Karachi  - The population of Palla fish in Sindh has declined to a significant extent. This phenomenon is attributed to the reduced flow of Indus River, resulting in the restricted upstream migration of Palla fish for breeding. The physical barriers in the form of barrages and dams have blocked the migratory passage. Though fish ladders (structures on or around artificial barriers such as dams or barrages) can help fish migrate but they are very rarely maintained along the Indus River and hence the stocks of Palla have been depleted at an alarming pace during the last 15 years, a report by WWF says. "Once claimed as comprising up to 70 percent of fish catch it now hardly constitutes 15 percent of the total catch fish in the area," report says. In Indus, Palla is found from the last week of January up to the middle of October, peak period being May to June. In the past, Palla fish has been reported to migrate as far as Multan (Punjab) before the construction of barrages on the Indus River, but, nowadays it only reaches up to Kotri Barrage (Ghulam Muhammad Barrage). The construction of various barrages on the Indus River has blocked the passage of the fish to its breeding grounds. Interestingly, even though the numbers in catch have declined dramatically (90%), catch data from Food and Agriculture Organization reports little change in the last 25-30 years and it appears that the damage to fish migration was done some time ago. However, very little is known about the population of Palla in Pakistan unlike Bangladesh and India, where a lot of research has been carried out but still scientists have very little information about the population and breeding ground. For example, some of the individuals of Palla in the Ganges are resident throughout the year and never reach the sea. Calls for fish ladders to be constructed to allow the fish to regain its swapping grounds upstream have largely gone unnoticed in Pakistan.  The construction of barrages on Indus River and decline in Indus water flow in Deltaic region has resulted in increased depletion of this otherwise prized fish species, study says.