Migration of African catfish and intensive use of chemical fertilizers in cardamom plantations have posed a threat to local fish species abundant in the river system originating from the high range area.
Periyar Lake, home to 36 fish species, has been identified as the largest one in the district to host endemic species including the Periyar trout.
As per an International Union for Conservation of Nature (IUCN) classification, seven species are critically endangered, six endangered, and eight vulnerable in the lake where six varieties are endemic.
Akbar Ali, Assistant Professor, Kerala University of Fisheries and Ocean Studies, told The Hindu that the introduction of hybrid varieties such as tilapia to the lake system resulted in high competition for food where the endemic species lose out. Moreover, tilapiahas a tendency to monopolise the lake, as it has high survival rate and lays egg at an early stage of growth. The spread of African Catfish in the lake was from private ponds where they were grown even after their breeding was banned by the Union government. A survey in the small tributaries to the lake pointed out high concentration of African catfish that lives on local fishes.
The two main threats to the endemic spices in the lake are African Catfish and tilapia, as per the studies. A Kerala Forest Research Institute study in 1999 also pointed out that there was high level of threat to local fish varieties in the lake owing to the introduction of hybrid fish.
It was mainly due to competition for food, the study said. Mr Ali said that though there was no report of natural breeding by African Catfish, it was opposite in the case of tilapia, which grew faster in the waterbodies. The Sucker Catfish, also reported in the waterbodies of Kerala, posed a threat to the endemic species, he said.
Though Matsayafed had deposited fish seeds in the reservoirs of Idukki, Anayirangal and Ponmudi with a view to increasing inland fish production, they could not be realised.
The spread of pesticide residue, introduction of hybrid fishes, and the migration of African Catfish are stated to be the reasons.
There were instances of large-scale fish kills reported from ponds in the cardamom plantations, especially after the application of pesticides.