British-based Fishtek Consulting has been hired to evaluate whether species indigenous to the river are able to use ladders that bypass the Xayaburi hydroelectric dam, the first to be built on the lower Mekong, said the Vientiane Times.
"Fish swimming ability tests began in May this year and are ongoing," it said.
The $3.5-billion dam has faced strong opposition from environmentalists and neighbouring countries concerned about its potential impact on fisheries.
To address these concerns, the majority Thai-owned contractor Xayaburi Power Company included fish ladders and elevators as part of the dam's construction design, but such devices have never been tested on species indigenous to the 4,880-kilometre Mekong.
Fishtek has been testing whether the fish can swim against a current travelling 1.2m per second, the Vientiane Times said.
"At this speed most of the fish species require several attempts to enter the flume," it said.
Construction was started in November 2012 and is now one-fourth complete, despite concerns about the dam's impact on fisheries, an important source of protein and income for millions of people living in the Mekong basin.