BILLINGS, Mont. - Wildlife advocates plan to challenge the approval of a new Yellowstone River dam to benefit farmers that critics say could kill off a dwindling population of a fish dating to the time of dinosaurs.

A bypass channel for the irrigation dam would be built near the Montana-North Dakota border to let endangered pallid sturgeon reach upstream spawning grounds.

But scientists don’t know if the fish would use the channel.

Get Breaking News Delivered to Your Inbox

Defenders of Wildlife and the Natural Resources Defense Council said in a court filing Friday they are negotiating with officials on how to proceed following last month’s dam construction approval by the U.S. Army Corps of Engineers and Bureau of Reclamation.

The groups have a lawsuit pending over the $57 million project.

U.S. District Judge Brian Morris temporarily blocked it in 2015.