conservation, economic growth and the use of genetically...

*Corresponding author: email: [email protected]

ISTA10 Special Issue

Selected Papers, ista10.2013.1050.Brummett, 12 pages

Conservation, Economic Growth and the use of

Genetically Improved Tilapia in Africa

Randall E Brummett

World Bank, 1818 H Street NW, Washington, DC

Key words: genetic management, introduction, indigenous biodiversity

Abstract

All Tilapias are native to Africa (and its environs), but have been distributed to

every continent not covered in ice. The global tilapia farming industry currently

produces 3.5 million tons which are worth almost $6 billion, over 80% of which is

produced outside of Africa. In Asia, and Latin America in particular, rapid gains

are being made by breeders in improving the performance of tilapias under

culture. These changes reflect shifts in gene frequency, which over time, increase

the genetic distance between captive and wild populations. Based largely on

evidence from salmon management in Europe and North America, environmental

concerns have been raised over the possible genetic damage that could occur to

natural tilapia biodiversity, should these improved strains escape, precluding their

use on their home continent. Substantial biological and ecological differences (e.g.

population size, life history strategy) between salmon and tilapia argue for caution

in using salmon at high latitudes as a basis for the extrapolation of genetic

impacts of escaped farmed tilapia in the tropics. Business planning analysis

indicates that the use of improved lines of tilapia for culture could have important

positive impacts on fish production and income generation, creating opportunities

especially for smaller-scale producers. Managed within zones so as to minimize

translocation of tilapia species within Africa, and selective breeding, with or

without other genetic modification, to produce a truly domestic tilapia with good

culture characteristics and low survivability in the wild, could make tilapia truly

the “aquatic chicken".

Randall E Brummett

Introduction

Globally, tilapia aquaculture has been expanding rapidly to now represent the third

most important finfish aquaculture species group after the Chinese and Indian Major

Carps (SOFIA 2012). Much of this growth has been fueled by the availability of fast

growing strains of the Nile tilapia (Oreochromis niloticus), improved through

selective breeding to produce fish that can uniformly reach market sizes of 800 g in

10 months or less. However, much concern has been expressed by the conservation

community that widespread introduction into Africa of selectively bred O. niloticus

could result in the destruction of conspecific and other indigenous tilapia biodiversity

that, given appropriate environmental impact regulation and breeding programs,

could also be used productively and profitably in aquaculture.

Quality of Farmed Stocks

Tilapias exhibit a high degree of phenotypic plasticity. When grown in small ponds,

they often mature precociously and these small adults are mistakenly stocked as

fingerlings (Figure 1).

Figure 1. Small-scale hatcheries rely on small ponds and mass spawning, often resulting in high rates of inbreeding and lowered performance.

Instead of growing, they reproduce, which in practice effectively amounts to

inadvertent selection for slow growth and/or early sexual maturation (Doyle 1983)

and can lead to declines of up to 20 percent in growth rate within six generations

(<three years) (Silliman 1975).

Since it is expensive, difficult, and sometimes illegal to get good broodfish,

individuals who seek to acquire and maintain a hatchery population, often resort to

using minimal numbers, thus building low genetic diversity into their production

systems from the outset (Eknath 1991). Often the effective number of broodfish

used to contribute to each subsequent generation (Ne) is less than 100-150 pairs

needed to maintain healthy genetic variability and minimize in-breeding

(Smitherman & Tave 1987). Even when numbers are sufficient, simply stocking 100

males and 100 females into a brood pond will not solve the problem. As male tilapia

are highly territorial and competitive for mates, only 30% of males dominate the

fertilization of the females (Fessehaye et al. 2006). Rates of inbreeding in such

systems are about double that expected in a randomly breeding population

(Fessehaye et al. 2006).

Many Nile tilapia introductions were of only a small number of individuals or of

limited family representation (Pullin 1988; Agustin 1999). The Bouaké research

station in Côte d’Ivoire is characteristic of how Nile tilapia genetic diversity has been

managed. Captured in Burkina Faso in the 1950s and transferred to Bouaké, what

was originally a pure population of O. niloticus was hybridized with other strains and

species and subsequently widely distributed in Africa and elsewhere (Thys van den

Audenaerde 1988). Transfers from Bouaké to Paraguay (1968), Sierra Leone (1970),

Venezuela (1971), Brazil (1971), Bénin (1979), Guinea (1978 and 1983), Mali

(1982), and back to Burkina Faso (1982) have been reported (Adou, in R.S.V. Pullin,

1988). The Bouaké case is by no means unique, with many such introductions and

crosses having been made over the years, either purposefully or by accidental

Conservation, economic growth and use of genetically improved Tilapia in Africa

mixing. For example, the Baobab Fish Farm Kenya, until recently (they are now no

longer operating) grew a hybrid strain of O. niloticus, Oreochromis spilurus, and O.

mossambicus. Kafue Fish Farm in Zambia maintains stocks of O. niloticus,

Oreochromis andersonii, Oreochromis aureus, and gets wild Oreochromis

mortermeri, and Oreochromis macrochir from the Kafue River. All of these species

easily cross with each other and perform less well than the original O. andersonii

stock (F. Flynn pers comm, Lusaka, Zambia, Sept 2003). In general, the worst

problem with such composite populations from the fish farmer’s point of view is that

they exhibit very high variability in performance among individuals and generations.

Another negative consequence of this mixing is outbreeding depression, through

which the fitness of populations that have adapted over time to culture conditions

decreases through dilution with genes from other populations or species. Today,

most research stations and private tilapia hatcheries around the world maintain and

disseminate strains comprised of mixtures of several species derived from a number

of sources.

Substantial declines in performance are associated with such hatchery

management practices. Genetic variability is the way in which selective breeding

improves growth. Typically, genetic variability of fish held in African hatcheries is 40-

70% less and growth rates 12-40% less than wild stocks (Table 1).

Table 1. Documented erosion of genetic variability and growth performance among African hatchery tilapia populations.

Introgression of O. macrochir genes reduced growth of O. niloticus by 20% (Micha et al. 1996)

Backcrossing 3 generations of red hybrid tilapias blocked reproduction (Behrends pers comm, Mussel Shoals, Alabama, USA, June 1984)

Genetic variability and growth rate down 50% and 12%, respectively, in stocks maintained on small-scale hatcheries (Morissens et al. 1996)

Wild fish 43% more genetic variability than those held on small-scale hatcheries (Pouyard & Agnese 1996)

50% loss of genetic variability in small-scale hatchery stocks (Agustin et al. 1997) Wild populations 5-40% better than African hatchery stocks, depending upon test

environment (Eknath et al. 1993) 70% loss of genetic variability among hatchery stocks (Ambali et al. 1999) 50% less growth in hatchery Vs Lake Victoria stocks (Gregory pers comm, Kampala,

Uganda, Mar 2003)

40% decline in growth of O. niloticus held on small-scale farms compared to wild conspecifics (Brummett et al. 2004)

Response to selective breeding can be reduced by up to one third by even

moderate reduction of genetic variation (Bentsen & Olesen 2002). Heritability, the

percentage of total variability that can be inherited, is of utmost importance to

breeders. Heritability (h2) for growth in tilapias is naturally low to moderate,

averaging less than 20 percent and can easily be swamped by low Ne (ICLARM/UNDP

1998). Even with Africa’s relatively small aquaculture industry, a 20 percent decline

in growth rate represents lost production on the order of 80,000 t, with associated

loss of profits on the order of $200 million per year to Africa’s fish farmers (FAO

2000)

Genetic Management

The first successful tilapia genetic improvement program was carried out by the

WorldFish Center (the International Center for Living Aquatic Resources

Management) and Central Luzon State University in the Philippines. The program

was based on a combined family selection program that resulted in increased growth

rate of 64% over eight generations, in a composite strain comprised of eight

different wild and farmed populations (Khaw et al. 2008). This genetically improved

farmed tilapia (GIFT) has been widely disseminated in Asia where it substantially

out-performs most local strains (Asian Development Bank 2005). Although illegal

Randall E Brummett

introductions are suspected, GIFT has not been legally re-introduced to Africa for

commercial farming. An on-station comparison of the GIFT and Akosombo strains of

O. niloticus that could lead to the first major introduction, is underway in Ghana.

Since the development of GIFT, a number of improved strains, some based on GIFT,

such as Big Nin from Thailand, and GET-Excel and FaST, from the Philippines have

been produced.

Growth comparisons have shown significant differences among both wild and

captive populations and relatively low genotype x environment (G X E) interaction in

the environments tested, indicating that lines improved under one set of culture

conditions will continue to perform well in other systems (Khater and Smitherman

1988; Eknath et al. 1993; Asian Development Bank 2005).

The territorial nature of O. niloticus lends to rapid formation of distinct sub-

populations, a characteristic that if reinforced by long-term geographical separation

and genetic drift has led to a certain amount of genetic divergence (Nyingi 2007).

There are seven subspecies of Nile tilapia in their natural ranges: O. niloticus

niloticus, the largest group representing populations in West Africa and the Nile River

valley; O. n. eduardianus in Lakes Edward, Kivu, Albert, and Tanganyika; O. n.

cancelatus in Ethiopia; O. n. barengoensis in Lake Baringo; O. n. vulcani in Lake

Turkana; O. n. sugutae in the Sugutu River of Kenya; and O. n. filoa in a hot spring

in the Awash River basin of Ethiopia (E. Trewavas, 1983). An eighth subspecies from

Lake Tana has been identified (S. Seyoum and I. Kornfield, 1992). This is based on

micro-satellite and mitochondrial DNA analyses. A ninth subspecies has been found

in a warm water spring in the Loboi Swamp near Lake Bogoria in Kenya (D. Nyingi,

2007). Overall, heterozygosity (less than 4.5 percent) and gene polymorphism (less

than 3%) is relatively low in Nile tilapia compared to some other fish species

(Gourène and Agnèse 1995). As hatchery managers have learned, Nile tilapia easily

hybridize with other oreochromiines both in the wild and in captivity, although some

of these crosses produce sterile offspring or skewed sex ratios (Wohlfarth and Hulata

1983; Agnèse et al. 1998).

Proper management of captive tilapia genetic resources is somewhat complicated

but not impossible. In those few cases where the genetic diversity of hatchery stocks

has been systematically managed either through the use of large effective breeding

numbers, rotational mating or controlled outcrossing, growth rates equal or exceed

those of natural populations (Pauly et al. 1988). Selective breeding can lead to

increases in growth rate of 10-15 percent per generation (Jarimopas 1990, Rognon &

Guyomard 1996, Vreven et al. 1998, Ponzoni et al. 2011).

Improvements in genetic quality undoubtedly increase production. In order to be

profitable, crop agriculture relies heavily on improved varieties. However, how these

improvements are made can have a large impact on the rate of progress, who

benefits, and how. There are two general approaches to improving the genetic

quality of fish raised in aquaculture: 1) import an exotic species or improved variety

developed elsewhere (centralized approach) or 2) locally develop a new species or

variety (decentralized approach).

Agricultural research and development have long relied on the option of

domesticating or breeding a new variety in a central location and subsequently

disseminating seed or broodstock. This approach requires the international and often

intercontinental transfer of genetic material thus risking both negative environmental

impacts and the possibility of G X E interaction, which renders improved genotypes

less competitive in culture systems that differ from those under which they were

bred.

The major advantage of the centralized approach is that complicated technologies

can be more easily managed in larger, more sophisticated facilities. Breeding

progress is faster. For example, the GIFT strain grows 20-70 percent faster than

most captive O. niloticus strains (ADB 2005). However, in Thailand and China, the

GIFT exhibits signs of G X E interaction and is not substantially superior to locally


adapted and bred strains under certain conditions. The GIFT was produced in four

years while the strains in Thailand and China were slowly selected over 30 and 20

years, respectively (ICLARM 1998).

Decentralized genetic improvement to produce specific lines of fish for specific

culture systems normally takes longer than the centralized approach.

Decentralization requires more people to be involved and is consequently less

efficient in terms of capital use. It also tends to be more difficult to implement in

short-term projects preferred by government. On the other hand, one of the key

constraints to improved genetic management of African aquaculture species is the

lack of skilled technicians and hatchery infrastructure. Without the capacity to

undertake proper management and breeding, the potential gains inherent in a new

strain will be quickly lost. The on-the-job training opportunities created by

decentralized genetic improvement projects are excellent means of creating both

new strains for culture and the capacity to manage them at the same time.

Unfortunately, progress to date on building this capacity in national hatcheries in

Egypt, Malawi and Ghana has been less than stellar partly due to poor funding, but

also and importantly, due to lack of commitment.

Economic Importance of Genetic Improvement

Tilapia are a global commodity, with 350,000 metric tons traded internationally for

over US$800 million each year (FAO 2013). The market is dominated by large-scale

producers, mostly in China, who are able to put frozen tilapia onto markets

throughout Africa at prices below local production costs, largely due to the use of

faster-growing strains (M. Amechi, pers. comm., Accra, Ghana, Feb 2013; P. Blow &

C. Chiwenda, pers. comm., Kariba, Zimbabwe, July 2013).

Competing with cheap imports of tilapia strains that grow nearly twice as fast as

local strains is impossible, especially for smaller-scale farms. Simple enterprise

budgets developed for tilapia farms in Cameroon illustrates the importance of

genetics in aquaculture profitability (Table 2a,b).

Table 2a. Pond production per hectare in Cameroon

of the existing farmed stock of Oreochromis niloticus over4 months.

Total Cost (FCFA*)

Annual Operating Cost

(FCFA)

Pond Construction & Equipment

3,100,000 310,000

Fingerlings 225,000 Transportation 40,000 Feed 360,000

Labor 200,000 Financing (1.5% per month)

295,860

Total Costs 1,430,860 Revenues 1,200,000

Net per Annum -230,860

The use of improved strains is essential for the success of African tilapia farming

at the farm level and, extrapolated to the existing African tilapia industry, could

easily double production to 1.5 million metric tons worth an additional US$2.2 billion

per year.

Risks of Using Improved Lines in Africa

Theoretically, the use of improved strains could represent a threat to indigenous

African tilapia populations in terms of declining abundance or reduction of genes with

Table 2b. Pond production per hectare in Cameroon of an improved line of Oreochromis niloticus that grows twice as fast as the existing farmed stock (2 months).

Total Cost (FCFA)

Annual Operating Cost

(FCFA)

Pond Construction

& Equipment

3,100,000 310,000

Fingerlings 450,000 Transportation 80,000 Feed 720,000 Labor 300,000 Financing (1.5%

per month)

499,460

Total Costs 2,359,460 Revenues 2,400,000

Net per Annum +40,540

*Central African Franc (US$1 500 FCFA)

Randall E Brummett

potential importance for future selective breeding programs. At present, there is

insufficient data available on tilapia ecology and/or genetic diversity to permit fully

informed decision-making in regard to the potential negative impacts on wild African

tilapia stocks of introducing or developing new improved strains. However, the

debate over the use of genetically modified fishes, either naturally through selective

breeding and hybridization (among other techniques), or through transgenetic

methods, has become global, and includes a large number of case studies

particularly from Europe and North America. Whether or not these data are sufficient

to adequately assess the risks involved in the use of selected tilapia strains for

aquaculture is debatable.

Genetic Introgression

Cultured populations of indigenous species will inevitably escape and breed with wild

fish. Declines in wild salmon runs have been attributed to the accidental escape of

cultured salmon and/or the purposeful introduction of hatchery stocks (Fleming et

al., 2000; Utter and Epifanio, 2002; McGinnity et al., 2003). Although impossible to

differentiate from the combined effects of landscape modification, pollution and over-

fishing, the process of genetic introgression, that is, the migration of genes from the

captive population into the wild population through inter-breeding has been given as

the principle mechanism behind these declines (Araki et al 2007). The overall degree

of adaptation or fitness of the wild population could be reduced by mixing the

genomes of captive fish that are specifically adapted to a hatchery or aquaculture

environment, with those that are specifically adapted to a particular river, or by

increasing the relative percentage of genes from one subset of the wild populations.

(Ryman, 1991). The magnitude of this potential problem is proportional to:

1. The degree and importance of the adaptation of the wild population to the

waterbody in question. In cases where only a narrow range of genotypes can

survive in a particular waterbody, the genetic variability of the fish population

narrows, rendering the wild population vulnerable to environmental changes,

one of which is the presence of large numbers of fish of other genotypes.

2. The relative sizes of the captive and wild populations. In cases where relatively

small (<1,000 individuals) wild fish populations that are highly adapted to a

particular river or lake, are inundated by tens of thousands of stocked or

escaped fish, such as is the case for many wild runs of Atlantic salmon (Salmo

salar). The consequences of the reduction in fitness could be catastrophic and

ultimately result in the extinction of the wild genome, even in cases where the

total number of fish in the waterbody has actually increased (McGinnity et al.,

2003).

3. The degree of difference between the genomes of the wild and captive

populations. Up to the point where they can no longer interbreed at all, the

more distant the relationship between the introduced and wild genomes, the

greater could be the reduction in fitness. This works both ways, with fitness of

both the wild fish and the captive populations (i.e., outbreeding depression).

4. The goals of having captive and wild populations in the same stream. If

preservation of the indigenous genome is considered of primary importance, the

fact that there is often more fish in a stocked waterbody (or one into which

cultured fish have escaped) may be less of a priority than the relative fitness of

the population.

In the case of small, highly adapted Atlantic salmon runs, with relatively narrow

genetic diversity, the risk of introducing large numbers of less well adapted hatchery

fish has been shown to reduce co-adapted gene combinations and whole lifetime

population fitness, at least in the short term (McGinnity et al., 2003). The case is

much less clear for species with relatively large and genetically diverse populations

such as sea bass, cod, red drum and red sea bream that have also been heavily

stocked or used in aquaculture (Youngson et al., 2001; Utter and Epifanio, 2002).


Incidences of ecological disruption attributed to the introduction of alien tilapias

have been widely documented (Lever, 1996; Canonico et al., 2005; De Silva et al.,

2006). Since fish introduced for aquaculture do escape to the wild, the introduction

of any strain of tilapia to watersheds for culture or capture fisheries where it is not

currently found, should be undertaken cautiously. It is recommended that countries

survey and characterize their indigenous tilapia biodiversity prior to considering such

an introduction.

The principal debate between wildlife conservationists and fish farmers regarding

transfer of genetically improved tilapias between African watersheds revolves around

the danger of genetic introgression with wild populations that probably contain

genetic diversity of important adaptive significance. These may be lacking in captive

populations and may be important in terms of adaptive capacity and/or the

development of future aquaculture strains.

Although there is no compelling empirical evidence arguing either for or against

the possibility of negative impacts resulting from the introgression of captive tilapia

strains into indigenous wild populations, the substantial biological and ecological

differences between tilapia and salmon (upon which most of the concerns over

genetic erosion are based) imply that the risks of introducing improved tilapias for

aquaculture might be significantly less than feared.

Referring to the list of decision making criteria listed above:

1. Tilapia are generalists, often exhibiting high degrees of phenotypic plasticity, but

not normally genotypically adapted to specific water bodies.

2. Wild tilapia populations are generally huge, in excess of millions of individuals,

while escapes from aquaculture are minimized, relative to intentional stocking

programs, for example, by farmers trying to protect their investments.

3. In terms of growth performance, hatchery populations of tilapia can differ by 40-

60% from wild populations, at least in terms of growth rate.

4. Food security and economic growth in impoverished communities are key

concerns. In addition to the ethical issues involved, failure to address food

security needs probably increases the threats to aquatic biodiversity posed by

over-fishing and environmental degradation.

In my view, only number three (3) gives substantial cause for concern. If, for

example, there are serious threats to a tilapia population of particular significance for

local capture fisheries, or of special value as a locally adapted race, the large

difference between captive and wild fish could represent a danger of eroding locally

adapted gene sequences (Deines et al. 2014).

On the other hand, the dangers associated with these genetic differences are

proportional to the absolute value of the difference, not whether the difference is

positive or negative. In addition, existing tilapia hatchery populations in Africa have

significantly diverged from wild populations, mostly negatively in terms of growth

performance. In consequence, the risk of doing nothing may be similar to the risk of

using centrally selected improved lines, without enjoying the productivity and

economic gains.

Other Tilapias

There are a number of species of tilapia in Africa, many of which have been tested in

aquaculture. A few have demonstrated economic potential. These include:

Oreochromis mossambicus and Oreochromis urolepis from the Lower Zambezi and

environs; Oreochromis macrochir in the South-Central and S-Western parts of the

continent; Sarotherodon galilaeus in the Center and West; Oreochromis andersonii in

the upper Zambezi and Kafue Rivers; Oreochromis aureus in the Nilo-Sudan zone;

Tilapia guineensis and Sarotherodon melanotheron in the coastal regions of West

Africa; Tilapia rendalli in South-Central; and Tilapia zillii in Northern and Western

Africa; and O. spilurus along the Eastern coast (Figure 2).

Randall E Brummett

Many of these possess important culture traits for aquaculture: O. andersonii is a

more placid and easily handled fish than O. niloticus; O. aureus is the most cold-

tolerant of the tilapias and when crossed with O. niloticus produces all male hybrids

that can be used in organic fish production; T. guineensis and S. melanotheron are

tolerant of brackish water and the latter tends to mature at a later age than other

species, potentially reducing the problems with precocious spawning (mentioned

above). T. zillii and T. rendalli are herbivores that can help control weeds in ponds.

However, aquaculture is largely a start-up venture in most of Africa and faces

many constraints. Having a fish like the GIFT strain of O. niloticus or one of several

other similar lines, that grows 40-60% better than the typical farm populations can

make the difference between success and failure. Improved lines of these other

species are needed to encourage farmers to adopt local species and thus reduce

pressure to further distribute O. niloticus. In addition, characteristics such as docility,

salt tolerance, cold tolerance, and the ability to produce all male hybrids, deserve

special concern and conservation effort.

Alternatives to Introduction

Some progress is being made in reducing incentives for importing alien lines and

species. For O. niloticus, there are existing breeding programs at Akosombo in

Ghana, and Abbassa in Egypt, that have reportedly produced 10-15% improvement

in growth over several generations in spite of many problems (Joseph Ofori, pers.

comm., Akosombo, Ghana, November 2013). For O. mossambicus, a breeding

program has been established at Stellenbosch University in South Africa. O.

shiranus, an indigenous species in Malawi, is being improved at the Malawi National

Aquaculture Center, specifically to provide Malawian fish farmers with alternatives to

O. niloticus, which is banned. There is also an on-going breeding program for O.

aureus in Egypt. At least two fish farmers in Zambia are working to improve the

performance of O. andersonii. However, it is imperative to do more, as pure

populations of several species are under serious threat from genetic contamination.

Among the most important species that remain, O. andersonii, O. macrochir, S.

melanotheron and S. galilaeus probably have the highest potential to become

economically viable aquaculture species.

Much emphasis has been placed on Best Aquaculture Practices and various

certification schemes that seek to ensure environmental and social sustainability in

the aquaculture sector. Consistent with these would be a zoning system to

encourage fish farmers in various sub-regions to culture only indigenous species. A

Figure 2. Natural distribution of tilapias used in aquaculture. M = O. mossambicus, U = O. urolepis, mc = O. macrochir , G = S. galilaeus a = O. andersonii, A = O. aureus, Sm = S. melanotheron, T. rendalli , T. guineensis, Z = T. zillii, S = O. spilurus (from Pullin 1988).


rough idea of what such a zonation might look like is shown in Figure 3. Any

definitive zonation would have to take into consideration the many sympatric tilapia

populations in Africa.

The Aquatic Chicken

Probably the only

permanent solution that

would protect the interests

of farmers, consumers, and

indigenous biodiversity,

would be the creation of a

truly domestic farm tilapia

that possesses good culture

traits while having low

survivability in the wild.

Aspects of this approach to

fish genetic management were first proposed by Moav et al. (1978) and reiterated by

Balon (2004), both working with common carp, Cyprinus carpio.

Tilapia has long been referred to as the “aquatic chicken” because of the

presumed role it could play in aquaculture and human diets. However, a major

difference between tilapia and chickens is that the former is essentially a wild animal

(“exploited captive” according to the usage of Balon) and when escaping to the wild,

can establish feral populations and interbreed with local tilapias. While chickens can,

and do, interbreed with their wild progenitors, escaped chickens are generally fat,

slow and dim-witted, and quickly done in by predators (Lawler 2012). Some strains

of common carp and red tilapia exhibit traits that collectively would make them a

true aquatic chicken:

Easy seinability

Sexual maturity above minimum market size

Rapid growth

Low aggression

Good carcass quality (e.g., low fat)

High fecundity

Bright color (easy for predators to see)

Slow-moving

Specific disease resistance

Grow under crowded conditions

Convert low-quality feed

Behavioral barriers to out-crossing

Some of these traits may be amenable to traditional selective breeding while

others, such as disease resistance and feed conversion, might be more easily

accomplished through direct manipulation of DNA. While currently anathema to

many in the conservation community, the long-term environmental benefits of

having a fully domesticated fish for tropical aquaculture would be a major

contribution to global agriculture and food security, while significantly reducing the

threat of introducing tilapias to new ecosystems.

Figure 3. Potential tilapia

aquaculture zones in Africa,

based on existing distribution of

indigenous species (from Lind et

al. 2012).

Randall E Brummett

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