Fish monitoring was conducted in three Mekong tributaries in Thailand that share a similar natural environment but differ in hydrological regulations. Despite similar historical fish assemblages reported prior to the regulation, these tributaries currently support contrasting assemblages for both adults and larvae. Fish assemblage in an unregulated tributary with no existing dams along its mainstem showed a highest species diversity with a clear seasonal fluctuation in the number of species. Fish assemblage in a tributary having a series of low-head dams was characterized by low species diversity and less seasonal fluctuation in the diversity. Lastly, fish assemblage in a tributary with a large hydropower dam near the Mekong confluence changed according to the operation of the dam. Short-distance as well as a few long-distance migratory species dominated the assemblage during a period when sluice gates were opened in the rainy season. A change from lotic to lentic systems and associated alternations to river flow by dams impact a life history of fishes, especially that of facultative riverine species. Although these fish are able to sustain populations in a lentic system, they typically exhibit lower productivity, slower growth rates, smaller maximum sizes and delayed maturation ages compared to their lotic counterparts.

1. Lessons from damming in the Mekong tributaries:
Effects on riverine fish community and biology
Tuantong Jutagate, Michio Fukushima
Pisit Phomikong and Wachira Kwangkhang

2. Rationales
• Ziv et al. (2012) reported that fish biomass in the Mekong basin would
decrease by about 0.3% (ca. 1700 t/yr) for each additional terawatt hour per
year produced by hydropower dams in tributaries and that about six species
will become endangered for every 1% of lost biomass.
• Few Mekong’s migratory fishes are known to complete their life cycle in
reservoirs (Baran, 2006).
• Thus, in this presentation, we demonstrate
(a) the fish assemblages in the dammed tributaries with different degrees
of regulation
(b) the performances of the Mekong’s migratory fishes that can live in the
new lentic (or semi-lentic) environment
Baran, E. 2006. Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems.
MRC Technical Paper No. 14, Vientiane, Lao PDR, Mekong River Commission.
Ziv, G., Baran, E., Rodríguez-Iturbe, I., and Levin, S.A. 2012. Trading-off Fish Biodiversity, Food Security and
Hydropower in the Mekong River Basin. Proceedings of the National Academy of Sciences 109 (15):
5609-5614.

3. The Songkhram River
(intact tributary)
The Gam River (cascaded
tributary with 5 low-
head dams)
The Mun River (blockage
by the large hydropower
dam)
Fish diversity and assemblages among the three rivers
• Previous studies showed similar fish species composition among the three rivers,
with more than 90% of common species (Srisatit et al., 1981; Duangswasdi and
Chookajorn, 1991; Boonyaratpalin et al., 2002)

4. Temporal changes in cumulative species richness
in each studied tributary
Songkhram
Gam
Mun
Case: The Songkhram River

5. The Migratory guilds of fishes in the Mekong River
Grey Fishes
White Fishes
Black Fishes

6. Temporal changes in cumulative species richness
in each studied tributary
Songkhram
Gam
Mun
The effectiveness of the measure has been
studies by using the “presence / absence” of
fish monitoring data of the Mun River from
1967 to 2007 and analyzing probability of
occurrence of individual species

7. Probability of occurrence
X1960s X1980s DC Impound X2001 Implement
020406080
Example of the species that benefit from the measurement I:
Jutagate et al. (2007; 2008)
Henicorhynchus siamensis
Bangana behri
Henicorhynchus lobatus
Mekongina erythrospila Scaphonathops spp.
Sikukia gudgeri
There were 32 species (with 8 species
had been reported on ascending the
fish ladder)

8. X1960s X1980s DurCon After X2001 Impleme
020406080100
Probability of occurrence
Jutagate et al. (2007; 2008)
Cosmocheilus harmandi Luciosoma bleekeri
Helicophagus leptorhynchus Pangasius micronema
Pangasius bocourti Micronema spp.
Example of the species that benefit from the measurement II:
There were 55 species (with 8 species
had been reported on ascending the
fish ladder)

9. Benefit to spawning
migration

10. Similarity of fish assemblage patterns among the three selected rivers
Fish diversity and assemblages among the three rivers

11. Fish diversity and assemblages among the three rivers

12. Larval fish diversity and assemblages among the three rivers

13. Assemblage I: Songkhram (August and December) and Mun (October)
Nineteen (19) larval species had the average %OP more than 40% in this assemblage.
They were mostly grey fish (11 species) such as Rasbora dusonensis, Rasbora
borapetensis, Clupeichthys aesarnensis, and Crossocheilus atrilimes.
Rasbora dusonensis
Clupeichthys aesarnensis
Rasbora borapetensis

14. Assemblage II: Songkhram (February, April and June)
The assemblage II was dominated by 6 grey fishes e.g. C. aesarnensis , Hampala dispar,
R. borapetensis) and 3 white fishes, i.e. Mystacoleucus ectypus , Henicorhynchus
siamensis and Barbonymus gonionotus.
Barbonymus gonionotus
Hampala dispar
Henicorhynchus siamensis

15. Assemblage III: Gam (All sampling months)
The average %OP of white fishes, in this assemblage, was significantly lower than the
other guilds. Only five larval species were dominant.
Parambassis siamensis
Trichopsis pumila

16. Assemblage IV: Songkhram (October)
Assemblage IV formed a unique characteristic, which no black fish larvaes
involved among the 30 highest %OP species . Fifteen dominant white fishes
included Sikukia gudgeri, Setipinna melanochir, Pangasius macronema
Notopterus notopterus
Sundasalanx mekongensis

17. Assemblage V: Mun (February, April, June, August and Decemebr)
The %OP grey fish larvae were significantly higher than the other two guilds in assemblage
V but only 2 species, i.e. R. borapetensis and C. aesarnensis were dominated.
Clupeichthys aesarnensis
Rasbora borapetensis

18. Table I Average ±SD of total length (mm) of commonly observed
fish species (N > 150) collected in the three rivers.
Fish species N P
River
Mun Songkhram Gam
Barbonymus gonionotus 335 0.179 130.5 ± 47.1 144.9 ± 86.5 126.3 ± 40.9
Barbonymus altus 156 < 0.001 75.5 ± 25.5b
94.6 ± 44.3a
86.9 ± 43.5ab
Henicorhynchus siamensis 450 < 0.001 130.5 ± 49.3a
134.0 ± 21.1a
87.8 ± 36.1b
Cyclocheilichthysa rmatus 162 < 0.001 87.1 ± 24.5a
100.4 ± 15.6a
81.2 ± 20.5b
Puntioplites proctozysron 712 < 0.001 86.3 ± 35.8b
142.0 ± 34.7a
145.1 ± 42.1a
Osteocheilus vittatus 1,175 < 0.001 123.8 ± 29.9b
123.8 ± 34.9b
135.8 ± 38.5a
Labiobarbus lineatus 776 < 0.001 99.0 ± 27.4b
117.9 ± 22.2a
112.5 ± 36.7a
Hemibagrus nemurus 165 < 0.001 64.5 ± 41.6c
203.5 ± 27.6a
174.1 ± 54.8b
Size variations of the adults of commonly found species

19. Changes in life history traits
(Jutagate et al., 2004; Suvarnaraksha et al, 2011)

20. • The riverine species, such as H. siamensis and Labeo chryophekadion can
establish populations in a lentic system but the production was too low,
compared to those in lotic condition.
• The reservoir residents showed slow growth rate and delay in age (size) at
maturity, compared to those lived in the rivers.
• The spawning ground is located in the
upstream of reservoir, i.e. the inlet area,
and highly related to the hydrological
cycle and related to flow.
Changes in life history traits