dams and Impoundments. Annual Review of Ecology and Systematics 8: 255-283. Chen, Y. and S. He, 1992. A new species, genus of Cyprinidae in Yunnan. Acta Zotaxonomica Sinica 17(2): 238-240. (in Chinese) Chen, Y., 1999. A new loach of Schistura and comments on the genus. Zoological Research, 20(4): 301-305. (in Chinese) Chen, Z., D. Huang and S. Xu, 2003. A New Record of Cyprinid Fishes in China — Barbodes gonionotus. Zoological Research 24(2): 148-150. (in Chinese) Chen, Z., J. Yang and W. Qi, 1994. Description of a new loach of Schistura from Lancang river basin, Yunnan, China. Acta Zotaxonomica Sinica 19(3): 375-377. (in Chinese) Cheng, Q., 1958. Research on Yunnan Fishes. Zoological Research 2(3): 153-165. (in Chinese)

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