THATTA, Sept 23: Climate change, pollution, reckless fishing as well as a scarcity of water in lower estuarine area have pushed the giant Indus prawn, as well as many other species, to extinction, according to a recent study carried out by the department of freshwater biology and fisheries of the University of Sindh.
Construction of dams and barrages on the river had also played a major role in the disappearance of migratory species in the Indus in particular and the rivers worldwide in general, said Dr Anila Naz Soomro and Prof Dr Wazir Ali Baloch who conducted the study.
The giant river prawn, scientific name Macrobrachium rosenbergii, is the largest known palaemonid in the world. Palaemonidae is a family of shrimp in the order of Decapoda. They are mainly carnivorous inhabiting all aquatic habitats except the deep sea.
The giant freshwater prawn is native to the Indo-west pacific region. However, it was introduced to other continents as aquaculture — the rearing of aquatic animals or the cultivation of aquatic plants for food — and gained popularity as a prime culture candidate due to its distinctive taste, fast growth rate, large market size and high resistance to diseases coupled with increasing demand in both domestic and international markets, says the study.
Today, the prawn is reported in over 77 countries and contributes heavily to global aquaculture both in terms of quantity and value. Moreover, its commercialisation for aquaculture in Asia has jumped rapidly with current production estimated at 205,000 tonnes valued at close to $1 million compared to about 18,500 tonnes in the 1990s.
The species is the most studied especially with regard to fisheries biology, aquaculture, taxonomy, morphology, development, anatomy, physiology, biochemistry, ecology and social behaviour.
It exhibits diadromy (migration of fish in either direction, from fresh to sea water or the reverse) with a river-estuary-river migration for breeding and especially for larval hatching in the estuarine area.
It was reported from Chittagong when Bengal was East Pakistan while its presence in Pakistan has so far remained a mystery for many researchers with no reported specimens from the Indus since 1980s, says the study.
In the past, no specimen of the giant prawn was found during surveys carried out by Prof Dr Baloch in the lower Indus although some fishermen had reported of occasionally catching “rare blue prawn” which was suspected to be Macrobrachium rosenbergii.
During surveys of Thatta fish market, the central collection point for most of the catch from the lower Indus, a team led by Prof Dr Soomro and Dr Baloch found a single specimen on July 1 this year, which was a blue clawed male measuring 32 cm and weighing 275 grams. It is the maximum reported length of the species.
The specimen was confirmed as the giant freshwater prawn, Macrobrachium rosenbergii, by Dr Soomro based on several taxonomic keys. This rare occurrence of the species is a clear indication that it is already endangered in the Indus, says the study.
The near extinct status of this species is attributable to environmental and habitat degradation, especially in view of drying out of the lower estuarine areas (the tidal mouth of a large river where the tide meets the stream) which are important for diadromous species.
The study points out that dams and barrages on the river have significantly reduced water levels downstream and lower estuarine areas, endangering many diadromous species.
It calls for declaring the giant prawn as threatened in order to conserve the species in the Indus and advises extensive research on the impact of habitat selection for the prawn in the face of drying up of lower reaches of the river.