tilapia cage culture in rwanda current status and prospects of future development

MINISTRY OF EDUCATION AND TRAINING
NHA TRANG UNIVERSITY
Institute of Aquaculture


Fidele KAMPAYANA
54CH345




Tilapia cage culture in Rwanda:
Current status and prospects of future development.






Master’s Thesis

Nha Trang, July 2014
2

MINISTRY OF EDUCATION AND TRAINING
NHA TRANG UNIVERSITY
Institute of Aquaculture


Fidele KAMPAYANA
54CH345

Tilapia cage culture in Rwanda:
Current status and prospects of future development.


Thesis for partial fulfillment of Master of Science in Aquaculture
FIELD CODE: 60620301

Supervisor:
Le, Minh Hoang Ph.D
Nha Trang University/ Vietnam
Co-supervisor:
Nguyen Tan Sy Ph.D
Nha Trang University/ Vietnam

NTU, July 2014
i

Declaration

I hereby declare that this is my original work and has not been produced elsewhere for
the award of a Masters in Aquaculture Science. Where other people’s ideas have been
used, they have duly been acknowledged.


Fidele KAMPAYANA

ii

Acknowledgement
For this opportunity I would like to express my heartfelt gratitude to all those that
supported me from the start of this thesis until the last minute. Surely, without your
contributions, this research could not be accomplished.
So, I say thank you to my supervisors, Le Minh Hoang, Ph.D Lecturer in Nha Trang
University and Sy Nguyen Tan Ph.D, the Vice Director of Aquaculture Institute in Nha
Trang University/Vietnam for their advice and thoughtful contributions towards the
accomplishment of this thesis. I must admit that with their guidance, writing this thesis
was made much easier. The Director of Aquaculture Institute in Nha Trang University
and particularly all lecturers of Aquaculture Master Class for their timely advice and
encouragement.
In a special way I thank the Director General of RAB/MINAGRI and particularly the
Fisheries and Aquaculture Program Manager, Dr Wilson RUTAGANIRA for accepting
me into their group members and for providing me all facilities and services needed for
this research. I am grateful to all Fisheries and Aquaculture Officials in lakes Burera,
Kivu, Muhazi and Ruhondo for accepting to work closely with me during my fieldwork.

Thank you for the reports and facilities you rendered. I also acknowledge the support got
from all respondents of this survey and interviews. Thanks for the information, data and
facilities you provided.
I wish to say “Com on” to all Vietnamese comrades for the comfortable accommodation
offered to me during my stay in their lovely country “Viet Nam” and my fellow course
mates who were supportive as we struggled together. I also acknowledge the moral
support of my family members, friends and relatives both in Rwanda and in Vietnam.
Finally, the AfDB through SFAR education loan fund for the scholarship awarded for my
Master courses.
Fidele KAMPAYANA
iii

Dedication

I dedicate this thesis to my last born daughter IZERE IGENA Darlene, my first son
IZERE IGABA Dharna and my beloved UWUMUKIZA Clarisse; mother of my two
children.

iv

Table of contents
Page
Declaration I
Acknowledgement ii
Dedication iii
Table of contents iv
Abstract vii
List of acronyms viii
List of graphs, figures and pictures IX
List of tables X
INTRODUCTION 1
Background information 1
Statement of the problem 2
Justification of the study 3
Objectives of the study 4
Hypotheses 4
Main parts of the thesis 4
CHAPTER 1: THEORETICAL FRAMEWORK AND CAGE CULTURE CONCEPTS 5
1.1. Historical and global overview of fish cage culture 5
1.2. Advantages and disadvantages of cage culture 7
1.2.1. Advantages 7
1.2.2. Disadvantages 8
1.3. Tilapia; Cage cultured species of choice 9
v

1.4. Cage design and construction 10

1.4.1. Classification of cage design and construction 10
1.4.2. Components of cage design and construction 12
1.4.3. Site selection and carrying capacity of cage culture 13
1.4.4. Classification of cage culture according to different management systems 16
1.5. Management practices of fish cage culture 17
1.5.1. Fish seed and stocking practices 17
1.5.2. Feeds and Feeding 19
1.5.3. Stock sampling 21
1.5.4. Harvesting 21
1.5.5. Cage maintenance and monitoring 22
1.5.6. Record keeping 22
1.6. Cage culture economics 23
1.6.1. Production costs 23
1.6.2. Returns 24
1.6.3. Break-even price 25
1.6.4. Market identification 25
CHAPTER 2: STUDY DESIGN AND METHODOLOGY 26
2.1. Scope and duration of the study 28
2.2. Survey 28
2.3. Interviews 29
2.4. Field studies and personal observations 29
2.5. Data analysis 29
CHAPTER 3: RESULTS AND DISCUSSIONS 30
3.1. Characteristics of the study area 30
3.1.1. Characteristics of Rwanda 30
3.1.2. Characteristics of the potential zones for tilapia cage culture 31
3.2. Current status of Tilapia cage culture in Rwanda 36
3.2.1. Characteristics of tilapia cage operators 36
3.2.2. Characteristics of Tilapia cage farms 40
3.2.3. Management practices 45

vi

3.2.3.1. Site selection and environmental monitoring 45
3.2.3.2. Seed supply and stocking practices 47
3.2.3.3. Cage inspection and maintenance 50
3.2.3.4. Stock sampling and fish manipulations 51
3.2.3.5. Feed supply and feeding practices 53
3.2.3.6. Harvesting practices and marketing system of harvested fish 55
3.2.3.7. Economic analysis of the current tilapia cage production in Rwanda 57
3.3. Perception of cage operators on the current and future prospects of tilapia cage culture
industry. 60
3.3.1. Appreciation of current productivity of tilapia cage culture 60
3.3.2. Constraints and problems of current tilapia cage culture in Rwanda 61
3.3.3. Needs for future development of Tilapia cage culture in Rwanda 64
CHAPTER 4: CONCLUSION AND RECOMMENDATIONS 67
4.1. Conclusion 67
4.2. Recommendations for prospects of future development of tilapia cage industry 67
References 69
Appendices I

vii

Abstract
The present study was conducted in order to contribute to the future development of
tilapia cage culture industry in Rwanda by reviewing the current production system and
its constraints and then propose possible solutions to overcome them. In order to obtain
the results of this research, 52 formal questionnaires have been personally administrated
to 52 tilapia cage operators (total population). In addition to the formal questionnaire, 28
in-depth interviews have been conducted with potential key informants in order to have
insight into the constraints of tilapia aquaculture development as well as the solutions that
can be considered to promote the future commercial tilapia cage aquaculture adoption in
Rwanda. The findings of this study revealed that 52 tilapia cage operators are located in
four lakes (Kivu, Ruhondo, Burera and Muhazi) and grouped into 23 tilapia cage-based
parks. A total of 656 floating cages were numbered in all parks; 80.5% of them were not
restocked (empty cages) due to many reasons provided by the operators such as lack of
funds to buy feed, lack of fingerlings and fear of high mortality rate. The stocking density
by most of the operators was ranged between 1,500 to 2,000 tilapia fingerlings of 10 to
50 g average weight per cage of 8m
3
. The harvest size of fish after 8 to 9 months of grow
out was approximated by most of the operators at 4 fish per kg (equivalent to 250g
average weight). During culture period, fish were fed twice or thrice a day with imported
floating pellet of 25 to 35% of protein content. The shortage of tilapia fingerlings, high
cost of quality feed and high mortality rate of fish were the major factors that affected
current tilapia cage production. Most of the operators did not undertake more innovative
and adequate management practices such as optimum stocking, stock manipulations,
sexing and grading, disease control and prevention, records keeping on inputs and outputs
of the cage culture operations due to lack of knowledge and know-how skills. However,
seed and feed production at farm and local levels, training and extension program based
on research and technological innovation were highly suggested by most of the operators

as main ways towards sustainable development of tilapia cage culture industry in
Rwanda.
viii

List of acronyms
AfDB: African Development Bank
BCEOM: Central Bureau for Overseas Equipments Studies (Bureau Central d’Etudes
pour les Equipements d’Outre Mer)
BMPs: Best Management Practices
DO: Dissolved Oxygen
DVO: District Veterinary Officer
EAC: East African Community
EIA: Environmental Impact Assessment
FAO: Food and Agriculture Organization of United Nations
FCR: Feed Conversion Ratio
GoR: Government of Rwanda
IFOM: International Federation of Organic Agriculture Movements
LVHD: Low Volume High Density
MINAGRI: Ministry of Agriculture and Animals Resources
MST: Mixed Sex Tilapia
NGO: Non Government Organization
NTU: Nha Trang University
PAIGELAC: Projet d’Appui à l’Aménagement Intérgé et à la Gestion des Lacs
Interieurs (Inland Lakes Integrated Development and Management Support Project)
RAB: Rwanda Agriculture Board
RAS: Recirculation Aquaculture System
SFAR: Student Financing Agency for Rwanda
SRT: Sex Reversed Tilapia
VTS: Vocational Training School

ix

List of graphs, figures and pictures
Page
Figure 1.1: Principal components of floating cage described by Vázques [28] 12
Figure 2.1: Schematic diagram of the contents 26
Figure 2.2: Schematic diagram of the study design and methodology 27
Figure 3.1: Rwanda map showing a hydrological network
(Source:http:// www.rema.gov.rw/soe/ chap7.pdf). 30
Figure 3.2: Map of Lake Kivu (source: Fisheries and Aquaculture Master Plan, 2011) 32
Figure 3.3: Map of Burera and Ruhondo
(source: Fisheries and Aquaculture Master Plan, 2011) 33
Figure 3.4: Map of lake Muhazi (source:
Fisheries and Aquaculture Master Plan, 2011) 34
Figure 3.5: Repartition of tilapia cage operators in lake kivu, Ruhondo,
Burera and Muhazi. Rwanda, 2014 37
Figure 3.6: Organization of tilapia cage operators in lakes Kivu, Ruhondo,
Burera and Muhazi. Rwanda, 2014 37
Figure 3.7: Reasons of tilapia cage culture adoption by 52 operators in Rwanda, 2014 39
Figure 3.8: Repartition of tilapia cages in lakes Kivu, Ruhondo, Burera,
Ruhondo and Muhazi. Rwanda, 2014 41
Figure 3.9: Photo of cage farm unit taken at the time of the field visits
in lake Kivu, 2014 42
Figure 3.10: Repartition of tilapia cage farm size among 52 operators in lake Kivu,
Ruhondo, Burera and Muhazi. Rwanda, 2014 43
Figure 3.11: Frequency of water quality monitoring 46
Figure 3.12: Climate changes and natural calamities affecting tilapia cage culture
in lake Kivu, Ruhondo, Burera and Muhazi. Rwanda, 2014 47
Figure 3.13 : Valuation of current tilapia cage culture productivity

among 52 respondents 61
Figure 3.14: Constraints affecting current tilapia cage culture production
in Rwanda, 2014 62
Figure 3.15 : Solutions suggested for the current constraints of tilapia cage culture
by 52 tilapia cage operation in Rwanda, 2014 65


x

List of tables
Page
Table 1.1: Classification of cage design and construction based on technical
characteristics. 11
Table 1.2: Recommended daily feeding rates, expressed as the percentage of body
weight, for Tilapia of different sizes. 20
Table 3.1: Physicochemical parameters of Lakes Burera and Ruhondo
(Source: BCEOM report 2008) 34
Table 3.2: Physicochemical parameters of Lake MUHAZI
(Source: BCEOM report 2008) 35
Table 3.3: Background information on 52 tilapia cage operators in Rwanda, 2014 38
Table 3.4: Number of cage-based parks, units and cages identified in lake Kivu,
Ruhondo, Burera and Muhazi. Rwanda, 2014 42
Table 3.5: Stocking density, size and source of fingerlings among 52 tilapia cage
operators in Rwanda 2014. 49
Table 3.6: Frequency of cage inspection and cleaning in lake Kivu, Ruhondo,
Burera, and Muhazi. Rwanda, 2014 50
Table 3.7: Frequency of fish sampling and stock manipulation purposes 52
Table 3.8: Type of fish feed used by 52 tilapia operators in Rwanda, 2014 54
Table 3.9: Frequency of daily feeding and feeding rate estimation methods
used by 52 cage operators in lake Kivu, Ruhondo, Burera and Muhazi. Rwanda 2014. 55

Table 3.10: Harvesting practices and marketing system of cage produced tilapia
by 52 operators in Rwanda, 2014 56
Table 3.11: Production parameters calculated for farm unit of 10 tilapia cages 58
Table 3.12: Economic indicators calculated for cage farm unit of 10 tilapia cages 59


1

INTRODUCTION
Background information
The aquaculture sector has increasingly played a vital role in meeting the growing global
demand for food [9]. This role has internationally widely recognized in recent years.
Cage aquaculture subsector has grown very rapidly during the past 20 years and is
presently undergoing rapid changes in response to pressure from globalization and
growing global demand for aquatic products [12].
Aquaculture, which is broadly defined as the culture of finfish and shellfish in water
systems is rapidly expanding in both size and culture systems and is therefore among the
fastest growing industries, second to biotechnology. In 2009 Das, Vass et al. [6] defined
aquaculture as the rearing fish and others aquatic organisms in the man-made ponds,
reservoirs, cages or other enclosures in lakes and coastal waters. Cage culture is when
fish are reared in from fry to fingerlings, fingerlings to table size or table size to
marketable size while captive in an enclosed space that maintains free exchange of water
with the surrounding water body [6]. Fish can be also cultured in one of the four culture
systems; ponds, raceways, recirculation and cages systems [16].
Rearing fish in suspended cages in reservoirs, rivers or lakes is a practice relatively new
in many parties of continents. Fish cage culture dates back in 18
th
century and were likely
used by fishermen as holding structure until fish could be accumulated for market. The
first true cages for producing fish were developed in the Southeast Asia around the end of

19
th
century [16], [2], and [10]. In recent years, cage culture has spread throughout the
world to more than 35 countries in Asia, America, Africa and Europe. By 1977, more
than 70 species of fresh water fish have been experimentally grown in cages [11]. Today
cage culture is practiced in many regions in the world and is a thriving industry in some
areas. Tilapia cage culture is the most freshwater cultivation practice which has
contributed substantially to livelihood, food demand and poverty alleviation.
2

Cage culture researches have been limited because of mostly large scale open pond
culture system was considered more economically viable, and therefore, receiving more
attention in research. However, factors such as increasing fish consumption, declining
wild fish stocks, and a poor farm economy have produced a strong interest in fish
production in cages by both researchers and commercial producers [12]. The present
studies entitled “Tilapia cage culture in Rwanda; Current status and prospects for future
development” aimed to review the current situation of cage culture in Rwanda and the
main constraints which may impede its future development.
Statement of the problem
With national population expansion, the demand for fish is steadily increasing and natural
fish production have declined during the last decade due to environment degradation and
overfishing [27]. Tilapia cage culture, most popular freshwater cultivation practice is
highly recommended in almost Rwanda water bodies due to its substantial contributions
to livelihoods, food demand and poverty alleviation [20]. Despite its socio-economic
importance, actually cage culture is almost none existing in Rwanda water bodies
whereas the country is blessed with an abundance of scattered inland water mass
comprising of lakes, reservoirs, ponds, streams and rivers which the total surface
estimated is 210,000 hectares.
Since 2000, Tilapia cage culture practice has been successfully tried and frequently
demonstrated in Rwanda. Unfortunately, after the period of first trials, this new

aquaculture technique had have not expanded and all infrastructures completely had
depleted without any trace. The practice is still remaining at the experimental and
demonstration levels.
For over the decade it has not yet translated to large scale, either on commercial or
subsistence levels despite many supporting projects. Meanwhile, the present study avails
to clarify the current situation of cage culture and its prospect by looking on the causes
which stunting a sustainable growth of cage culture industry in Rwanda.
3

Justification of the study
For supplying an increase fish demand, Rwanda is straggling to reach the level of sub-
Saharan fish per capita consumption of 6.7 kg per person per year while Rwandan annual
fish consumption is still at 1.5 kg per caput. However, national fish production stands at
only 13,000 metric tons of annual fish production whereas capture fisheries contribution
is 9,000 metric tons and only 4,000 metric tons is aquaculture production [20]. Despite
the dormancy of fish farming in Rwanda during and nearly after the tragedy of 1994,
MINAGRI under PAIGELAC project has revitalized aquaculture sector since 2006 by
providing material and technical support to fish farmers. Since 2010, Tilapia cage culture
pilots and demonstration projects were reinitiated in selected potential water bodies, until
now, no scientific study has been conducted on them.
Tilapia cage culture production systems appear to be well-suited for adoption by both
small and large scale producers because the initial capital investment is not high. Because
of declining catch and catch per effort of numerous Rwandan lake fisheries, large
numbers of small-scale fishermen have to be attracted to cage culture systems where the
investment required is comparable to that of a fishing unit. The increased production
resulting from all these enthusiasms will have impacts on animal protein supply and will
respond considerably to stead fish demand in the country.
Because of the project potential for generating income to no land or poor farmers and
protein to consumers, the present study has been chosen to review the current status of
the initiated tilapia cage culture projects and to draw a persuasive baseline to new

investors for profit maximization and technology transfer to overall local communities
based on the experience of previous adopters. Possible constraints to further expansion of
cage culture in Rwanda are also needed to be identified.
4

Objectives of the study
The main goal of this study is to contribute to future development of Tilapia cage culture
in Rwanda; through identification of technical and socio-economic limitations for
adoption and expansion of this new technology. Specifically, the present study has the
following specific objectives:
- To identify the current production systems of Tilapia cage culture in Rwanda
- To identify all problems affecting Tilapia cage culture production in Rwanda,
- To propose possible solutions to overcome the hindrances for further expansion of
Tilapia cage culture industry in Rwanda.
Hypotheses
Fish cage culture is not common practice in Rwanda water bodies. It is relatively a new
technology, though most of the farmers and other investors are still reluctant to get
involved. The most likely reasons for the failure to entirely popularize cage culture in all
national water bodies are in one hand, the production systems currently applied could not
lead to profit maximization and in the other hand, a lack of farmer’s hands-on knowledge
and skills would be a key constraint to cage culture industry development in Rwanda.
Main parts of the thesis
This study is present into five chapters; the first chapter is the introduction which
provides the general background on cage culture, the problem statement of tilapia cage
culture in Rwanda, the justification of this subject, and finally the objectives and
hypothesis of the study. The second chapter is a literature framework which describes the
concept of tilapia cage culture. The third chapter shows the design of this study and
describes the methodology used for this study in order to achieve its objectives. The forth
chapter presents the results and discussions which are closed with the last chapter of
conclusion and recommendations resorted from the findings of this study.

5

CHAPTER 1: THEORETICAL FRAMEWORK AND CAGE CULTURE
CONCEPTS
1.1. Historical and global overview of fish cage culture
Fish cage culture practices dates back to the late 1800 in Southeast Asia in freshwater
lakes and rivers [16]. Cage culture technology took a significant development at
commercial scale industry since 1960s in major Asian countries and in Europe and
Americas [3]. Cage culture technology was also introduced in some African countries in
1970s but it is still in its infancy [12]. The rapid development and financial success of
cage farming systems known in Europe, Americas and Asia was driven by a combination
of factors [10], such as available resources (including water, land, labor, energy), well
established hatcheries respecting breeding techniques that produce good quality and
sufficient quantity of various marine and freshwater fish seed, availability of supporting
infrastructures such as feed manufacturers, fish processors by using easy and cost
effective technology, applied research and development initiatives and supports from
institutions, governments and universities and the private sector ensuring refinement and
improvement of culture techniques, increased corporate investment and supporting
government regulatory environment.
Currently many fish species have been cultivated in various designs and sizes of cages in
Asia, Europe and in other parts of the world. In 2005, FAO reported an estimated of 40
fish families cultured in cages of which only five families (Salmonidae, Sparidae,
Carangidae, Pangasiidae and Cichlidae) make up 90 percent of the total production and
one family (Salmonidae) is responsible for 66 percent of the total production. Also 80
different species are reported to be presently cultured in cage, only one species of those
Salmo salar accounts 51 percent of all total cage production while Oreochromis niloticus
accounts only 4 percent [7] and [8]. Salmonids (cold water fish species) are commonly
cultured in Europe and Americas while Tilapia and Carps are predominated fish species
cultured in cage in Asia and sub-Saharan African countries.
6

FAO declared a lack of statistical information concerning a global production of cultured
species within cage systems and concerning the overall growth of the sector. The recent
data published by FAO (2007) were provided only by 62 countries worldwide and are
relevant for the year 2005. The total cage aquaculture production reported was amounted
to 2,412,167 tons [8]. From the reported data in 2005, FAO ranked the top 10 major cage
culture producers excluding Chine which always comes at the top; After Chine (991,555
tons), Norway was ranked at the top with 652,306 tons of total production, respectively
followed by Chile (588,060 tons), Japan (272,821 tons), UK (135,253 tons), Viet Nam
(126,000 tons), Canada (98,441 tons), Turkey (78,924 tons), Greece (76,577 tons),
Indonesia (67,672 tons) and Philippines (66,249 tons).
In Sub-Saharan African region, the countries which provided statistical data on cage
culture to FAO are Benin, Gabon, Ghana, Mauritius, Mayotte (France), Mozambique,
Réunion (France), Zambia, and Zimbabwe. While Rwanda together with Côte d’Ivoire,
Kenya, Madagascar, Nigeria, South Africa and Uganda were classified in a group of
countries that are known to be actively engaged in commercial cage aquaculture [12].
In most of East African Countries, aquaculture is still at premature state. In Uganda and
Kenya the sector is more advanced, whereas in Rwanda and Burundi is less developed.
Small scale aquaculture is a dominated system with extensive production of Tilapia
and/or African catfish in earthen ponds [27].
In all East African countries, the needed infrastructures for aquaculture development are
not existed or not sufficient such as hatcheries and fish feed factories and markets. IFOM
(2013) reported a declining of wild stocks in inland lakes and river systems as well as of
the marine sources in five East African Countries (Burundi, Kenya, Tanzania, Rwanda
and Uganda) caused by unsustainable exploitation. Also due to growing population, the
per capita fish consumption is extremely low in particular in Rwanda and Burundi.
7

1.2. Advantages and disadvantages of cage culture
The advantages and disadvantages of cage culture are adjudged by its comparative

performance with other land based culture systems in terms of level of technology
required for construction, ease of management, adaptability, quality of the fish reared,
resource use, social implications, and economic performances.
Like other aquaculture systems, cage culture has many advantages and disadvantages
[35] and [10].
1.2.1. Advantages
Construction of cage is comparatively simple and requires smaller financial investment to
artisanal type than sophisticated modern ones.
Observation of the stock is easy in cages; therefore feeding and routine management are
easy and require less labour.
Cage reared fish are protected from predators and competitors. Fish are superior in
quality in terms of condition factor, appearance and taste.
Cage is transferable to sites with better environmental conditions. Cage can be stocked
with fish at higher densities than ponds. Cage culture offers high production per unit area
or volume, greater opportunity for expansion and allows the use for aquaculture of
existing water bodies.
Cages make use of existing water bodies and thus it can be given to non-land owned
people of the community (fishermen) whose income is affected by many reasons in
fishing sector. It therefore acts as an alternative income for such groups.


8

1.2.2. Disadvantages
Pond fish can make use of naturally occurring food, while cage grown fish only have a
limited access on natural food since they cannot forage on their own. Cage grown fish
therefore needs to be fed by the farmer to a much higher extent. The food that is given to
the cage grown fish also has to be nutritionally complete, e.g. contain proper amounts of
all necessary vitamins and minerals.
Many farmers want to optimize the stocking density when fish are grown in cages instead

of ponds. A high stocking density creates a stressful environment for the fish and
damages the immune system. The risk of disease is therefore high. The risks will be
increased further if the farmer fails to provide the fish with optimal water conditions and
a satisfactory diet. Cage culture can introduce or disrupt disease and parasite cycles,
change the aquatic flora and fauna and alter the behaviour and distribution of local fauna.
If proper water exchange is not there, the uneaten feed and metabolic waste released from
cages will lead to eutrophication of the site.
Barnacles can be attracted to the cages and for that additional protection has to be
provided such as predator nets. Poaching is easy because fish are confined in a small
area. Marine cages face problems like fouling and storms can damage the cages; therefore
more expensive.
When cages are installed indiscriminately, its impact on environment and biodiversity is
adverse and it will influence on current flow and increase local sedimentation. Since
cages occupy open water sources, it may affect navigation in the area, or reduce
landscape value of that area and are vulnerable to pollution from any source.

9

1.3. Tilapia; Cage cultured species of choice
Tilapia, especially Nile tilapia (Oreochromis niloticus), better known as aquatic-chicken,
has become the second most important fish species in world aquaculture after carps
overtaking salmonids [9]. Tilapia, although native to Africa, have been introduced around
the globe and its farming is growing rapidly especially in most Asian countries because
of their fast growth, ease of breeding and accept a wide range of feeds including
planktons from natural sources, high disease-resistance and tolerance to poor water
quality and low dissolved oxygen levels [34, 37].
Tilapias are a good source of protein and a popular target for both subsistence and
commercial aquaculture in Rwanda. Although tilapia as the most preferred species in
Rwandan culture, it has been cultured since 1940s fostered by Belgian colonial
administration and is considered almost like a native species. It was raised in inland

ponds and lakes. Two main fingerlings production centres were constructed during this
period; at Ecole des Assistants Agricole Butare (now known as Rwasave Fish Station) in
1952 and Kigembe Fish Station in 1954 [32].
The introduction of good brood from lake Albert in Uganda by Inland Lakes
Development and Management Support Project (PAIGELAC/MINAGRI) and the
development of Sex Reversed Tilapia (SRT) seed production technology in Kigembe
National Fish station have increased fish production in existing fish ponds and lakes.
Through these dynamic projects interventions also a new dimension in tilapia aquaculture
in water reservoirs and cages has been recently developed.


10

1.4. Cage design and construction
Fish Cages are enclosures used as a rearing facility for fishes. It has enclosed bottom and
sides. It can be made of wood, net screens or wire mesh. Sizes can range from one to
thousand square meters. Cages have an enormous diversity of designs and classifications
[2, 5, 15, 28, 31].
1.4.1. Classification of cage design and construction
Beveridge [2] described four basic types of cages: fixed, floating, submersible, and
submerged.
Fixed cages: Consist of a net supported by posts driven into the bottom of a lake or river;
they are comparatively inexpensive and simple to build, but their use is restricted to
sheltered shallow sites with suitable substrates. However, they are limited in size and
shape.
Floating cages: Consist of a buoyant frame or collar that support the net bag whose
shapes and sizes vary according to the purpose of the farmer. Rigid materials such as GI
or PVC pipes, timber, bamboos and plastic pipes can be used as frames. Flotation
materials such as empty plastic drums and jars can also be utilized. Floating cages are
less limited than most other types of cages in terms of site requirements and can be made

in a great variety of designs, and they are the most widely used ones.
Submersible cages: Rely on a frame or rigging to maintain shape. Submersible cages
were designed to take advantage of prevailing environmental conditions. During bad
weather, the cages are submerged to avoid destruction by strong waves.
Submerged cages: these enclosures are underwater the whole duration of the culture
period. Submerged cages allow the use of site exposed to strong winds. Fewer materials
are needed for framing and flotation and may yield better per cubic meter. However, cage
size is limited and working area is absent.
11

The submerged cages can be wooden boxes with gaps between the slats to facilitate water
flow and are anchored to the substrate by stones or posts. They are used in flowing
waters, while net bag designs are used in lakes [2].
Another classification system was proposed by Huguenin [31] considering technical
characteristics summarized in table 1.1.
Table 1.1: Classification of cage design and construction based on technical
characteristics.
Way of operating
Surface
Submerged

Place of operating
Marine
Estuarine
Freshwater

Means of support
Fixed to bottom (usually via pilings)
Floating (buoyancy)

Type of structure
Rigid (usually structure and mesh)
Flexible (usually mesh only)

Access for servicing
Cat walked
No catwalks (usually boat/barge serviced)

Operating parameters; Biomass loading/
Feeding practices
Intensive
Extensive
Fed (Hand/Auto)
Unfed

Environmental severity
Sheltered
Exposed
Open water
Source: Modified from Huguenin [31]
Loverich and Gace [15], on the other hand, classified the cages into four classes
according to the effects of the currents and waves as follows:
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Gravity cages: rely on buoyancy and weight to hold the shape of the cage and volume
against externally applied forces.
Anchor tensioned cages: rely on anchor tension to hold their shape.
Self-tensioned and supporting cages: the self-tensioning structure resists net
deformations.
Rigid cages: self-supporting structures made of jointed beams and trusses.

1.4.2. Components of cage design and construction
The different classes of cages described above can be built in several types and sizes;
however most of them present common components [29, 31, 38]: floating system,
mooring system, anchor system, net cage and services system (Figure 1.1).

Figure 1.1: Principal components of floating cage described by Vázques [28]

The design of the cage is directly related to the chosen site, inshore or offshore. In this
respect, as stated by Loverich and Gace [15] in their analysis of currents and waves
effects on several classes of cages for offshore, the most suitable cage is a self-supporting
cage.
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In all the cases: inshore, offshore, sheltered or not, the cage structures must withstand the
forces of the currents, waves and winds, while holding stock securely. This is the
engineering task [28]
1.4.3. Site selection and carrying capacity of cage culture
Historically, site selection has been based largely on available space and constraints to
productivity (e.g. circulation or food availability). Foremost, the success of any
aquaculture endeavor is the selection of best site [1, 13, 14, 18]. The common practical
considerations in site selection for cage farming are the following:
Shelter: A suitable area should be protected from strong winds and waves. Sheltered
sites are preferred for cage culture.
Water currents: Stagnant waters are used for cage farming. However, sites with
sufficient currents can offer good water exchange for replenishment of oxygen and
removal of waste metabolites. However, excessive currents may lessen the volume of the
cage, add weight to the supporting structures and moorings and may contribute to feed
losses.
Water Quality: The site must be free or far from sources of industrial, agricultural and
domestic pollution. Water runoff from rivers will cause high turbidity, abrupt salinity

fluctuations and possible destruction of cages caused by run-off debris. Turbidity brought
by water run-off can affect 2 to 15 kilometer radius of a coastal area from the mouth of
the river and may last for 3 to 6 days. High turbidity may disrupt feeding of fish and clog
or irritate the gills which can lead to bacterial infection.
Water depth: Water depth should be 2 to 3 meters for freshwater. In marine
environment, deeper sites are preferred for sufficient water circulation and acceptable
water quality. In addition, sea cages have deeper net bags.