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Exotic fish species has dominated the aquaculture industry causing a great threat to the indigenous fish culture. The entry and spread of the invasive exotic fishes have altered the stability of the native ecosystem in the open water bodies. Such replacement and wide spread of the invasive fish species will not only lead to loss of aquatic biodiversity but will also initiate extinction of indigenous fish species form the ecosystem.
Exotic fishes are the non-native fish species occurring outside of its natural range. These exotic fish when get establish and begin to reproduce in the new environment, it is considered as invasive species. Normally, invasive species includes all taxa of organism, ranging from microscopic insects to large mammals and can invade any ecosystem. According to IUCN, invasive species is an alien species which becomes established in natural or semi-natural ecosystem or habitat causing threat to native biological diversity. Invasive fish species are those fish species that does not occur naturally in a specific aquatic environment and whose introduction does or are likely to cause harm to the indigenous fish species living in the aquatic system or may cause economic or environmental harm to the ecosystem.
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Rupchanda (Red-bellied Pacu)
Exotic fish species were introduced to several countries mainly for aquaculture purpose, for aquarium keeping, sport fishing, for controlling unwanted organism in the aquatic ecosystem and to increase the aquaculture production as they grow fast and can acclimatise in any condition,. But when these species escape from the confine system to the natural ecosystem cause harm to the other indigenous fishes causing a threat to the aquatic biodiversity.
During last several decades, over 300 species of exotic fish species have been brought to India for aquaculture experiment (most of the food fishes, such as Golden carp, Tench, Common carp, Tilapia, Silver carp, Grass carp, Tawas, etc.)aquarium keeping (live bearers, egg- layers), sport fishing (Brown trout, Brook trout, Rainbow trout, sockeye salmon, etc.)unauthorised introduction (Bighead carp, Piranha, African catfish, Red tilapia), weed control (Grass carp, Tilapia) and mosquito control (Guppy, top minnows).
Several exotic fish species are now well established in the natural water bodies and aquaculture system of India. Not all the exotic fish species are invasive in nature; introduction of sport fishes has been found considered as non-problematic in Indian waters, the three exotic food fish (Common carp, silver carp and Grass carp) are well established in India, included in the composite fish culture. But several reports prove the devastating impact of these fish species in the natural water bodies such as rivers, lakes, beels and other water bodies causes decline of native fish species.
The introduction of invasive exotic fish species brought about a worldwide concern as it resulted in a wide range of problems such as prolific breeding, predation or competition with the native fish species affecting indigenous biodiversity, extirpation of indigenous fish species, diseases transmission, contamination of local genetic pools, habitat loss and these could also lead to irreversible changes in the aquatic ecosystems and result in extinction of species. There are numerous reports on the catastrophic impacts of exotic introduction both at ecosystem and species level.
The potential risk not only affect the quality or level of biodiversity but also the socio-economic aspects of the human community that depend on aquatic ecosystems for their livelihood. The indiscriminate and unmonitored entry and transfer of the exotic species is particularly alarming, raising several concerns including producing hybrids by crossing with indigenous fish species and thus contaminating the fish germplasm.
India has a rich biodiversity with 2936 of fish species (11% of total fish species) including more than 300 exotic fish species. The invasive exotic fish species have been found to encroach the natural water bodies and adversely affect the native fish species in the ecosystem. Most of the entries of the invasive exotic species into the open water bodies have been due to accidents and floods. Several cases on impact of invasive species in reservoirs, lakes, wetlands and rivers have been observed. Among them, one of the invasive fish species dominating the aquatic ecosystem is the Tilapia fish (Oreochromismossambicus), which is now found in all water bodies from cultural pond to wetlands.
The tilapia was first introduced into the pond ecosystem in 1952 and soon that by the end of 1960s, most of the reservoirs of south India (Tamil Nadu and Kerala) were regularly stocked with this fish. The warm water of India has provided a conducive habitat for the tilapias and it has established a secure position in all water bodies. The tilapia matures early, a prolific breeder, creating over-population and dominating the ecosystem, leads to virtually elimination of the indigenous fishes (carps, catfish, murrels, etc.) present with tilapia.
Tilapia is also creating problem in pond culture system in Northeast India, once the tilapia is introduced in the pond, it becomes difficult to remove the species from the system for other species culture, even after proper dewatering of pond, creating a problem in local fish culture, growth and feeding, as the fish dominate the system and compete for food and space.
Tilapia (Photo courtesy: Niti Sharma)
African catfish (Clarias gariepinus), one of the exotic invasive fish species poses significant threats to aquatic biodiversity through large scale extinction of endemic fish species. It has been found in all water bodies from cultural tank, streams to rivers to wetlands causing declining of indigenous fish species. These fish have also been reported to contaminate the aquatic germplasm by creating hybrid with native catfish species. The fish is a nocturnal predatory fish feeds primarily on living, as well as dead, animal matter including fish, invertebrates and small birds. It has the ability to survive in shallow mud and can tolerate poor oxygenated water giving advantage over the native fish species. In Assam, the African catfish has been recorded in several streams and beels. Recently, during a field visit to a beel, we have observed that local people staying near to the beel areas are growing African catfish in their backyard drain ‘nalas’, feeding them with household sewage. This fish has a great chance to escape to the nearby beels during the flood or during heavy rain. So, there is an urgent need to prevent the spread of the invasive species that poses great risk to the environment, economy or human health.
Piaractus brachypomus, commonly known as the ‘red-bellied Pacu’ highly invasive fish species were introduced in 2004 from Bangladesh for aquarium and aquaculture purpose. The breeding and culture of the fish begin at West Bengal and gained popularity in other states. In 2011, the fish was recorded in Vembanad Lake and Periyar River in Kerala, Dimbhe reservoir and Gadhi River in Maharashtra, Rivers in Andhra Pradesh, etc. In Assam, the fish widely entered in the fish market, locally known as ‘Rupchanda’. Rupchanda is now cultured and reared in the state. Initially, the seeds of the species were entering through Bangladesh and West Bengal, but at present the seed production of this species has been taking place in most of the part of the state. It’s a matter of concern for any ecologist or anyone associated with nature and biodiversity is that, the species has been recorded of late from natural water bodies of Assam by Fisheries Resource Management Department, College of Fisheries, Raha. If ‘Pacu’ gets established in the water bodies of Assam, the impact may be huge on our natural indigenous flora and fauna.
Other than the food fish, the exotic ornamental fish also found their way to natural water bodies from the aquarium. These happen mainly due to accidental escape during flood from the rearing or breeding tanks or release of unwanted healthy ornamental fishes when hobbyists are tired of them, or when the fish become too large to be accommodate in the aquaria. For example, the suckermouth catfish (Pterygoplichthy spardalis), a popular aquarium fish native to South America has spread into the open water bodies in several states Kerala, Vandiyur Lake, Madurai, Tamil Nadu, Andhra Pradesh, Bihar, West Bengal etc. The presence of suckermouth catfish showed negative impacts on the indigenous fish varieties, including decline in other aquatic biota such as aquatic insects, composition of aquatic vegetation and also damaging fishing gears.
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Suckermouth catfish (Photo courtesy: A. Bijukumar)
Guppy (Poecilia reticulate), one of the common live bearers fish is now found in every aquarium and all water bodies to local drains in India. These fish can easy breed, get mature in 3-4 months and give birth to 100 young ones at a times. It is widely spread in many countries perhaps to control disease –controlling mosquitoes, but in years, it has created problem by dominated over the native fish species by utilizing their food and space. It has been found that guppies were weak in controlling mosquitoes and got the chance to introduce into local ecosystem are large and that guppies can easily establish population and damage the aquatic ecosystem. Many other ornamental fishes are escaping form the close system to the natural water bodies, slowly creating their own space in the local system and overruling the indigenous fish population.
In today’s world, fish diversity constantly faces lots of challenges due to water pollution, siltation of river, over-exploitation, illegal fishing and the impacts of exotic fish species, reducing the native fish population, thereby threatening the fish diversity and even leading to extinction of fish species. The exotic fish species has dominated the aquaculture industry causing a great threat to the indigenous fish culture. The entry and spread of the invasive exotic fishes have altered the stability of the native ecosystem in the open water bodies. Such replacement and wide spread of the exotic fish species will not only lead to loss of aquatic biodiversity but will also initiate extinction of native fish species form the ecosystem. Control measures should be taken for release of invasive species in open water bodies from confined tanks. Public awareness should be created on potential negative impacts of these species; should develop proper quarantine procedures; conduct research to evaluate negative impacts of already introduced exotic species and socio-economic benefits of using native and less risk exotic species; developing culture techniques for indigenous species; should develop appropriate policies; management tools taking both positive and negative impacts into account; develop specific measurement for developing zones that allowed to culture exotic species and to control fish movement; strengthening the manpower and build the capacity for implementing policies and regulations.
Despite the uncontrolled introduction of invasive fish species across the world, there is no systematic study on most of the exotic fish species, its potential damage or impact on the native environment. To assess the impact that an exotic species may have in the introduced habitat requires organized and consolidated research programs, which is presently lacking across the globe. Management and detection of invasive fishes require coordinated nationwide efforts.
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