The natural reproduction of major carp species has been in sharp decline and conversely, species of smaller fish have been multiplying in Rangamati’s Kaptai Lake – the largest manmade freshwater lake in Bangladesh.
A recent study of Bangladesh Fisheries Research Institute (BFRI) revealed that the production of four commercially important carp species like rui (Labeo rohita), catla (Catla catla), mrigel (Cirrhinus mrigala), and Kalbaus (Labeo calbasu) was 81% during 1965-66.
However, the production of carp has alarmingly diminished to 5% over 2015-16.
On the other hand, smaller fish like Chapila (Gudusia chapra), Kachki (Corica soborna), and Mola carplet (Amblypharyngodon mola) continue to be the dominating fish species in the artificial lake, which was primarily created for hydro-electric power generation back in 1961.
The production of smaller fish species was only 8% in 1965-66, but has increased phenomenally to 92 percent in 2015-16.
With a total surface area of 68,800 hectares, the commercial exploitation of fish resources from the lake began back in 1965. Presenting a comparative picture of fish production, a study shows that the lake produced 4,566 metric tons of fish in 2002-03 and 9364 metric tons in 2015-16.
Of the total fish production, the share of four species of carp fish was 258 metric tonnes in 2002-03. However, the contribution plummeted to seven metric tons in 2015-16.
On the other hand, 3,399 metric tonnes of Chapila and Kachki were fished in 2002-03 with the production of the two smaller fish species rising to 5619 metric tonnes in 2015-16.
Apart from carp and other aforementioned species, some others have also declined in the lake. Fish such as sylon, sarpunti (Puntius sarana), bagair (Bagarius bagarius), pangas (Pangasius pangasius), ghaura, and mohini bata have already become extinct in the lake while deshi mohashol, modhu pabda, poa (Otolithoides pama), phasa (Setipinna phasa), shada ghonia, and gulsha are on the verge of extinction.
At present fish like kalbaus (Labeo calbasu), chapila (Gudusia chapra), kachki (Corica soborna) and mola carplet (Amblypharyngodon mola), telapia (Oreochromis mossambicus), ayer (Sperata aor), and bata (Labeo bata) are dominating.
Reasons for decline of carp
BFRI’s scientific officer Abul Bashar said the pH level (slightly alkaline) of the lake is ideal for fish cultivation, and it has not had outbreaks of fish pests.
“However, fish of major carp species are declining sharply in the lake due to a number of both manmade and natural factors. Carps lay eggs in water bodies with strong currents. Massive silt deposition reduced the water flowing capacity of the lake, drastically reducing their breeding,” explained Bashar who led the research work.
To substantiate his claim, the researcher pointed out that the depth of a certain spot of the Barkal Channel of the lake was found to be 30.3 ft in September, but 4.2 ft in May.
“The migration patterns of four rivers linked to the lake – Karnaphuli, Maini, Kasalong, and Rengkhong – have regrettably changed due to silt deposition. Moreover, an increase in agricultural activities in the lake’s adjoining areas, soil eroded from nearby areas finding its way into the lake, and overfishing are also reasons behind the drastic fall of carp fish production,” he added.
There are four major breeding grounds of carp fish in the lake—Kasalong Channel, Mainimukh; Barkal Channel, Jagannathchari; Chengi Channel, Naniarchar; and Rengkhong Channel, Bilaichari.
“The breeding grounds and migratory channels of carp fish should be restored through dredging. Besides, the number and area of sanctuaries in the lake should be increased and expanded to boost the stock of these fish. The carps usually breed between May and July, and we should not net the brood fish during this period,” said the researcher.
In his research paper titled “Increasing Fish Production in Kaptai Lake and Tolerant Management Strategy for Conserving Biodiversity”, the BFRI’s Director General Dr Yahiya Mahmud mentioned that 26 percent of the total fish collection comes from a natural fish trap which is locally known as “Jak”.
According to this research article, all fish irrespective of size and species get caught in “Jak” in the lake. As a result, fries of carp fish get killed in the traps. Brood fish also get caught as they flock to the natural traps as a last resort in their search for water during dry seasons. Although using a “Jak” is an illegal method for catching fish, many people use these traps. It is important to put an end to this malpractice, he said.