Fish Spawning Habitats in theMekong and 3S Rivers in Cambodia
Touch Bunthang and Chheng Phen
Inland Fisheries Research and Development Institute (IFReDI),Fisheries Administration (FiA), Cambodia
5th APBON Web Seminar
21st Jan 2021

Outline
I. Cambodia Inland Fisheries
II. Pressures on fish and fisheries
III. Study of fish spawning in Mekong and the 3S Rivers

I. Cambodia Inland Fisheries
Cambodia Fisheries Production in 2020=935,900 Tons

Aquaculture
Production
43%

Inland Fisheries
Production
44%

Marine Fisheries
Production
13%

Area (1,000 ha)

Land use categoryRain-fed rice field
Flooded rice field (receding and floating rice fields)

3,215.6
371.8

Flooded grassland/marsh and swamp

302.8

Flooded shrub land
Flooded forest

533.6
20.6

Open water (Mekong, Tonle Sap and main rivers)
Reservoir
Total:

Rain‐fed rice fields

Reservoir

416.9
11.8
4,873.1

Open water

Flooded forest

Shrubland

Floodedgrassland/
marsh/
swamp

Floodedrice fieldCambodia’s Capture Fisheries 500,000 tonnes annually
Fish providing 76% ‐ 81% of total animal protein intake
for the population• Floodplain 40.000 km2 = 22.1% total• Tonle Sap Lake 1,500km2 in dry season, 16,000km2 inflooding season• Flooded forest 670,000 ha• Connectivity between different habitats• Influences by Mekong flood regime• At least 498 fish species recordedin the Cambodia’s Mekong River• Tonle Sap Lake hosts 296 fishspecies ranked third in the world
after Lake Malawi (433 species),and Lake Tanganyika (309 species).
• Tonle Sap Lake production is >60% of total fish production
13%
Rainfall30%
TS tributaries
57%

Mekong water

Factors driving Cambodia inland production
HYDROLOGY
Water level(higher flood = more fish)

FLOODPLAINENVIRONMENT

Flood duration(longer flood = more fish)

Flooded vegetation(less trees = less fish)

Flood timing(early flood = more fish)

Built structures(more structures = less fish)

Flood regularity(Anoxic mortality)

drivesAffectEXOTIC SPECIES

influences

Fish stocks

condition

Access tofeeding and breeding areas
Access to refuges(ponds, sanctuaries, deep pools)(better protection = more fish)allowsyields

FISH CATCHMIGRATIONSFISHING
Number of fishers
Impossible to control
Fishing intensity
Difficult to assess

White Fish

Gray FishBlack Fish1

II. Pressures on fish and fisheries
 Increasing demand Habitats degradation
 Disrupt connectivity
 Over fishing Illegal fishing's Climate changes and mining, etc.

Removing flooded forest for agriculture development inTonle Sap Lake

10

Building Dike within the floodedforest area
Destruction of Flooded Forest

Cutting flooded forest for firewood• Clear flooded forest areas forcrop cultivation

11

Mosquito net
fishing gear

Rat Tail Traps

Electro‐cute fishing gears
Hydro Power Dam12

Biggest potential threat isconsidered hydropower dams
 9 planned dams on LMB(mainstream) – Two are operation.
 23 existed dams (> 20 MW) ontributaries
 Many more tributary dams areprojected or under construction

Xayaburi(2012)

Donsahong(2014)

Dam will totally blocks
migration route of fishes
both upstream and down
streamAbout 87% of wellknown fish species
in the Mekong are
migratory species.

Upstream developments
will result in floods
arriving later
DELAY

Upstream developments
will shorten the durationof the flood

Upstream developments will
reduce the water level andthe surface area flooded87
Baseline scenario
Intensive basin development

5Extreme basin development

4321AprilMarchFebruary

January

DecemberNovember

October

SeptemberAugustJulyJune

0May

Water level (m)6Introduction to Mekong and 3S river system in Cambodia

Introduction to 3S river system in Cambodia• Sekong originates in Vietnam, a long stretch in Lao PDR, joint with
Mekong at Stung Treng Town,• Sesan originates in Vietnam, in Cambodia it joint with Sekong about
15 km above Stung Treng Town, and• Srepok originates in Vietnam, in Cambodia it joint with Sesan about
30 km above Stung Treng Town.

III. Study of fish spawning in Mekong and the 3S Riversystem• Fish caught in the Sesan, Srepok andSekong near the confluence with theMekong and in the Mekong mainstream
near Stung Treng• Fish assessed for presence of eggs andwhat stage of development (early, late)• Used to determine their ‘readiness’ tospawn – used as an indicator of spawning
locationsStudy of fish spawning in Mekong and the 3S River system• Evaluated the location and seasonal
spawning readiness of 124 species
caught by the study• Used cluster analysis to determinepatterns in spawning ready behavior
across species, location and timing• Identified species that are following
similar patterns of spawning behavior• Determined relative importance of the4 rivers for different species groups andtiming of spawning.

Long
Distance

Short
Distance

Sedentary

Study Results• Both migratory and non‐migratory species of Mekong fish spawn in
the 3S river system• The majority of species classified as “long‐distance migrant” were
found in ‘spawning‐ready’ condition in the Mekong river• Many of the smaller, more sedentary species were heavily reliant ona)b)
the Sesan Srepok and Sekong.
Important fish species from the Mekong and 3S river a) Cirrhinus microlepis, b) Pangasiuslarnaudii, c) Cyclocheilichthys enoplos, d) Henicorhynchus lobatusc)d)

Ward cluster analysis shows groupings of species following similar patterns of behavior – importance of rivers and season

Spawning in
the Sesan andMekong

Spawning in
the Srepok or
Sekong andMekong

Long‐distancemovements /Confined to
the Mekong

Spawning in
all rivers ‐MoreSedentaryspecies

Key Findings• Many migrant species show location
specific spawning behaviors, which
confirms the importance of the 3S‐
Mekong confluence for spawning.
• Over half the fish sampled were large‐
bodied, long‐lived, long‐distancemigrants – all of which were found in
spawning ready condition in the Mekong.
• The mainstream is particularly important
as a spawning ground for large migrant
species.

Key Findings• The confluence of rivers is important forbiodiversity with 124 species captured by thestudy many of which were found to be inspawning‐ready condition.
• This demonstrates the importance of theSrepok and Mekong at the confluence forproviding spawning locations.
• 98% of the small, short‐lived, migrant species,
such as Trey Riel species were spawning readyin the Mekong and one or more of the otherrivers in the system.

Key Findings• The Srepok is an important spawning location
for a majority of the more sedentary species
such as Mystus species.
• Few species were spawning ready in theSekong river near to the confluence with theMekong. However, the habitat in the lower
Sekong has been shown to be less suitable forspawning (no availability of sandy banks, rockpools, leafy overhangs) compared with thehabitat available further upstream

Mystus cf. wolffii

Srepok River near Stung Treng

Sekong River near Stung Treng

Conclusions
The study results taken
together with other evidence of
the importance of the Sekong
and its high biodiversity,
suggest that fish will migrate
further upstream to spawn.
This study confirmed thespawning peak period is June toAugust for most species, butthat many species will also
spawn outside of this peak.

Figure 3: Seasonal pattern of spawning‐
ready female fish abundance by river.

Likely Impacts of development on spawningIt is likely, the proposed hydropower dams on the lower parts of the 3S
rivers will have impacted many species but even more will be impacted
by mainstream dams.
There will be a disproportionate
impact on large‐bodied, long‐
lived commercially importantcyprinids and catfish as well as,
many of the short‐distancemigratory species that are heavily
relied on for subsistenceNo successful solution exists to get larval and juvenile fish downstream ofdams
Unless a satisfactory solution is developed those species trapped above
dams, such as Lower Sesan 2, are likely to become isolated and may notsurvive the transition from running river to a reservoir.
Development of a fish pass to move fish can potentially act as an ecologicaltrap to those species using the fish pass as they will be trapped in a non‐viable habitat
Species diversity is important for maintenance of fish production particularly
under changing climate conditions
The Khone Falls is a natural and historic barrier to many species, keeping afree‐flowing river south of the falls is very much still relevant despiteprevious barrier effects from dams upstream in Laos and at the falls itself

Future• Understanding the importance of river connectivity and monitoring
dam impacts is key• Important to determine which dams to build and which to avoid• Important to understand how and for which species to mitigate damimpacts• Continued work on fish pass design will be critical to mitigate thenegative impacts of the Sesan 2 damThank you for your attentions