Invasiveness of introduced fish species on native biodiversity
ICAR-RC for NEH Region, Manipur Centre, Imphal – 795004, India
First the concept of ‘introduced fish’ must be clear and the synonymous term of ‘alien or exotic or non-native species’ are having the similar meaning. This species does not belong to the present habitat or water body and come into existence in the new environment because of accidental or deliberate human activity and due to some natural calamities or causes. All this term is very different from the “invasive’ species where the former does not have very little to any effect on the ecological balance of the aquatic body or sometimes may contribute in enrichment of the system ecosystem. But the later has a different story, and causes negative effect on the local ecosystem.
The motive behind the introduction of new species legally or illegally in India can be summed up for ornamental fishery, improvement of wild stocks and broadening biodiversity, aquaculture production, biological control measures( like mosquitoes) and finally recreational purposes. Firstly, an impact assessment of species introductions must be done requiring information from number areas such as the biology, ecology and genetics of the target species.
Some of the common traits of invasive species
- Short life span
- Rapid growth
- Rapid sexual maturity
- High fecundity
- Euryoecious (ability to colonize a wide range of habitat types)
- Eurytopic (wide range of physiological tolerances)
- Gregarious behaviour
- Wide genetic variability
- Disease tolerance
The following possible future negative impacts may arise by altering availability or quality of food, competition for food and resources, change of habit, affecting gene flow, development of stunted natives, emergence new diseases in wild and within the aquaculture sector.
The intensity of impact on native species lies on the objective of its introduction. Analyses of the FAO database on introductions of aquatic species (DIAS) reveals that aquaculture development was the most often cited reason for fish introductions, and that government organizations were responsible for more introductions than any other group. Table 1 presents information that most of the ecological effects of introduced species reported were negative, however the socio-economic impacts were reported to be more often beneficial and there were more positive socio-economic benefits reported than negative ecological impacts.
Table 1. Effects of introduced fish on Ecological and (Socio-economic) environments by reason for the introduction. Data represents number of records from FishBase
1 Other includes: accidents, bait, forage, to fill niche, research, diffusion
It is generally agreed that release of exotic fish creates competition with native species for space, food and other resources. The potential impact occurs when the exotic species can successfully started reproducing and adapt in the new environment. A good example here is the tilapia species which is notorious for their prolific breeding while African catfish were known for their voracious predatory habit. Thus, these new fishes can be expected to pose a high risk and have certainly cause some serious concern to some indigenous species due to their feeding, reproductive and life history strategies in many parts of the world including natural water bodies like rivers in India. Although the future outcomes of an introduction are difficult to predict, it is possible that they may coexist with indigenous species due to superiority in trait, spatial or temporal variability in habitats or strategies for their survivability and existence. Successful introduction of an alien fish species is often difficult to achieve. Considerable technical expertise is often required and many attempts fail. The change in hydrological behaviour of the rivers due to the construction of dams also contributes to the establishment of this new species. In India, over 300 alien fish species including 291 ornamental species, 31 aquaculture species and 3 larvicidal fishes are recorded. They include:
Aristichthys nobilis: This is a banned species in India and has initiated displacement of native Catla catla leading to decline in commercial catches. Possibility of natural hybridization of A. nobilis with Catla catla is on the possible look out by the scientist.
Cyprinus carpio: Well established population of this species has led to declines of Indian major carps from the Ganga River contributing to their increased production. Although consider a good candidate in Indian aquaculture their presence in river causes a major concern.
Oreochromis niloticus: The ability to adapt and breed naturally has augmented the population of tilapia in rivers causing sharp declined in catches of Gangetic carps although positively contributing towards overall production of the Ganga River. If the trend continues then the native species may be suppressed.
Clarias gariepinus: Another banned species and a very highly carnivorous species can cause serious loss to aquatic biodiversity especially fishery. It has been reported that this species have naturalized in some small streams connecting some major river and is a big concern for fishery sector.
Hypophthalmichthys molitrix: Consider to be an important species in composite culture for Indian aquaculture this species has naturalized their population in natural water bodies and is adversely competing with one of the commercially important major carp species, Catla catla.
Gambusia affinis: Introduced in many part of the country as a biological measure for controlling malaria, its presence has reduced natural zooplankton population thereby competing the food regime of the natives and likely displacing local fish species.
Pterygoplichthys perdalis and P. disjunctivus: This two species has been reported to establish in some major river of India and breed naturally.
Pygocentrus nattereri: The Red-bellied Piranha which is well known for their highly predatory nature has been recently reported from the Godavari River.
Thus, the species of interest must be judiciously selected for of any of the mentioned purposes as it might lead to unwanted impact on the native biodiversity. Local and government rule and guidelines should be followed with stringent implementation at various check point to avoid any illegal activities. The people and local must be aware of the importance of their native species and side by side the negative consequences that might cause from the exotics.
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