She said data from an acoustic monitoring device installed upstream from the rocks suggest about 700 fish, mostly chinook and some sockeye, have passed through.

A second acoustic device is expected to be installed on Thursday while other options are being explored to save the fish, including the trucking option, Antcliffe told a conference call.

"What we don't know is how many fish we would expect to migrate through at this time of year,'' she said.

"The water is very turbid and you cannot see the fish in the water, and until further acoustic monitoring devices are on the downstream side, it will be difficult to tell.'

Technical staff and engineers are monitoring the area by helicopter because it's unsafe for crews to do any work in the remote area, Antcliffe said.

An incident command post has been set up in Lillooet, with representatives from First Nations and the federal and provincial governments.