Maximum sustainable yield assessment in nui coc reservoir

MINISTRY OF EDUCATION AND TRAINING
NHA TRANG UNIVERSITY

MAXIMUM SUSTAINABLE YIELD ASSESSMENT
IN NUI COC RESERVOIR

By: Mrs. Stéphanie KAMONDO
Nha Trang University, Khanh Hoa province

Master Thesis

July, 2014


MINISTRY OF EDUCATION AND TRAINING
NHA TRANG UNIVERSITY

MAXIMUM SUSTAINABLE YIELD ASSESSMENT
IN NUI COC RESERVOIR

Thesis submitted in fulfillment of the requirements for the award of Master of
Science in Aquaculture

By: Mrs. Stéphanie KAMONDO
54CH347
Nha Trang University, Khanh Hoa province

First Supervisor:

Dr. Anh T. Bui, Ministry of Agriculture and Rural
Development, Vietnam

Second Supervisor:

Prof. Sena S. De Silva, Deakin University, Australia

July, 2014


CANDIDATE DECLARATION
I certify that the thesis entitled “MAXIMUM SUSTAINABLE YIELD
ASSESSMENT IN NUI COC RESERVOIR” submitted for the degree of Master of
Science in Aquaculture is the result of my own work and that where reference is
made to the work of others, due acknowledgment is given.
I also certify that any material in the thesis which has been accepted for a degree or
a diploma by any University or Institution is identified in the text.

Full Name:

Stéphanie KAMONDO

Signed ………………………………..
Date:

i


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ACKNOWLEDGEMENTS
First and above all, I praise God, the Almighty for providing me this opportunity

and granting me the capability to proceed successfully.
This is a great opportunity to express my gratitude to my first supervisor Dr. Anh
Bui The for making this research possible. His support, guidance and advice
throughout the research project as well as his painstaking effort in proof reading the
drafts, are greatly appreciated. Indeed, without his guidance, I would not be able to
put the topic together.
My special and heartily thanks to my second supervisor Professor Sena De Silva for
his support and suggestions.
I gratefully acknowledge the financial support I received from the Government of
Rwanda through the Ministry of Agriculture and Animal Resources under the
Rwanda Agriculture Board. I also thank the Vietnamese technical team for their
support and valuable recommendations to the former PAIGELAC /RAB.
To Nha Trang University, I extend my gratitude for the academic and technical
support beyond giving me the opportunity to pursue my studies. Especially, I would
like to thank the Director of Aquaculture Institute, Dr Pham Quoc Hung for his
support and collaboration.
A special acknowledgement goes to the Research Institute for Aquaculture No.1,
particularly to Dr. Nguyen Van Tien, Mr. Nguyen Duc Tuan and Mrs. Nguyen Thi
Hoa for their kindness, friendship, guidance and support. The library facilities,
office, internet access as well as accommodation have been indispensable.
Acknowledgements to the people in Nui Coc Fishery Station for their collaboration,
particularly Mr. Le Quy Hung (the Director) and Mr. Pham The Hoang
(Accountant).
I would like to thank my adorable husband Irenée NIYIBIZI for his love,
understanding and personal support at all times. The great patience, assistance and
encouragement shown by my father Faustin RWAMFIZI NYANGEZI during these


ii

two years are greatly appreciated. My lovely children, mother, brothers, sisters and
brother in law have given me their equivocal support throughout, as always, for
which my mere expression of thanks likewise does not suffice.
Last but not least, many thanks to my classmates, all friends and relatives, your
friendship and encouragements were a real support during this period.


iii

ABSTRACT
Nui Coc Reservoir was constructed to be used mainly for two purposes: to control
flooding and to irrigate 11,500 ha of agriculture land and industrial zone. The latter
was recognized by the Government of Vietnam as an important provider of animal
protein and employment opportunities for people living in its vicinity. However as
in many other Vietnamese reservoirs, the biodiversity of fish fauna is deteriorating.
There is a decline of economically significant species in number and production.
The aim of this study was to collate available fisheries data such as catch data,
fishing effort, CPUE, fishing gear and estimate the Maximum Sustainable Yield
(MSY) for better management strategies in the future. This thesis research also
attempted to estimate the stocking efficiency in the water body.
Through primary and secondary data collection, catch assessment based on catch
and effort data was the main approach used to achieve the mentioned objectives.
MSY of two different periods (P1 and P2) were estimated applying Schaefer model.
A gradual decline in fish production trend during a twenty two-year period (19782010) was observed due to some interactions factors and inappropriate/poor
management of the reservoir. Two main fishing seasons (March-July and
September-November) were detected and this is related to the water level
fluctuations and the type of fishing gear used in Nui Coc reservoir. Stocking activity
as a cost effective strategy was found to have significant impact on fish yield when
implemented on a regular basis. However a number of factors should be taken into
consideration to optimize fish yield.

Some management strategies such as fishery regulation and regular stocking
activities are given as recommendations for long-term sustainability of the water
body resources.


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TABLE OF CONTENTS
ACKNOWLEDGEMENTS ...................................................................................... i
ABSTRACT .......................................................................................................... iii
TABLE OF CONTENTS ....................................................................................... iv
LIST OF TABLES ................................................................................................. vi
LIST OF FIGURES ............................................................................................... vii
ABBREVIATIONS ................................................................................................ ix
INTRODUCTION ................................................................................................... 1
CHAPTER 1- LITERATURE REVIEW ................................................................. 3
1.1. Overview on world inland fisheries ............................................................... 3
1.2. Reservoir fisheries in the world ..................................................................... 6
1.2.1. Overview on reservoir fisheries resources ......................................................... 6
1.2.2. Reservoirs classification ................................................................................... 8
1.2.3. Fish production in reservoirs ............................................................................. 8
1.2.3.1. Phase 1- Early growth production ............................................................... 9
1.2.3.2. Phase 2- Stabilization ............................................................................... 10
1.2.3.3. Phase 3- Sustainable production ............................................................... 11

1.3. Reservoir fisheries in Vietnam ..................................................................... 12
1.3.1. Water resources in Vietnam ............................................................................ 12
1.3.2. Reservoir fisheries development in Vietnam ................................................... 14
1.3.3. Reservoir fisheries management in Vietnam ................................................... 15
1.3.3.1. Government line management .................................................................. 16

1.3.3.1.1. Indirect GoV management ................................................................. 16
1.3.3.1.2. Provincial management ...................................................................... 16
1.3.3.1.3. Enterprise licensing management ....................................................... 17
1.3.3.1.4. Open access ....................................................................................... 17
1.3.3.2. Co-operative line management ................................................................. 17
1.3.3.3. Private sector line management ................................................................ 18

CHAPTER 2- METHODOLOGY ......................................................................... 19
2.1. Study area.................................................................................................... 19
2.2.1 The fishery ...................................................................................................... 21
2.2.2. Fish yield ........................................................................................................ 21
2.2.3. Historical development of Nui Coc fisheries ................................................... 22
2.2.3.1. Progression period - State subsidies (1976 – 1985) ................................... 22
2.2.3.2. Recession period (1986 – 1990) ............................................................... 22
2.2.3.3. Improvement period (1990 – 2000) .......................................................... 23

2.3. Data collection ............................................................................................ 24

Delet


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2.4. Fishing gear ................................................................................................. 25
2.5. Fishing effort ............................................................................................... 27
2.6. Catch per Unit of Effort ............................................................................... 27
2.7. Estimation Production (Y) ........................................................................... 28
2.8. Maximum Sustainable Yield (MSY)............................................................ 28
2.8.1. Standardization of effort ................................................................................. 29
2.8.2. Relationship of stocking density to fish yield .................................................. 30

2.9. Data analysis ............................................................................................... 30
CHAPTER 3- RESULTS AND DISCUSSION ..................................................... 32
3.1. Current fisheries management system in Nui Coc Reservoir ........................ 33
3.2. Fish production and MSY (relative CPUE and relative effort) ..................... 34
3.3. Fishing seasons............................................................................................ 39
3.4. CPUE and effort by gear.............................................................................. 43
3.5. Stocking efficiency ...................................................................................... 48
CHAPTER 4- CONCLUSION AND RECOMMENDATIONS ............................. 52
4.1. Conclusion .................................................................................................. 52
4.2. Recommendations ....................................................................................... 52
APPENDICES....................................................................................................... 62


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LIST OF TABLES
Table 1.1. World capture fisheries production in 2011 and 2012 (using data from
[16]) ........................................................................................................... 5
Table 1.2. Inland capture fisheries: Top ten producers in the world [16]. ................. 6
Table 1.3. Records on large reservoirs by regions [20] ............................................ 7
Table 1.4. Inland water bodies of Vietnam [39]. ................................................... 12
Table 1.5. General status of reservoir fisheries in Vietnam in 1993 [44]. ............... 13
Table 2.1. Some physical and chemical features of Nui Coc Reservoir [6] ............ 20
Table 2.2. Fingerling stocked and fish production in Nui Coc Reservoir from 1978
to 2000 [10]. ............................................................................................. 23
Table 2.3. Summary of the features of the main types of gear used and the species
of fish caught in Nui Coc Reservoir: ......................................................... 26
Table 2.4. Unit effort of fishing gear used in Nui Coc Reservoir ........................... 27
Table 3.1. Relationships between the relative CPUE and total yield to the relative

effort......................................................................................................... 37
Table 3.2. Relationship between fishing effort and CPUE for different fishing gears
in Nui Coc Reservoir ................................................................................ 43


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LIST OF FIGURES
Figure 1.1. World capture fisheries production in 2011 and 2012 (using data from
[16]) ........................................................................................................... 5
Figure 1.2. Records on large reservoirs by regions [using data from 20] .................. 8
Figure 1.3. The representation of the fish productivity changes ............................. 11
Figure 1.4. Reservoir fisheries productivity by regions in Vietnam in 1993 (using
data from [44]).......................................................................................... 13
Figure 2.1. Map of Nui coc reservoir showing its location (Source: Google map data
2014) ........................................................................................................ 19
Figure 2.2. Nui coc reservoir photograph showing the study area .......................... 19
Figure 2.3. Fluctuation in water level of Nui Coc Reservoir in 1999 [10] .............. 20
Figure 2.4. Fishing gear utilization [using data from 42, 48-50]. ............................ 21
Figure 2.5. Yearly fish productivity (kg/ha) in Nui Coc Reservoir in the period of
1978 - 2003 [10, 48, 50]. .......................................................................... 22
Figure 3.1. Schematic representation of the current Nui Coc Reservoir fisheries
management structure (Thai Nguyen province) ........................................ 34
Figure 3.2. Variation of the Fish production during 1978 to 2010 period ............... 35
Figure 3.3. The relationship between the relative effort (100 m2 of net used) and the
total yield (kg) for the period of 2001-2003 ............................................. 36
Figure 3.4. The relationship between the relative effort (100 m2 of net used) and the
total yield (kg) for the period of 2006-2008 ............................................. 36
Figure 3.5. The relationship between the relative effort (100 m2 of net used) and the
CPUE (kg per 100 m2 of net used ) for the period of 2006-2008 .............. 37

Figure 3.6. Monthly changes in fish yield using gill net during the period P1 (20012003) ........................................................................................................ 40
Figure 3.7. Monthly changes in fish yield using gill net during the period P2 (20062008) ........................................................................................................ 40
Figure 3.8. Monthly changes in fish yield using covert net during the period P1
(2001-2003) .............................................................................................. 40


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Figure 3.9. Monthly changes in fish yield using covert net during the period P2
(2006-2008) .............................................................................................. 40
Figure 3.10. Monthly changes in fish yield using trammel net during the period P1
(2001-2003) .............................................................................................. 41
Figure 3.11. Monthly changes in fish yield using trammel net during the period P2
(2006-2008) .............................................................................................. 41
Figure 3.12. Monthly changes in fish yield using seine net during the period P1
(2001-2003) .............................................................................................. 41
Figure 3.13. Monthly changes in fish yield using seine net during the period P2
(2006-2008) .............................................................................................. 41
Figure 3.14. Effect of reservoir water fluctuations on fish yield ............................. 44
Figure 3.15. The relationship between CPUE (kg) and effort of different fishing
gear, gill net and seine net in units of 100m net used per day, covert net and
trammel net in unit of 100m2 per day in the period of 2001-2003 ............. 45
Figure 3.16. The relationship between CPUE (kg) and effort of different fishing
gear, gill net and seine net in units of 100m net used per day, covert net and
trammel net in unit of 100m2 per day in the period of 2006-2008 ............. 46
Figure 3.17. The relationship between the number of fingerlings stocked and the
catch in Nui Coc Reservoir from 1978 to 2009 ......................................... 49
Figure 3.18. The relationship of the fish yield in year n, n+1, n+2 and stocking
density in year n in Nui Coc Reservoir ..................................................... 49

ix

ABBREVIATIONS
CPUE

Catch per Unit of Effort

DARD

Department of Agriculture and Rural Development

FAO

Food and Agriculture Organization

f MSY

Fishing Effort which provides the maximum yield

GoV

Government of Vietnam

ha

Hectare

IFEP

Institute of Fisheries Economics and Planning, Hanoi, Vietnam

IMC

Irrigation Management Co Ltd.

Kg

Kilogram

MARD

Ministry of Agriculture and Rural Development, Hanoi, Vietnam

MoF

Ministry of Fisheries, Hanoi, Vietnam

msl

Maximum supply level

MSY

Maximum Sustainable Yield

P1

Period extending from 2001 to 2003

P2

Period extending from 2006 to 2006

PAIGELAC

Inland Lakes Integrated Development and Management Support
Project

PGO

Provincial Government Organization

PRA

Participatory Rapid Appraisal

RAB

Rwanda Agriculture Board

RIA1

Research Institute for Aquaculture No.1

SD

Stocking density

TNIMC

Thai Nguyen Irrigation Management Co. Ltd.

%

Percentage


1

INTRODUCTION
In recent years, the number of reservoirs has been steadily growing all over the
world. The purposes of reservoir construction may include irrigation, hydro-electric
power generation, flood control, public water supply, recreation and navigation [1,
2] but they are never impounded for fishery development per se. Fisheries are
beginning to be recognized as an important secondary user of reservoir water
resources. In certain instances, it is reported that the income from the fishery
exceeds that from the intended primary function of the reservoir, such as for
example in Ubolratana reservoir, Thailand [3]. In the light of increasing population
growth, reservoirs fisheries are becoming increasingly important providers of
animal protein and employment opportunities, particularly to poorer sectors of the
community, which also often happen to be rural. Unlike in the past, reservoir
fishery activities are considered as a significant avenue for resettling displaced
persons particularly exemplified in the case of Saguling and Jatinuhur reservoirs in
Indonesia [4].
The current number of reservoirs in Vietnam is much higher than in the past
because many new reservoirs have been constructed [5], with about more than 6000
reservoirs built in recent years (information provided by Directorate of Irrigation,
2014). Most of reservoirs in Vietnam were built after 1954 and depending on their
functions, reservoirs may be classified for irrigation, hydro-electricity, flood

control, industry and drinking uses [5-7]. However, most of them are constructed to
serve several purposes. Reservoir construction is continuing, in response to the
growing demand for energy, water for crops and these water areas can usually be
used secondarily for fisheries [8]. Fisheries have an important role in providing
livelihoods for a large numbers of persons, particularly those living in the vicinity
of the reservoirs.
Nui Coc Fishery Station is a subsidiary of Thai Nguyen Irrigation Management Co.
Ltd. (IMC), under the management of People's Committee of Thai Nguyen
Province. The latter is in charge of promoting aquaculture development on the


2

reservoir as well as creating better conditions for the surrounding inhabitants to
exploit the reservoir resources. Fishing activity was stopped in this water body by
the Thai Nguyen IMC from 2012 to 2013 due to dam rehabilitation work but it is
well known there is a socio-economic link between the enterprise and the
neighboring reservoir population whose livelihoods depend largely on the fishery
resources.
However, it is noted that biodiversity of fish fauna in most existing reservoirs in
Vietnam is deteriorating. A decline in the number and production of economically
significant species has been recognized in most reservoirs throughout the country
including Nui Coc reservoir

[5, 6]. Nguyen [9] noted that 70-75% of fish

production in north and central Vietnam was fish of low value [5]. At the same
time, lack of systematic information on fisheries data from Nui Coc reservoir is a
major limitation for the management of the fish stocks and this is mainly due to the
high costs, qualification of staffs and limitation in management [10].

Information on fish yield and species composition, major gear types and their
effectiveness, estimates on total fishing effort, marketing and management aspects
(gear limitation and closed seasons if any, landing size limits, etc…), are crucial for
effective and sustained management of fishery resources. The main purpose of this
study was to determine the optimal level of sustainable exploitation to support
better planning for the future. This is not only needed for the sustainable
management but also in order to assist the Government to meet its broader
development objectives of increasing the availability and affordability of fish
supplies to the rural poor and generate livelihood opportunities for communities
living in the vicinity of the reservoirs. Therefore, the present study was undertaken
in Nui Coc reservoir with the following objectives:
a.

To collate the information about fishing gear, seasonal harvesting,
effort, CPUE, mean landing sizes, catch data, current fisheries
management policy, etc.

b.

To estimate the MSY of the reservoir in order to have a better fisheries
management strategy in the future.


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CHAPTER 1- LITERATURE REVIEW
1.1. Overview on world inland fisheries
Inland fisheries are defined as any activities conducted to extract fish and other
aquatic organisms from inland waters [11]. Fish from inland water capture fisheries
are an important source animal protein, especially in landlocked countries and for

riparian populations.
Beard Jr et al [12] noticed that inland fisheries provide a vital source of commerce,
employment, nutrition and recreation for people throughout the world [13, 14]. This
is particular so in subsistence and artisanal inland fisheries, where fisheries
constitute a “safety net” for millions of rural poor people by providing an essential
source of food and supplement income.
Fish and other aquatic resources are captured from a great variety of inland systems,
including perennial lakes, which have combined surface area of about 1.7 million
km2, nearly 1 million km2 of which are accounted for by large lakes (˃ 100 km2).
Swamps, marshes and other wetlands throughout the world amount to about 4
million km2, of which CIS and the Baltic States claim the greatest portion. The
world’s main channel river length amounts to about 269.000 km. The highest
density of rivers occurs in South America, the lowest in Oceania [15].
Global production in 2012 as derived from the FAO fishery statistics database
showed a new maximum production of 86.6 million tonnes when the highly
variable anchoveta (Engraulis ringens) catches is excluded. The share of global
inland capture production in the total global capture production remains below 13
percent (11.6 million tonnes) in 2012 (see table 1.1 and figure 1.1) and inland
waters continue to be the most difficult subsector for which to obtain reliable
capture production statistics [16]. Some 11,500 fish species (41 percent of all
fishes) are exclusively freshwater and about one percent are diadromous. Only
about 100 fish species or species groups are listed in FAO statistics as making up
inland capture.


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In Asia, inland fisheries currently account for about 10.5%, and what is important to
note is that it has been increasing over the past decade or so albeit to a smaller
extent. Similarly, when one considers the inland fishery sector itself, it is evident

that Asia contributes in excess of 50% of world production. Asia has the highest
relative share of the inland capture fisheries production and reservoirs, but the
second-lowest (next to Oceania) river density index [2].
Inland fisheries are jeopardized by lack of research-based understanding of how
other human activities affect the production potential of rivers and lakes [17].
Owing to incomplete understanding of how inland waters function, policy makers
and managers often fail to intervene when fisheries are in decline, until the
ecosystem has virtually collapsed [18]. According to FAO [16], the contribution of
inland fishery resources to food security is greatly underreported because of the
large number, dispersion, variety and dynamic nature of inland water bodies and the
diversity of the aquatic fauna. It has been stated that actual catches may be at least
twice as high as submitted to FAO by inland fishing countries. Failure to improve
basic data gathering and reporting on inland fisheries can result in the following:
-

Insufficient political justification by fishery administrations to invest in the

fishery management and to effectively protect inland fishery resources from
degradation of the environment and loss of habitat;
-

Lack of awareness of policy makers in other sectors of the importance of

inland fish and fishery resources to food security and income generation which
consequently reduce the capability in the fisheries sector to compete for resources
and for access to water;
-

Interests and needs of inland fish producers, processors, transporters and

marketers often being ignored or neglected, particularly at local levels; and
-

Lack of a suitable knowledge base for dialogue and co-operation among

countries sharing inland water fishery resources and having lake and river basins in
common.


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Table 1.1. World capture fisheries production in 2011 and 2012 (using data from
[16])
2011

2012

Million tones

Million tones

11.1

11.6

4.50%

(excluding anchoveta)

74.3

75.0

1.00%

Anchoveta

8.3

4.7

-43.60%

Marine Capture

82.6

79.7

-3.50%

World total

93.7

91.3

-2.60%

Inland capture

Variation
2011 - 2012

Marine Capture

Figure 1.1. World capture fisheries production in 2011 and 2012 (using data from
[16])


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Table 1.2. Inland capture fisheries: Top ten producers in the world [16].
No

Country

2012 (tones)

1

China

2,297,839

2

India

1,460,456

3

Myanmar

1,246,460

4

Bangladesh

957,095

5

Cambodia

449,000

6

Uganda

407,638

7

Indonesia

393,553

8

Tanzania, United Rep of

314,945

9

Nigeria

312,009

10

Brazil

266,042

Total 10 major countries

8,105,037

Total other countries

3,525,283

World total

11,630,320

Share 10 major countries

69.70%

1.2. Reservoir fisheries in the world
1.2.1. Overview on reservoir fisheries resources

As mentioned above, reservoirs are relatively new aquatic ecosystems in the global
landscape, although reservoirs construction has played a major role in shaping
certain ancient civilizations. Reservoirs represent an important economic and
environmental resource that provide benefits such as flood control, hydropower
generation, navigation, water supply, commercial and recreation fishing,
particularly in developed countries.
Avakyan and Iakovleva [19] noticed that worldwide there are about 60,000
reservoirs with a volume larger than 10x106 m3, representing a total volume
exceeding 6500 km3 and a surface of about 400.000 km2. Over 2,800 of these
reservoirs have a volume larger than 100x106 m3 and account for about 95% of the
6500 km3 combined reservoir’s volume. Of these larger reservoirs, for which


7

records are more reliable, 915 (1,690 km3) occur in North of America, 265 (971
km3) in Central and South America, 576 (645 km3) in Europe, 815 (1,980 km3) in
Asia, 176 (1000 km3) in Africa, and 89 (95 km3) in Australia and New Zealand (see
table 1.3 and figure 1.2) [20].
Costa-Pierce [21] reported that the reservoir acreage was 5,376,618 ha in Asia and
was predicted to reach 16,798,000 ha by 2010 which is tantamount to a 212%
increase in reservoir acreage in approximately 25 years or so [2].

Weimin [22] mentioned that China has a long history of reservoir fisheries activity
and many reservoirs have been constructed since the 1950s for various purposes.
The earliest reservoir in China, Dong Qian Hu, was constructed about 1000 years
ago in Zhejiang Province. However, there were only 20 reservoirs in the whole of
China until 1949, when the People’s Republic of China was founded. Reservoir
fisheries are a major component of freshwater in China with a cultivable surface
area of more than 2 x 106 ha [23], amounting to 40% of the total cultivation fresh
water area. More than 86 000 reservoirs have been constructed with a total storage
of 4 x 1011 m3 [23]. MAFBC [24] reported that in terms of area, lakes and reservoirs
account for the largest part of freshwater aquaculture with a total area of 2.72
million ha, followed by pond culture with 2.33 million ha.
Table 1.3. Records on large reservoirs by regions [20]
Region

Number of reservoirs Volume of reservoirs (Km3)

North of America

915

1,690

Central and South America

265

971

Europe

576

645

Asia

815

1,980

Africa

176

1,000

Australia and New Zealand

89

95

Total

2836

6,381


8

Figure 1.2. Records on large reservoirs by regions [using data from 20]
1.2.2. Reservoirs classification

One main difficulty that confronts fishery scientists is the different classification of
reservoirs adopted by different nations. For example, De Silva [2] mentioned that in
India, according to Srivastava et al. (1985) reservoirs are classified as small (< 1000
ha), medium (1000 to 5000 ha) and large (˃ 5000 ha). Whereas in China the
classification is based on capacity (De Silva et al., 1991) where small, medium and
large reservoirs refer to capacities of less than 10 x 106, 10-100 x 106 and more than
100 x 106m3, respectively. In Sri Lanka reservoirs are classified as perennial and
non perennial (=seasonal tanks), and generally the latter rarely exceed 20 ha.
In Vietnam, based on size, the reservoirs may be classified as large, which have
more than 10,000 ha of water surface, medium of 1,000 to 10,000 ha, and small,
those less than 1,000 ha [5].
1.2.3. Fish production in reservoirs

Scientists began to study the possible use of reservoirs for fishery purposes in the
early 1920s. Some viewed reservoirs as “biological deserts” because of low fish
productivity after the first few years of high production [25]. Intensive forms of
management were applied in the hope of solving the problems [1].
Based on many studies done previously, there is a common belief that the fish yield
tend to be higher during the first few years after impoundment and then after this
period start to stabilize at a certain lower level. Thus, fish productivity in reservoirs
can be summarized in three main phases (see figure 1.3).


9

1.2.3.1. Phase 1- Early growth production

The fishery productivity of reservoirs typically rises rapidly to a maximum within
the first few years of their existence [26]. The increase is believed to be the result of
better environmental conditions which favour fish growth. During the filling period
of the reservoirs, water leaches nutrients from newly inundated soils, submerged
plant debris, and other organic matter. The impounded water is of high fertility,
encouraging the growth of bacteria, phytoplankton, zooplankton and benthos which
in turn serve either directly or indirectly as food for fish; consequently fish feeding
on these organisms become more abundant as do also predacious species which in
turn feed on the small fish.
Huang et al [27] noticed that, after impoundment of Chinese reservoirs, the flow
decreases, the surface area widens, the water column deepens, the transparency
increases, nutrient accumulate and fisheries production increases. Migratory species
gradually disappear because of obstruction from dams but economically valuable
species grow quickly and their yield increases rapidly at this stage.
Pirozhnikov [28] observed that production of fish in the main Dnieper reservoir in
the USSR was nearly four times greater than the annual catches from unregulate
Dnieper. In the inundated section of Barren River (USA) the fish stock increased
from an average of 125 kg/ha/year before impoundment to 218 kg/ha/year by the
first year post impoundment [29]. The second and third years after impoundment
had standing stocks averaging 225 and 270 kg/ha/year, respectively. Turner [30]
estimated that the average yield of fish harvested increased from about 4 kg/ha
before impoundment to 17 kg/ha in the first two years of impoundment in Rough
reservoir. Mabaye [31] reported that fish production in Lake Kariba in Africa
increased several-fold from pre-impoundment conditions, and fish landings
increased from 454 t in the second year to 1, 814 t in the fifth year of impoundment.
In southeast Asia and the Indian sub Continent, the yield of fish reservoirs follows
more or less the same pattern found in reservoirs elsewhere. Fish production in
reservoirs in Thailand has greatly increased to much higher levels than found during

10

river conditions previously [32]. Bhukaswan and Pholprasith [33] reported that fish
production in Ubolratana Reservoir had increased from 474 t in the first year of
impoundment to 2,100 t in the fourth year and the maximum yield of 2, 480 t
occurred in the ninth year. The catch landing of the Bhumipol Reservoir increased
from 583 t in the second year to 1,089 t in the third year after the closure of the
Bhumipol dam [34]. Fish yield of Rihand Reservoir in India reached its peak in the
fourth year of impondment [35]. Annual fish landing from Gandhi Sagar Reservoir
also in India reached its first peak in the fourth year but the maximum yield of the
697 t of fish was obtained in the 13th year after the closure of the dam [36]. Sarnita
[37] reported that fish yields in reservoirs in Indonesia reached their peaks during
the first several years of impoundment. He pointed out that maximum catch was
recorded in the fourth year after impoundment in Jatiluhur, the ninth year in lake
Darma and twelfth year in lake Rawa Pening.
1.2.3.2. Phase 2- Stabilization

After a few years of impoundment, as the water continue to be filled and used in the
reservoir, the fish yield will decline and stabilize at a low level due to the high
water exchange rate and low food availability, the macrophyte population which is
not formed any more or is less developed, and, some fish being unable to find
suitable spawning grounds or suitable feeding areas for their larvae.
High fish production in reservoirs is usually not long sustained. It remains high only
for one to several years, declining rapidly in ensuing years to a much lower level
which may be maintained or may gradually rise to somewhere near half the
magnitude of the initial phase [26, 38].
During this phase, pelagic piscivorous fish gradually develop and become
dominant. Predation from piscivorous fish and capture pressure on economically
valuable fish are strengthened. The development of economically valuable fish is

prohibited and the trash fish populations with high rates of reproduction grow
quickly to compensate for losses from predation. Since trash fish have a low market
value and are hard to capture, their rapid development decreases the production


11

potential of reservoirs [27]. It is believed that fish production reflects the general
fertility of reservoirs which includes a number of processes such as:
1. The leaching of nutrients and other elements from the soil;
2. The decomposition of organic matter predominantly of plant origin
flooded by rising water;
3. The loss of the nutrients locked up in the bottom sediments;
4. And the reduction in the bottom fauna due to rapid sedimentation
1.2.3.3. Phase 3- Sustainable production

The fish populations and their food organisms become naturally adjusted to the
permanent basic fertility of the basin and in additional nutrients from the inflows
and watershed runoff [1]. It is usually recommended to apply some management
strategies at this stage to optimize the fish yield of the reservoir:
-

Reduction of the fishing pressure;

-

Protection of fish brood stocks and their spawning grounds;

-

Protection of fry and fingerlings by fixing the minimum fish size and
capture rate;

-

Restocking activities;

-

Intensive fish culture (Cage culture, pen culture…).

A simplified representation of the change can be represented as:

Figure 1.3. The representation of the fish productivity changes


12

1.3. Reservoir fisheries in Vietnam
1.3.1. Water resources in Vietnam

Total inland waters in Vietnam are estimated to be 1,379,038 ha; of which small
water bodies such as pond areas contribute 4% whilst rice field contribute to about
39% (548,050 ha). Large water bodies such as rivers, streams, lakes and reservoirs
which collectively amount 397,500 ha contribute a significant proportion comparing
to that of the total aquaculture area (28%) [39].
According to data collected by the Research Institute for Aquaculture No.1 (RIA1)
in 1993, there were 768 reservoirs in 38 provinces with a combined area of 215 549
ha in Vietnam [40]. The Institute of Fisheries Economics and Planning (IFEP) in
1994 indicated that there are about 2470 reservoirs with a total area of 183 579 ha in

the country. Among these, reservoir greater than 5 ha in area total 1 403, with a
total area of 181 176 ha [41].
Table 1.4. Inland water bodies of Vietnam [39].
Inland water bodies

Area (ha)

Percent (%)

Ponds

58,088

4

Large water bodies

397,500

28

Rice fields

548,050

39

Inter-tidal

290,700

21

Bays, lagoons

84,700

8

However, as mentioned above, the current number of reservoirs must be higher than
in the past because many new reservoirs have been constructed in recent years.
There are about 4000 community reservoirs [42] of which 460 are medium or large
in size, with water volume of more than a million m3 [9] . They are usually located
in hilly and mountain areas in the north and central part of Vietnam. According to
the Directorate of Irrigation-Ministry of Agriculture and Rural Development of
Vietnam (MARD), the current number of reservoir is about more than 6000 [43]. In
the last 20 years, a lot of reservoirs have been built in Vietnam.


13

Table 1.5. General status of reservoir fisheries in Vietnam in 1993 [44].
Regions

Northern
Northern Central
Southern Central
Central Plateau
Eastern Mekong
Total

Total
reservoir
area (ha)
63677
20775
11196
12424
73105
181177

Stocked
No
Area
(%)
(%)
3.4
10.3
33.9
8.9
7.1
43.9
3.2
3.2
19
1.3
7.6
8.1

Production

Total catch Productivity
(t)
(kg/ha)
370.4
56.4
92
50.0
192
39.1
59.5
150.6
314
330.9
1027.9
70.1

Figure 1.4. Reservoir fisheries productivity by regions in Vietnam in 1993 (using
data from [44]).
Based on the size, the reservoirs may be classified as large, which have more than
10,000 ha of water surface, medium of 1,000 ha to 10,000 ha and small, those of
less than 1,000 ha. The country has very few large reservoirs; Thac Ba (Yen Bai
Province – 19,050 ha), Hoa Binh (Son La and Hoa Binh Provinces – 16,792 ha), Tri
An (Dong Nai Province – 32,400 ha), Thac Mo (10 600 ha) and Dau Tieng (Tay
Ninh Province – 27,000 ha). The medium reservoirs consist of Yaly (Gia Lai and
Kon Tum Provinces – 6,450 ha), Phu Ninh (Quang Nam Province – 3,200 ha) and
Nui Coc (Thai Nguyen Province – 2000 ha), Easoup Thuong (Dak Lak Province –
1270 ha and Dong Mo (1,100 ha). The density of small reservoirs is very high in the