Monday, December 20, 2010

Fish Introduction Impacts


The proposal by the Fisheries Department to stock fish seeds in reservoirs, especially those in reserve forests, has been objected severely by the conservationists. The proposal seeks stocking Indian Major Carps (IMCs) in the 16 reservoirs of Kerala, including Kallada, Kakki, Pamba, Anayirangal, Neriyamangalam, Ponmudi, Kandala, Shenkulam, Mattupetti, Bhootathankettu, Peringalkuthu, Sholayar, Parambikkulam, Karapuzha, Banasurasagar and Pazhasii, located within the reserve forest areas.

The conservationists oppose the move primarily because of the fact that the Western Ghat rivers are the aquatic biodiversity hotspots. Any introduction of IMCs, which are highly competitive and voracious feeders would lead to the extermination of the highly threatened indigenous fish fauna, notwithstanding their impacts on other aquatic organisms, which remain less documented. 

The introduced fishes would be aggressive, prolific breeders which would move out of the reservoirs and expand their range along the river courses, both upstream as well as downstream. They would easily push out of their niches the indigenous species and prey upon the local species of fishes, their fingerlings or spawn. They would voraciously feed on the aquatic biomass and successfully compete with all the indigenous species of freshwater fauna. Dr. Sathis Chandran, a  noted ecologist, says that most of our reservoirs are in the Western Ghats and the catchment areas upstream of the reservoirs are Reserved Forests. At least twenty large reservoirs in the State are within National Parks and Sanctuaries where the primary objective of management of the area is wildlife protection. Introducing exotic species is obviously violation of the Wildlife Protection Act. Another seven reservoirs are inside wet evergreen forests with very high biodiversity potential where the species richness remains inadequately documented. Therefore this move has to be abandoned.

A recent study conducted by Mr Robin Kurian Abraham in the southern Western Ghats shows that the protected areas act as the refugee for highly endemic fish fauna and one of the greatest threats is the presence of exotic species. For example, in Kulathupuzha river the Common carp introduced for aquaculture has escaped downstream and is in the process of replacing the threatened mahseer fish. This is an indication of possible replacement of indigenous fish by the larger and highly competitive carps.  Exotic fishes such as Tilapia (Oreochromis mossambicus), Common carp (Cyprinus carpio), African catfish (Clarias gariepienus), and IMCs have gained entry into the reservoir ecosystems of India through accidental or deliberate introduction. Among them tilapia has already entered into all the upstream rivers of Kerala, including forest streams and established viable populations (Dr. Bijukumar, Dept. of Aquatic Biology and Fisheries, University of Kerala).

Convention on Biological Diversity states: "Inland water ecosystems are particularly vulnerable to invasive alien species. There is limited success in the prevention, eradication and control of invasive alien species in inland water ecosystems with fewer methods available than for the control of invasive species in terrestrial systems, particularly for organisms, which are submerged (such as fish)". The rivers of the Western Ghats are freshwater biodiversity hotspots, as each stream may serve as an evolutionary centre as far as highly endemic freshwater fish fauna are concerned. The water bodies associated with the Western Ghats harbours 288 species of freshwater fish of which 118 are endemic. A larger diversity of new freshwater fishes has been recorded from the aquatic ecosystems associated with the Kerala part of the Western Ghats. For example, a relatively small watershed of Chalakkudy river, covering less than 1700sq km has more than 100 species of fishes, many of which are endemic to the river! Moreover, the upper reaches of most of the river basins in Kerala are within forests and have not been exhaustively surveyed for their biota. 

In the Government's 'Guidelines for Culture-based Fisheries Management in Small Reservoirs in India', under Box. 3, it is stated "In India, fish transferred on the trans-basin basis within the geographic boundaries of the country is not considered as exotic, with no restrictions on such transfers. Thus, Indian major carps such as Catla is not regarded as exotic in peninsular rivers, despite the fact that the species is outside its normal range of distribution and peninsular rivers have habitats, distinctly different from the Ganges and Brahmaputra river systems. The small west-flowing drainages of the Western Ghats…. have ichthyofauna different from the Ganga and Brahmaputra river systems. Catla, Rohu and Mrigal have been stocked in the peninsular reservoirs for many decades now, varying results. In some of the reservoirs in Southern India, they have established breeding populations. The country's policy on stocking reservoirs,though not very explicit, disallows the introduction of exotic species into reservoirs." There is a clear affirmation in the above passage that west flowing drainages of the Western Ghats are substantially different from the perennial, glacier-fed, floodplain rivers of the Ganga-Brahmaputra river systems, which is the original habitat for the Major Carps. So, when such large bodied floodplain species are regularly introduced in large numbers, to the seasonal river systems of the Western Ghats, which harbour a rich diversity of significantly different, but remarkably unique fish species adapted to the seasonal streams, there is bound to be a marked negative impact on many of the smaller endemic fishes and on other important aquatic organisms like tadpoles of numerous species of endemic frogs, molluscs, crabs, freshwater prawns, larvae of macro-invertebrates and aquatic vegetation, says Robin Kurian Abraham.


Majority of the endemic fishes in Kerala find their home in the pristine streams and rivers within the reserve forests of Kerala. The recent recording of Piranha from Chalakkudy river, African catfishes from many water bodies including Periyar lake and Nile tilapia from Bhrathapuzha river is a matter of concern to the freshwater aquatic diversity, taking into account the greater endemism of fish fauna in these systems. The present case of introduction of Indian Major Carps (IMCs) should also be discussed in a holistic ecosystem perspective, as there are reports that introduction of fish to different habitats within the country may also lead to loss of biodiversity. Though there are such reports in Asia, such studies, unfortunately, have not initiated in India and this demand for a precautionary approach. (Dr. Bijukumar, Dept. of Aquatic Biology and Fisheries, University of Kerala).




There are at least 61 species of threatened fishes (listed as either critically endangered or endangered) that occur in the rivers of Kerala. Although explicit information regarding the factors that drive native fish species towards endangerment is still unclear, there is a general consensus that habitat loss and exotic species could be the possible reasons. There is bound to be competition by the introduced major carps with indigenous fish species, since they are introduced in huge numbers. For instance, the Mrigal, which is a bottom feeder, would compete with local bottom feeding species like members of the genus Hypselobarbus, which have several endemic species in the Western Ghats. Column feeders such as Puntius spp. would be in direct competition with introduced Rohu, who use the same mid-column niche.

According to recent researches, one of the major threats for native freshwater fishes, after habitat destruction, is the introduction of non-native species.

Majority of the endemic freshwater fish in Kerala are restricted in their distribution to the Western Ghats rivers, especially in the reserve forest areas. Moreover, it is also important to note that all the fishes reported new to science from Kerala in the recent past have their restricted distribution in the forest streams. Based on his recently concluded a scientific study Mr Robin Kurian Abraham sys that streams, rivers and reservoirs inside Protected Areas managed by the Forest Department are critical habitats for many indigenous freshwater fish species. These species, many of which are endemic to the Western Ghats, are severely under pressure outside Protected Areas, from threats like sand mining, dynamite fishing, overharvesting and the release of untreated domestic, medical and chemical wastes into rivers. So substantial populations of many native and rare species seek refuge inside Wildlife Sanctuaries and Reserved Forests, whereas their population is drastically low, outside of the sanctuaries and reserved forests.


The IMCs are represented by three species: Catla (Catla catla), Rohu (Labeo rohita) and the mrigal (Cirrhinus cirrhosus; earlier Cirrhinus mrigala). Catla catla grows to a maximum length of 182 cm and a weight of over 38 kg and is a surface and mid-water feeders, feeding mainly omnivorous with juveniles feeding on aquatic and terrestrial insects, detritus and phytoplankton. The column feeder Labeo rohita attains a maximum length of 200 cm and weight of 45 kg, and prefers plants as their food. The mrigal Cirrhinus cirrhosus grows to 100 cm in size and 13 kg in weight and is a bottom feeder. It is important to note that majority of indigenous fish in the Western Ghats never attain this length, and feeds as voraciously as IMCs. Further, by exploiting all the available niche, they are strong competitors. Therefore, we also need to consider the fact the smaller rivers in Kerala are ecologically not suitable for the IMCs and in the long run, culturing them in the reservoirs of Kerala would not be economically feasible in the long run, as the current motive of stocking is ONLY to 'enhance' fish production.

The costs and benefits of invasive alien species are mixed because alien species are often a major source of income, food or livelihood for local communities and often support major economic activities (such as aquaculture production). The challenge, therefore, is how to maximize the benefits whilst minimising the risks. Greater attention needs to be given, where feasible, to promoting the use of native species. However, the use of native species should include precautions to conserve the genetic diversity of native populations. In this context conservation of indigenous biodiversity should be given priority.


Most of these introductions, including those within the country, have been made regardless of the consequences for the ecosystems, as the effect on ecological interactions between species, hybridization with native species, loss of biodiversity, and introduction of new pathogens and diseases among others. The replacement of non-native species with native species is an issue that must be considered, especially in countries with high freshwater fish biodiversity where it is easier to find suitable candidates. Considering the possible negative impacts of IMCs to the indigenous fish fauna, which shows greater endemism, the move to stock them in the reservoirs within the reserve forests need to be stopped as a precautionary principle.

1 comment:

dotfish said...

I am happy to come across you blog post. Its really scary that some arm chair scientist are planning really stupid things without even thinking about the consequences. U must highlight your article in various mediums to make public awareness in need of protecting native species and nature around us. Congratulations and keep up the good work