At least 20 of the 30 alien fish species brought to the country over the last seven decades for rapid aquaculture expansion have escaped to open waters threatening the existence of many native fishes.
The escaped alien fishes are mostly recognised as highly invasive for they have rapidly invaded open waters in some of their host countries after making similar escape, causing the extinction of native fishes and destruction of aquatic ecology and livelihood based on it.
‘Some commercial farmers however greatly benefitted from cultivating exotic fishes and the mad race among them to increase profits amid poor government monitoring might have helped the exotic fishes escape,’ said Dhaka University’s fisheries department professor Monirul Islam.
The government however is unaware of the spread of most of the exotic species to open waters.
‘We know exotic fishes like African magur and piranha are being illegally cultivated but are unaware of the other exotic fishes escaping to open waters,’ said Department of Fisheries director general Quazi Shams Afroz.
The culture of the alien fishes has significantly raised overall fish production but greatly harmed food security for they are less nutritious than many of the threatened native fishes and are often laced with excessive heavy metal residues, researchers and fisheries experts said.
Bangladesh is 5th in world aquaculture production recognising contribution from some of the world’s worst invasive alien fish species threatening to alter the country’s aquatic biodiversity.
‘The negative impact of alien fish invasion on local fishes may take a while to surface for the new fishes are competing local ones in a new environment,’ Bangladesh Agricultural University’s fish, biology and genetics professor Mostafa AR Hossain told New Age.
Once the alien fishes get accustomed to the new environment, he said, they will grow exponentially, with their strength growing from generation to generation, leaving native fishes in severe crises of food and space.
An official account of how many of the alien fishes brought for aquaculture escaped their supposed confinement is absent but scattered national and international research found that at least 20 of them naturally breeding in open waters.
Open waters such as rivers, floodplains and beels were the last sanctuaries where native fishes were cornered after aquaculture largely removed them from closed water bodies such as ponds, haors and baors.
Before aquaculture boomed in the 1980s, the closed water bodies formed an important part of Bangladesh’s diverse aquatic habitat giving shelter to many native fishes during dry season.
Most of the native fishes once abundant in closed water bodies can no longer be found there with commercial farmers regularly drying them up to culture invasive exotic fishes.
Even pesticide is applied to wipe any traces of other aquatic life to prepare closed water bodies where exotic fishes are reared with contaminated feed, excessive antibiotics and growth hormones.
‘Exotic fishes are popular for aquaculture for their capacity to grow exponentially in worst environmental conditions,’ said Dhaka University’s zoology professor M Niamul Naser.
The escape of the exotic varieties of fishes in open water ecosystems, he said, could bear disastrous consequences for native fishes struggling to cope with challenges such as river encroachment, unplanned infrastructure development blocking natural water and industrial pollution.
The exotic fishes found naturally breeding in Bangladesh’s open waters are tilapia, common carp, black carp, mirror carp, bighead carp, scale carp, leather carp, grass carp, silver carp, Thai sharpunti, African magur, Vietnamese shol, Vietnamese koi, suckermouth catfish, mosquito fish, guppy and gurami.
The exotic fishes were mostly brought in for aquaculture since 1950s and some of them are present in more than one species.
‘Alien fish invasion is among the three top reasons behind the decline and extinction of indigenous species in many countries,’ said Rajshahi University’s fisheries department assistant professor Shams Galib.
Department of fisheries in their latest yearly fisheries statistical report admitted a gradual reduction in inland freshwater fish capture over the last four decades.
The report said that the share of inland capture fisheries to overall fish production fell to about 28 per cent in 2018-19 from nearly 63 per cent in 1983-84.
The government account showed only half a dozen indigenous fishes were significantly contributing to the total fish production though there are 260 freshwater fishes in the country.
In 2015, the International Union for Conservation of Nature announced that at least 10 native fish species became threatened following a rapid decline in their number in the 15 years since 2000, taking the overall number of threatened fish species to 64.
The IUCN noted alien fish invasion as one of the potential causes of the decline in the number of native fishes but did not elaborate on it.
Baim, chapila, chela, kachki, foli, bele, ayre, chota shingi, telchita, koksa, kala bata, tit punti, ghora mach, mohashol, chital, gajar, tengra, rita, pangas, chaka, bot shingi, ghaura, shol, baghair and khorka are the native fishes IUCN found under threat.
Researches revealed that many of the threatened native fishes contained greater amounts of minerals and vitamins than exotic fishes cultured in the country though their protein contents are almost equal.
A 2015 research published in the Journal of Food Composition and Analysis after analysing nine native and exotic fishes with high fat contents concluded that native ones are better for growth and development.
‘One would have to consume 200 kilograms of silver carp to get the same amount of vitamin A found in a kilogram of Mola fish,’ said Mostafa.
The greatest drawback, however, in growing exotic fishes in loosely regulated aquaculture in a flood-prone country like Bangladesh is their predatory nature.
Banned seven years ago, African magur, scientifically called Clarias gariepinus, is still cultivated in Bangladesh, especially in pockets of floodplains during the dry season. The fish, a voracious predator feeding on everything coming across its path from fishes to livestock, is sold at its infancy to unsuspecting buyers as native magur.
Red piranha, scientifically called Pigocentrus nattereri, an omnivore, is also banned for 13 years but still cultured and sold in the market, even in the capital, as rupchanda.
Tilapia has become a headache to local fisheries in southern and western Bangladesh and is also breeding in Kaptai Lake, a vast open water body in south-eastern Bangladesh.
Grass carp, common carp and silver carp are widely caught in many rivers across Bangladesh almost around the year.
‘A night of fishing could end up catching 100kg of grass carp nowadays,’ said Ripon Kumar Rajbanshi, a fish trader from Mirzapur, Tangail, a central Bangladesh district.
Carps are also widely found in the Padma and northern Bangladesh. Common Carp are bottom feeders constantly poking at river bed turning water turbid. Turbidity reduces sunlight penetration in water and buries fish eggs when matters suspended in water settle.
‘Common Carp also erodes banks of rivers, where many native fishes nest during summer,’ said Dhaka University’s fisheries department professor Monirul Islam.
Grass carp, a voracious weed eater, can reduce aquatic flora to a great extent. The floral loss could lead to drastic fall in dissolved oxygen, making life difficult for native fishes.
Monir said that aquaculture contributed greatly to ensure people’s access to animal protein in a developing country like Bangladesh but the achievement might turn out to be a tragedy in the long run if it comes at the cost of local fishes.
United States Geological Survey said that their government has been trying to control exotic carps for a century but the fishes are still present in almost every state to the detriment of indigenous fishes.
The USGS said that the Asian carp invasion was threatening their $7-billion fishing industry.
The harmful impacts of tilapia are largely unknown. Like other exotic fishes, tilapia can survive with low oxygen and is known for altering aquatic ecology and feeding on small fishes.
In 2000, the IUCN released a list of world’s 100 worst alien invaders including eight fish species and three of them — common tilapia, common carp and mosquito fish — are naturally breeding in Bangladesh.
Mosquito fish was brought for controlling mosquito but have spread from the capital’s sewerage drains to fish farms in Mymensingh. IUCN called it an extremely aggressive predatory fish that feeds on fish eggs and likened its potential negative impacts to that of the Nile perch, the fish that caused the extinction of more than 200 fish species.
Exotic fishes also bring diseases and two of the worst ever fish diseases — Red Spot Disease and White Spot Syndrome — are also attributed to alien fish import.
The decline in the number of taki, shol, magur, punti and indigenous carps are attributed to the spread of red spot disease in the 1980s.
The white spot syndrome has hit hard the shrimp industry.
‘We have made some momentary achievements but it seems we are set to be a loser in the long term, as long as nutrition is concerned,’ warned Mostafa.
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