River dolphins of the Indus River in Pakistan are in decline, and environmental experts believe the causes of the population losses are dams and irrigation.

Many forms of freshwater marine life have experienced population declines due to the loss of habitat. This study examined historical records to determine when the population of the animals began to decline.

"The historical range of the Indus dolphin has been fragmented into 17 river sections by diversion dams," researchers wrote in a paper announcing the results of their study.

Of those 17 regions of the river where the dolphin once lived, 10 are currently without any members of the species. The dolphins still exist in six of the areas, and results from the last area were inconclusive.

The Indus River Dolphin is an endangered species. River dolphins originally lived in the oceans, but were forced into inland waters by competition from other dolphin species. As generations of dolphins were born, they evolved into animals that could easily live in fresh water. They tend to have longer snouts than their ocean-dwelling cousins, but cannot see as well. This could be caused by the dark, murky conditions of the rivers where many of the species live. The Yangtze River Dolphin, which once lived in China, has not been spotted for several years. Many biologists believe the species went extinct almost a decade ago.

Researchers examined seven possible causes of the loss of river dolphin populations. These included dams and dry season river discharge. They looked at the location of the dolphins in the river, compared to banks.

Dry-season river discharge, due to irrigation at diversion dams, was found to be the primary reason for the decline of dolphin population in the river.

The Indus River has been modified more than almost any waterway in the world.

"This important study shows that it is river habitat fragmentation by dams, and removal of river water for irrigation that has caused the massive range decline of the Indus River freshwater dolphin. This increased understanding of species decline in fragmented river systems is especially important because hundreds of new dams and water developments are planned or are under construction in many of the world's rivers and large losses of aquatic biodiversity can be expected," Gill Braulik, of the University of St. Andrews, said.

Study of how dams and the construction of irrigation systems affected dolphins could be used to predict how other marine animals could react to similar projects in the future.

Investigation of the decline of dolphin population was detailed in the online journal Plos One.