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Is Tilapia Becoming An Invasive Fish in Sri Lanka?

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Is Tilapia Becoming An Invasive Fish in Sri Lanka?
Saman Athauda
Faculty of Agriculture,University of Peradeniya, Peradeniya, Sri Lanka
Introduction
Sri Lanka is one of the most important biologically diverse hotspot in the world despite
being a small island nation covering an area of 65,610 km2 due to it biological diversity.
As a result there is a high proportion of endemic aquatic fauna and flora, which
contribute to the biodiversity richness of the country.Twenty species of invasive alien
fauna, consisting of 15 vertebrates and 5 invertebrates, have been identified as spreading
in the natural and semi natural ecosystems in the different bioclimatic zones of Sri Lanka
(Bambaradeniya, 2001). The vertebrates include nine species of freshwater fishes, one
reptile and five mammals. Among the total Invasive Alien Species (IAS) documented in
Sri Lanka, 10 species of fauna are included in the list of 100 of the world’s worst IAS
(http:// www.indiaenvironmantportal.org.in/node/38152, cited on 10.02.2009). The
introduction of the above species to the country is deliberate or accidental. However, both
the freshwater fisheries and the ornamental fish industry are responsible for the
introduction of freshwater invasive species to Sri Lanka.
The Rainbow trout (Salmo trutta and Salmo gairdneri) is the first deliberately introduced
exotic fish to Sri Lanka from Europe and North America respectively as a sport fish in
1882-1893. They were introduced to some streams in the Horton plains and other up-
country streams, but the population now under control and recorded as the only exotic
freshwater fish species recorded from the Horton plains at present (Pethiyagoda, 1999;
http://www.dailynews.lk/2006/08/28/fea06.asp, cited on 14.02.2009).
The tilapia (Oreochromis mossambicus and Oreochromis niloticus) that introduced to Sri
Lanka in 1952 and 1956 respectively in order to enhance the freshwater food fish
production now becomes dominant group of fishes in Sri Lanka. There is no argument
that introduction of Tilapia triggered the development of an inland fisheries in Sri Lanka.
Though the tilapia still the dominant group of fishes in Sri Lanka’s inland water bodies,
and has invaded everything from saline estuaries to rivers, the performance of tilapia as
commercial table fish has been discouraging since recent past mainly due to its o
ver population and
dwarfing. Not only less productive but also tilapia has been blamed to cause problems for
native aquatic flora and fauna in several ways due to its presence in large number in the
water resources.
However, the eradication of this tilapia species without having any alternative cheaper
animal protein source for especially poorest sector is the challenge for present day
Scientists, Economists as well Conservationist.
Biology of Tilapia makes it more resilience
Tilapia is a superior competitor for the resources, being non-selective in diet and prolific
breeder, can live in many different types of water from lakes and rivers to sewage canals.
Even though, tilapia is considered a freshwater species it has shown a great tolerance
towards salt and can be found in both estuaries and coastal lakes under brackish
condition. It inhabits a temperature range of 13.5-33 0C and has fast growing ability in
many different types of waters.
Among tilapia species, the Mozambique (Oreochromis mossambicus) tilapia that native
to African region which introduced to Sri Lanka in 1952, is a highly adaptable species
that can survive and even thrive in many different environments. This is naturally a good
thing for the tilapia itself, but it is also what makes it such a risky introduction to non-
native waters. It will for instance breed even in environments where food is scare, and
each female can rear several large broods each breeding season. The offspring can start
breeding when they are no older than 3-4 months. They can tolerate wide range of water
temperature and also very tolerant when it comes to poor water quality, pollution and
oxygen scarcity. Being an opportunistic omnivore it will explore a wide range of different
food source, from plankton and rooted plants to invertebrates and fish fry.
Tilapia can cause problems for native flora and fauna in several ways. They will compete
with other species for food and can disrupt the ecological balance of aquatic ecosystem.
Tilapia can also cause turbidity in clear waters since they are fond of digging which make
turbidity in water. Turbidity will reduce the amount of available light in the water, which
affects all organisms relying on photosynthesis. Tilapias are also famous for their
opportunistic feeding style and many species can make use of a wide range of food
source, from aquatic invertebrates and small fish to decomposing organic matters, plants
and plankton. In addition to this, many species of tilapia breed rapidly and grow fast.
These specific characteristics make them more resilience in any aquatic ecosystem, which
create food scarcity for other creatures sharing that ecosystem due to high competitive
habit of Tilapia.
The Mozambique tilapia has been nominated by the IUCN Invasive Species Specialist
Group (ISSG) as one of the species belonging to the group 100 of the World’s Worst
Invasive Alien Species”.
Tilapia as an Invasive Species
Global scenario
In the United States, established populations of tilapia occur in the many different parts
of the country. There is for instance a thriving population of Oreochromis mossambicus
in Salton sea, an inland saline lake in Southern California. Therefore, several states (15
states of USA) take measures to control Population. Further south in the America, tilapia
has been introduced to the sensitive Galapagos Islands where their remoteness has
resulted in a very high degree of endemic are now threatened by the introduction of
foreign species, such as Tilapia fish.
In Singapore, the problem with tilapia as an invasive species may be on its way of
solving itself. During World War II, the Japanese introduced Oreochromis mossambicus
to Singapore. Soon, it became a common fish in both fresh and brackish water, as well as
in the sea off the northern coast. Since the late 1980s, these feral population have
however been declining dramatically and this is believed to be caused by the introduction
of other tilapias, probably the hybrid Oreochromis mossambicus x Oreochromis niloticus
and possibly also Oreochromis urolepis hornorum and Oreochromis aureus. When
Oreochromis mossambicus mates with certain species and hybrids, the resulting batches
tend to have a much skewed sex ratio where almost all fishes are male.
Sri Lankan scenario
Together with Tilapia there are about 35 other species of fish, which are more or less
invasive have been introduced to Sri Lanka since 1880s. All of them alien and thriving
now, naturalized in Sri Lanka aquatic systems. Both fresh water fisheries and the
ornamental fish industry are responsible for the introduction of fresh water invasive
species to Sri Lanka either deliberate or accidental. Tilapia is now the dominant group of
fishes in Sri Lanka’s rivers, and has invaded the entire dry zone, most estuaries and
lagoons in the dry and wet zones and the large streams, reservoirs and rivers of wet zone
lowlands up to 600 m elevation (Pethiyagoda, 1999). Only tilapia-free habitats being the
shallow, shaded rainforest streams in hill country, which, for some reasons these fish
haven’t still invaded.
Being a superior competitor for the resource and being non-selective in diet and prolific
breeder tilapia has allowed to colonize easily in the tanks, reservoirs and slow flowing
rivers, while displacing the native inhabitants such as Labeo porcellus and Labeo
dussumeiri (Bambaradeniya, 2000). Tilapia breeds throughout the year, but more
frequently during the rainy season, which overlaps with the breeding season of the most
the indigenous fish. The diet of small tilapia comprises zooplanktons which are feed
resources of the indigenous fish make food scarcity for some indigenous species make
them to prey on their own young causing threat to the native population indirectly. It
seems extremely probable that the decline of red-in labeo (Labeo lankae) and the spiny-
eel (Macrognathus aral) have been due to competition with tilapia (Pethiyagoda, 1999).
Therefore, it appears tilapia has been responsible for at least single species extinction in
Sri Lanka (Pethiyagoda, 2006, http:// www.indiaenvironmentportal.org.in/node/38152,
cited on 10.02.2009), but no control measure is adopted and even no critical assessment
has been made of its impacts on indigenous fishes since after the introduction to Sri
Lanka in 1950’s.
The presence of native fishes in the river systems had dwindled whenever the presence of
tilapia was found in large numbers. This might be due to its early sexual maturity,
continuous breeding habit, high survival rate of young’s and its carnivorous behavior on
eggs and juveniles of other fishes and other aquatic fauna. Tilapia can cause problems for
native flora and fauna in several ways. They will compete with other species for food and
can disrupt the ecological balance causing turbidity in clear water since they are fond of
digging. Turbidity that creates by digging will reduce the amount of available light in the
water, which affects all organisms relying on photosynthesis as well those who depend on
primary productivity as well as zooplanktivores in aquatic ecosystem.
However, food habit of tilapia population in Sri Lanka reservoirs did not confirm to a
particular pattern. In different seasons in certain reservoirs, the dietary habits changed
from a complete detritivory habit in the wet season and phytoplanktivory in the dry
season while blue-green algae did not constitute an important component of the diet
except tilapia living in highly eutrophic, Colombo Lake (Bere Wewa) where the blue-
green algae were the main food of Tilapia. Tilapia that might inhabitant in a temperature
range of 13.5 to 40 oC but, growth decline rapidly at temperature below 20 oC, which
restricted it population spreading to hill country water streams in Sri Lanka due to their
cooler temperature and fast water flowing nature.
The contribution of tilapia species to the inland fishery exceed 70% in few decades back,
but over the last two decade it has been gradually reduced to just over 50% and replaced
by Indian Major Carps which was introduced to Sri Lanka in 1980s. However, Tilapia
still play a major role in inland fishery sector being a dominant group of fishes, where the
society that largely depends on freshwater fishery, especially poorest rural community in
the country. Therefore, instead of the eradication of this fish species from the country,
adoption of mitigation measures for their unwanted characteristics and developing them
towards the less invasive and high productive population of fishes is the challenge for
present day scientists, conservationists and economists.
Strategies to make Tilapia a Non-invasive Fish Species in Sri Lanka:
In view of the above considerations, the initial approach is to produce monosex (all male)
tilapias through hybridization. The cross between the female Oreochromis mossambicus
and male Oreochromis hornorum generates a nearly all-male hybrid population of tilapia.
This technique is based on creation of males with two “Y chromosomes” (super male).
Mair et al. (1997) adopted this technology in 1991,which gave relative advantages
including commercial viability of producing genetically all male tilapia (GMT)
population over hormone-induced sex inversion. Thus, the hybridization strategy could
minimized overpopulation in the culture environments and also increases the productivity
of populations because of faster rate of growth of male tilapia. It has been proven that the
yield of GMT was 30-40% high when compared to the normal mixed-sex cultured tilapia.
In addition, the mean grow-out period also shorter compared to GIFT (Genetic
Improvement of Farmed Tilapia) fish and sex-reversed tilapia, which is presently cultured
in commercial level (Eknath and Hulata, 2009). Hybridization followed by hormonal sex-
reversal method can be applied to produce all-male seed stock in large scale. This can be
practiced in the breeding stations of National Aquaculture Development Authority
(NAQDA) of Sri Lanka, where seeds are producing at present for the stocking in seasonal
tanks as well perennial water resources in the country.
Conclusions and Policy Implications
The production of all-male tilapia systems could be introduced as a remedial measure to
overcome the invasive behavior of tilapia in freshwater bodies in Sri Lanka. Feeding or
immersion of steroid hormones, hybridization and YY super male technology are the
technologies to be adopted as mitigation measures to bring down the present population
levels of tilapia in Sri Lanka to produce a non-invasive male-skewed population. These
changes in tilapia population would make it the most promising fish species in Sri Lanka
by fulfilling the socio-economic and environmental aspects of the people of the country,
as well as the inland capture fishery through high production.
References
Bambaradeniya, C.N.B. (2000). Alien invasive species in Sri Lanka. Loris, Journal of
Wildlife and nature protection society of Sri Lanka. 22(4): 3-7.
Bambaradeniya, C.N.B. (2001). The status of invasive alien species in wetland
ecosystems of Sri Lanka. Sri Lanka Biodiversity Review. 1(1): 61-75.
De Silva, S.S, Subasinghe, R.P., Bartley, D.M., Lowther, A. (2004): Tilapia as alien
aquatic in Asia and the Pacific: a review, FAO Fisheries Technical Paper, 453. FAO,
Rome.
Eknath, A.E., Tayamen, M.M., de Vera, M.P., Danting, J.C., Reyes, R.A., Dionisio, E.E.
(1993): Genetic improvement of farmed tilapias: the growth performance of eight strains
of Oreochromis niloticus tested in different farm environment, Aquaculture, 111: 171-
188.
Eknath, A.E. and Hulata, G. (2009): Use and exchange of genetic resources of Nile
tilapia (Oreochromis niloticus). Review in Aquaculture, 1: 197-213.
Mair, G.C., Abucay, J.S., Skibinsk, D.O.F., Abella, T.A., Beardmore, J.A. (1997):
Genetic manipulation of sex ratio for the large-scale production of all-male tilapia,
Oreochromis niloticus. Canadian Journal of Fisheries and Aquatic Science, 54: 396-404.
Pethiyagoda, R. (1999). Fishes in trouble: The decline and fall of Sri Lanka’s fresh water
fish fauna, Loris, 22: 56-64.
Pethiyagoda, R. (2006). Conservation of Sri Lanka fresh water fishes. In The faunaof Sri
Lanka (Ed: C.N.B. Bambaradeniya), pp. 103-112. World Conservation Union, Gland.
... Yes, and also listed in the 100 worst IAS of the world Tilapia (Oreochromis mosambicus), although used a common food fish to stock inland water bodies in Sri Lanka, is a known IAS in tropical countries and is listed as one of the 100 worst IAS of the world (Athauda, 2010). It is a superior competitor, a generalist in terms of environmental tolerance, an opportunistic omnivore that feeds on a wide range of faunal and flora species, breeds rapidly, and causes turbidity in water bodies as it digs up the substrate (Athauda, 2010 ), a rare migrant, was observed in Vidattaltivu. ...
... Yes, and also listed in the 100 worst IAS of the world Tilapia (Oreochromis mosambicus), although used a common food fish to stock inland water bodies in Sri Lanka, is a known IAS in tropical countries and is listed as one of the 100 worst IAS of the world (Athauda, 2010). It is a superior competitor, a generalist in terms of environmental tolerance, an opportunistic omnivore that feeds on a wide range of faunal and flora species, breeds rapidly, and causes turbidity in water bodies as it digs up the substrate (Athauda, 2010 ), a rare migrant, was observed in Vidattaltivu. Many of these lay their eggs on the ground, and therefore, are highly vulnerable to predation by feral dogs. ...
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The otoliths (N = 12) of freshwater invasive species tilapia (Tilapia mossambicus) collected from two water bodies located at Kolkata and Bangalore, India, were analyzed for stable isotopes (δ18O, δ14C) and major and trace elements in order to assess the suitability of using otoliths as a tracer of aquatic environmental changes. The stable isotope analysis was done using the dual inlet system of a Finnigan-MAT 253 isotope ratio mass spectrometer (Thermo-Fisher, Bremen, Germany). Concentrations of major and trace elements were determined using a Thermo X-Series II quadrupole mass spectrometer. The stable isotope composition in tilapia otolith samples from Bangalore and Kolkata water bodies are quite good agreeing with that of the respective lake/pond and rain water. Elemental composition revealed in a pattern of Ca > Fe > Na > Sr > K > Ba > Cr > Mg > As > Mn > Zn > Co > Cu > Cd > Pb. The otoliths from Kolkata pond water are more enriched in Ba, Zn, Pb, Mn, Se, Cu, Zn, Cd, and Ni whereas Cr and As were found to be higher in otolith samples from Bangalore lake. The enrichment factor (EF) values of Cr were higher for both the sampling location in comparison with other metals, although all the studied metals exhibited EF values >1. The PCA shows clustering of metals in the otolith which are related either with the metabolic and physiological attributes or waterborne source. The study demonstrated the potential of stable isotope techniques to distinguish otolith specimens from varied climatic zone, while elemental composition recorded the quality of water at both the locations. The role of climate driving the quality of water can be understood by detailed and continuous monitoring of otolith specimens in the future. Future method allows reconstruction of climate and water quality from old specimens from field exposures or museum collection.
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