Impact of Introduced Nile tilapia (Oreochromis niloticus) on Non-native Aquatic Ecosystems
The global invasion of non-native aquatic ecosystems by Nile tilapia (Oreochromis niloticus) is well documented and coincides with their increased use as an aquaculture species. Aquaculture can be defined as the farming of fish or other aquatic organisms and it varies considerably in terms of production practices. Generally, freshwater finfish, such as Nile tilapia, are reared in inland ponds (closed systems). However, in several countries, floating cages are increasingly used to rear Nile tilapia in open water bodies. In such systems, escape is inevitable. The Nile tilapia is considered an omnivorous species and it ingests zooplankton, phytoplankton, or debris present in rivers. As a consequence, the release of Nile tilapia into non-native aquatic ecosystems may result in competition for food and space, thereby damaging native species. The wide environmental tolerance and high reproductive rate of Nile tilapia facilitate its use for aquaculture, but also render the species highly invasive. Here, we review the high frequency of Nile tilapia in non-native biodiversity and indicate the existence of the species under feral conditions in every country in which it has been introduced through farming systems.
Received: December 01, 2012;
Accepted: February 07, 2013;
Published: March 16, 2013
Rapid human population growth has necessitated increased food production from
agriculture, livestock and aquaculture (Vicente et al.,
2011a) However, the expansion of land for crop production results in many
environmental problems, making enhanced productivity the key means of increasing
food production. In particular, there is considerable emphasis on breeding programs
(Fonseca-Alves et al., 2011).
Introductions of non-native fish species can reduce biodiversity and modify
local community dynamics in freshwater systems. Exotic species have been identified
as the third leading cause of extinction of vertebrate species in aquatic environments
(Groombridge, 1992). Introductions of exotic species generally
threaten the stability of ecosystems, resulting in extinction through long-term
predation and competition and leading to replacement of native species by exotic
species. Other documented effects are hybridization with native species, disruption
of the food chain and environmental degradation (Williamson,
1996; Cox, 1997).
The Nile tilapia, Oreochromis niloticus (Linnaeus 1757), is
responsible for reducing local biodiversity, through competition with other
aquatic species for available food resources. Lack of predation and adaptation
to changing environmental conditions increase the impact of Nile tilapia on
the ichthyological composition (Leveque, 2002; Vicente
et al., 2011a).
The Nile tilapia is an omnivorous species indigenous to Africa and it is found
mainly in the basins of the Nile, Niger and Tchad and in lakes of the Middle
East (Leveque, 2002). In North and South America, introductions
of Nile tilapia stem from aquaculture facilities and also from historical introductions
for recreational angling. The Nile tilapia has been introduced in more than
100 tropical and subtropical countries, to improve fishing productivity and
facilitate the development of aquaculture (Coward and Bromage,
2000; Leveque, 2002).
The ease of reproduction of the Nile tilapia encourages farmers to acquire
the species and populate their tanks at a low investment cost. Furthermore,
the Nile tilapia constitutes a rough species, which occurs in a wide range of
environmental variations, tolerating extreme limits of temperature and oxygen,
as well as the presence of various pollutants (Beyruth
et al., 2004).
FEEDING BEHAVIOUR AND REPRODUCTIVE TRAITS OF NILE TILAPIA
Adult Nile tilapia feed predominantly on phytoplankton. If phytoplankton is
not abundant, the adults feed first on zooplankton and thereafter on debris.
Seasonal variations also influence the type of diet. During the rainy season,
debris is predominantly consumed, whereas, in dry seasons, consumption of phytoplankton
prevails (Beveridge and Baird, 2000).
Food is ingested by filtration and retained by the gill rakers, which are characteristic
of microphagous species and are located in the arches. Within the pharyngeal
cavity, food can be ingested or rejected (Beveridge et
al., 1993). The accepted material is broken into smaller fragments by
pharyngeal bones and forwarded to the oesophagus (Beveridge
and Baird, 2000).
The reproductive behaviour of Nile tilapia is highly influenced by the mode
of reproduction. In the genus Oreochromis, for example, males build nests
for spawning and develop secondary sexual structures (Turner
and Robinson, 2000).
Yamamoto (1969) revealed that steroid hormones may be
used to modify the phenotypic sex of Nile tilapia. Androgen hormones have been
widely used to produce all-male populations of several tilapia species (Macintosh
and Little, 1995; Green et al., 1997). Various
techniques have been used in an attempt to curb overpopulation. Such methods
include stock manipulation, utilization of tilapia fish predator cultures and
monosex creation (Phelps and Popma, 2000). Of these,
monosex creation is most frequently used. The production of single-sex male
tilapia, by adding the hormone androgen 17α-methyltestosterone to the larval
diet, is considered to provide optimal sex reversal of tilapia (Turner
and Robinson, 2000).
USE OF NILE TILAPIA IN AQUACULTURE
The Nile tilapia is feral in every country in which it has been cultured or
introduced and where local conditions allow the species to establish (Courtenay,
1977; Costa-Pierce, 2003). Nile tilapia comprise
83% of the global production of tilapia (FAO, 2002) and
are responsible for the dramatic expansion of tilapia in recent decades (Bentsen
et al., 1998; Gupta and Acosta, 2004). The
large size at the first reproduction, rapid growth rate and versatile feeding
habits with a basal position in the food chain (Costa-Pierce,
2003) justify the predominance of the Nile tilapia in tilapia production
(Gupta and Acosta, 2004). As a consequence of its considerable
potential for aquaculture, the species has undergone several breeding programs,
which have generated different lineages.
The rearing of tilapia in cages, especially in small volumes, has increased
considerably in recent decades and may become the most important aquaculture
system in many countries. According to Beveridge (2004),
the technique has several advantages over traditional farming, including low
initial investment, utilization of available aquatic resources, enhanced production
control, elimination of problems associated with excessive reproduction and
ease of handling (Shinohara et al., 2012). Tilapia
have several favourable characteristics for aquaculture, including rapid growth
rates (Hassanien et al., 2004), especially in
males (Toguyeni et al., 2002), high feed conversion
rates (Kubitza, 2000) and disease resistance (Ardjosoediro
and Ramnarine, 2002) at high densities (Gall and Bakar,
1999) and low concentrations of dissolved oxygen (El-Sayed
and Kawanna, 2004). Roughness (Yi et al., 1996),
ease of obtaining fingerlings (Coward and Bromage, 2000)
and high market acceptability (Wille et al., 2002)
are additional desirable features of the species for aquaculture.
IMPACT OF NILE TILAPIA INTRODUCTIONS ON ECOSYSTEMS
The terms introduced species, exotic species, alien species, non-native species,
non-indigenous species and allochthonous species have the same biological significance
and according to the European Inland Fisheries Advisory Commission (EIFAC) and
correspond to any species transported and released by humans outside of their
natural range, intentionally or accidentally (Vitule, 2009;
Fonseca-Alves et al., 2011). Another important
concept and distinct from the above is that of invasive species. The International
Union for Conservation of Nature (IUCN), for example, defines invasive species
as any organism introduced by humans in places outside their native range that
was established and dispersed, causing a negative impact on other species or
ecosystem (ISSG, 2011).
The introduction of exotic species and extinction of native species, coupled
with environmental changes, influence the loss of biodiversity, through increasing
genetic similarity and taxonomic and functional biota, on a global and regional
scale. This phenomenon affects evolutionary and ecological factors (Olden
et al., 2004) and is known as biotic homogenization (McKinney
and Lockwood, 1999). The construction of canals, international trade, recreation
and aquaculture (Naylor et al., 2001) are the
main reasons for the mixing of allopatric groups. Olden
and Poff (2003) describe many possible scenarios for increasing similarity
quenching and biotic introduction of species. In fish, the pattern is related
to the degree of disturbance caused by human activities, such as human settlements
and usage of land and water resources (Mendonca et al.,
2012). For example, the construction of dams fragments rivers, thereby changing
the processes and dynamics of flow and threatening limnological biodiversity
(Power et al., 1996). The same characteristics
that increase the ability of the Nile tilapia to become a potentially invasive
pest in various environments are responsible for the economic importance of
the species in aquaculture (Peterson et al., 2005).
Rapid growth rate, high prolificacy, varied food habit, high resistance disease
and year-round spawning, in addition to excellent flavour, make tilapia fish
farming increasingly important in neotropical regions (Melo
et al., 2006).
Canonico et al. (2005) reviewed the potential
impacts of tilapia on ecosystems into which they are introduced. These include
local extinction of native species, predation of eggs and young of other fish
species (Goudswaard et al., 2002), alteration
of the dynamics of nutrients and eutrophication (Starling
et al., 2002), destruction of vegetation from the lake bottom and
introduction of parasites (McCrary et al., 2001).
According to Casal (2006), the Nile tilapia has been
introduced in 85 countries, with establishment reported in 58% of these countries
and adverse ecological effects in 14%. Based on the considerable capacity of
the species for colonization and its potential environmental impact, the protection
and management of aquatic environments is crucial when introducing tilapia into
ecosystems (Esselman, 2009).
Organic residues produced as a result of microbial action provide essential
nutrients for the development of plankton and macrophytes. When present in excess,
these residues cause eutrophication of the environment and changes in the composition
and abundance of many aquatic organisms (Vicente et
al., 2011b). Kestemont (1995) collated information
about the negative effects of aquaculture on the biological environment and
highlighted the following: changing values of water temperature; increased chemical
biochemical oxygen demand; elevated levels of phosphorus and suspended solids;
decreased concentrations of dissolved oxygen; chemical contamination; accumulation
of sediment rich in organic matter; pollution; erosion and increased risk of
When a new species is introduced into an ecosystem, there is always the risk
of it being able to escape and settle in the natural environment, resulting
in possible adverse effects to native species or even the functioning of the
ecosystem (Gozlan et al., 2010). The effects
resulting from introductions can be devastating, causing biological invasions
are considered a major cause of biodiversity loss (Vitule,
2009; McGeoch et al., 2010).
Fish are among the group of aquatic animals over introduced worldwide (624
species) and 91% of the sources of introductions are related to farmed fish
(Gozlan, 2008). Nile tilapias are used for aquaculture
because they are extremely hardy, physiologically tolerant and characterized
by multiple spawning and parental care (Agostinho et al.
2007). The negative effect of the Nile tilapia on the native fauna of worldwide
has been extensively reported. The Nile tilapia changes native community structure,
reduces abundance of planktonic microcrustaceans, lowers water transparency
and increases the abundance of microalgae (Attayde et
Many non-native freshwater fish are invading Latin America and others parts
of world (Vitule, 2009) and the characteristics of
successful invaders and their impacts on native species have becoming increasingly
known (Espinola et al., 2010; Dos
Santos et al., 2011). However, controlling the non-native fishes
is difficult, mainly because physical or chemical removal of the invaders is,
in addition to its potential adverse impacts on local species, overall ineffective
in large ecosystems or for species with high reproductive rates (Garcia-Berthou,
2007; Sato et al., 2010).
Gozlan (2008) evaluated the threats posed by introduced
freshwater fishes on non-native aquatic ecosystems. Through on his analysis
of datasets from the FishBase and from Food and Agriculture Organization (FAO),
he concluded that the majority of intentional non-native fish introductions
associated with aquaculture and its societal benefits, have not been reported
as having an impact on aquatic ecosystems. Consequently, Gozlan
(2008) advocated the protection of introductions that have beneficial outcomes
and a more systematic ban of species freshwater fish presenting a higher risk.
As a consequence of its wide environmental tolerance, high reproductive rate, rapid population growth and ease of cultivation, the Nile tilapia has become a model of livestock farming in several countries. However, the same characteristics that make the species attractive for aquaculture render it highly invasive, with considerable potential for becoming a pest in aquatic environments where it is introduced. The risks of tilapia introductions must therefore be rigorously evaluated and weighed against the potential socio-economic benefits.
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