This document discusses the introduction of exotic fish species around the world and their impacts. Some key points: 1) Exotic fish species have been introduced for various purposes like improving fisheries, aquaculture, and mosquito control. However, they can negatively impact native species through genetic changes, competition for resources, and spreading of diseases. 2) In India, many exotic trout and carp species from places like the UK, Sri Lanka, and Thailand have been introduced since the 1860s for experimental culture and stocking water bodies. 3) The impacts of exotic fish introductions include reduction of native populations and genes pools, hybridization leading to genetic pollution or species extinction, and alteration of aquatic ecosystems through pred

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2. Introduction
• Exotic fish species have been receiving international
attention in different forums including the important
ones like Convention on Biological Diversity and also in
the FAO Code of Conduct for Responsible Fisheries.
• While much of the recent attention has been mainly
focused on the adverse impacts of alien fish
introduction.
• There is realization that all alien species perform bad
under every circumstances.

3. Why are they introduced?
• Exotic species of the fishes were introduced in many
parts of the world with high hopes of promoting……..
1. Improving local fishery potential and for broadening
species diversity in aquaculture programmes,
2. Sport fishing,
3. For aquarium keeping,
4. Improving aquaculture or open water fishery
productivity,
5. Controlling of unwanted organisms (mosquitoes).

4. Exotic fishes transplanted
in
INDIA

5. Species Home Year Purpose
1.Brown Trout (Salmo trutta
fario)
U.K. 1863- 1900 For planting streams, lakes and
reservoirs.
2. Loch Leven Trout (Salmo
levensis)
U.K. 1863 For planting streams, lakes and
reservoirs.
3. Rainbow Trout (Salmo
gairdneri)
Sri Lanka &
Germany
1907 For planting streams, lakes and
reservoirs.
4. Eastern Brook Trout
(Salvelinus fontinalis)
U.K. 1911 For planting streams, lakes and
reservoirs.
5. Sockeye Salmon
(Oncorhynchus nerka)
Japan 1968 For planting streams, lakes and
reservoirs.
6. Atlantic Salmon (Salmo
salar)
U.S.A. 1968 For planting streams, lakes and
reservoirs.

6. Species Home Year Purpose
1) Golden Carp
(Carassius carassius)
U.K. 1870 Experimental culture
2) Tench
(Tinca tinca)
U.K. 1870 Experimental culture
3) Gourami
(Osphronemus
goramy)
Java &
Mauritius
1916 Experimental culture
4) Common Carp
(Cyprinus carpio)
[German Strain]
Sri Lanka 1939 Experimental culture
5) Tilapia
(Oreochromis
mossambicus)
Africa 1952 Experimental culture
6) Common Carp
(Cyprinus carpio)
[Bangkok strain]
Thailand 1957 Experimental culture
7) Grass Carp
(Ctenopahryngodon
idella)
Japan 1957 Experimental culture and
weed control
8) Silver Carp
(Hypophthalmichthy
s molitrix)
Hong
Kong
1959 Experimental culture
9) Tawes
(Puntius javanicus)
Indonesia 1972 Experimental culture

7. Species Home Year Purpose
1) Guppy
(Poecilia reticulata)
South
America
1908 Mosquito control.
2) Top Minnow
(Gambusia affinis)
Italy 1928 Mosquito control.
Species Home Purpose
1) Live bearers
(27 species)
From various countries Aquarium keeping
2) Egg layers
( 261 speceis)
From various countries Aquarium keeping

8. Impact On Bio-diversity
• Exotic or alien fishes make some bad impact on
aquatic biodiversity. Those impacts may be
categorized as :
1) Genetic Impact,
2) Ecological(including biological) Impact &
3) Socio-economical Impact.

9. • The genetic impact of introduction of exotic fishes
on native fishes can be classified into 2 categories :-
A. Reduction of effective population size by the
ecological, biological & genetic effect of
introduction. &
B. Alteration/extinction of gene pools of the
species/stocks by crossbreeding or hybridization
& backcrossing.

10.  One of the reasons for the increased frequency of
hybridization is introduction of exotic species.
 Hybridization may lead to hybrid vigor or heterosis due to
over dominance and heterozygocity at many loci or may
lead to genetic pollution.
 Thus, the result of hybridization in general and with exotic
species in particular is unpredictable in advance.
 Hybridization between an alien species and a native one
leads to genetic introgression. This is the transmission of
genes from one species to another through hybridization.

12.  Hybridization between exotic and native species
has not only brought in genetic contamination
but even resulted in species extinction in some
cases.
 Apache trout (Oncorhynchus apache) & Gila trout
(O. gilae), two native species of Southwest USA,
have faced extinction primarily due to
hybridization with the introduced species,
Cutthroat trout (O. clarkii) & O. mykiss.

13.  Loss of traits leading to economic value decline
takes place in uncontrolled hybridization between
exotic and native species.
 Hybridization between Bighead (Aristichthys
nobilis) & Silver carp (Hypophthalmichthys
molitrix) showed beneficial properties in terms of
growth, food conversion & disease resistance.
 But in further uncontrolled hybridization of these
fishes in later generation, the offspring lost the
acquired beneficial traits.

14.  A genetic bottleneck is the sudden and drastic
decline in numbers. It effectively samples
(although not necessarily randomly) a few
individuals from a larger gene pool, resulting in
a remnant population with less overall
variation.
 Loss of variation has two components like
reduction in variance of qualitative traits and
loss of specific and usually rare alleles.

15.  It is probably the most serious and yet most
nebulous problem in small populations of
endangered fishes.
 It is mating of individuals related by common
ancestry that share common genes due to descent
than individuals randomly selected from the
population.
 Fitness characters with low heritability are
generally effected in consanguuious matings
including severe body deformity, growth reduction,
behavioral changes and reproductive failures.

16. Competition of exotic fishes with the native species
for living space with same niche preference, for
food with fishes of similar types of feeding habits,
or of omnivorous feeding habits ,or predation on
native fishes by these, spreading parasites and
pathogens, thereby are some common ecological
concerns.

17.  Top carnivores are often viewed the most
significant threat as introduced fishes.
 Although the generality of this statement is not
borne out by an analysis of the records in the
Database on Introductions of Aquatic Species
(DIAS).
 Predation directly reduces population size of the
prey species, and may cause cascading ecological
effects, such as increased plant growth when
herbivores are removed by top predators.

18.  Competition can occur between alien and resident species for
food, habitat, mates, or other essential resources.
 Tilapia (Oreochromis spp.) and especially the Mozambique
tilapia, Oreochromis mossabicus, are considered to be a threat
to native diversity in many areas where they have been
introduced and most of the impacts have been reported for
inland waters.
 O. mossambicus in the Philippines and Pacific islands competes
for algae and other resources and has displaced preferred
species of mullet, Mugil cephalus, brackishwater shrimp,
Penaeus merguiensis, and milk fish, Chanos chanos in brackish
water fish ponds.

20.  The spread of pathogens along with species transported or
traded in aquaculture is a serious concern that is being dealt
with by several international agencies such as FAO, World
Health Organization (WHO), World Trade Organization
(WTO) and the International Office of Epizootics (OIE).
 Of particular concern to exotic species is that the level of
uncertainty will be higher with new introductions on what
pathogens may be present and may cause problems in the
new environment.
 For example, along with abalone that the California
aquaculture industry imported from South Africa came a
sabellid worm parasite that caused no problems in South
Africa but has had devastating effects of abalone under
culture in California; the impact on other Californian
mollusks is unknown.

21.  Many species of freshwater animals greatly modify
aquatic habitats when placed into a new area, e.g.
beavers, crayfish, common carp, and grass carp.
 The Asian clam, Corbicula fluminea, was probably
introduced into the USA by Chinese immigrants as a food
item (though possibly not for intentional farming) in
1938.
 The clam has since spread widely to inland and coastal
areas of 38 states in the USA. The most significant affect
is in biofouling of freshwater systems, but the clam can
grow in such large numbers as to alter the flow and
substrate in streams and lakes and can remove large
amounts of phytoplankton from the water column.

22. • The economic impacts can be seen at two levels -
1. capture fisheries and
2. aquaculture.
• Since the exotic fishes never fetch higher price than the
native varieties and also the decline of native fish
production is observed in the presence of exotic species in
natural waters , the total economic returns declined for
the stakeholders of the capture fisheries.
• In aquaculture however it provided immediate gain, in
most cases without consideration of the long term
ecological consequences.

23. • The global experience and the present status of
introduction of exotic species in different
counties including India their ecological,
biological and genetic impact analysis in this
contribution indicates their generally deleterious
effect on autochthonous species.
• In addition to direct devastating eco-biological
impacts, it has also been seen that some fish are
even extinct owing to loss of genetic variability
and heterozygosity.

24. • Consequence Of Alien Fish Introduction –
- by P. Das.(In Fishing Chimes, Vol 26 No. 10,January
2007).
• Plecostomus multiradiatus an armored catfish from
freshwater ponds near Kunnamkulam, Kerala and its
possible impact on indigenous fishes – by Ajithkumar,
C.R., C.R. Biju and R. Thomas (1998). LAK News,
Limnological Association of Kerala, 1-2.
• Fish fauna of Bharathapuzha (Nila river), Kerala. J.
Bombay nat. Hist. Soc – by Bijukumar, A. and S. Sushama
(In press).