RIVERTON (AP) - Hundreds of thousands of fish diverted into irrigation canals each year die because they are not able to return to streams and reservoirs before the canals dry up, a study on the Wind River Indian Reservation found.

"Irrigators, sportsmen and wildlife managers have always known that fish are lost in irrigation canals each year, but we were amazed by the number of fish we found in every canal we sampled," said Dave Skates of the U.S. Fish and Wildlife Service, which participated in the research.

Brendan McGinn, a University of Colorado graduate student, led the study of eight canals on the reservation during the 2002 and 2003 irrigation seasons.

The study revealed that more than 500,000 fish were diverted from reservoirs and rivers into the canals and ditches during the period and were unable to make it back before the water was cut off to irrigation waterways in the fall.

The fish suffocated as the canals and ditches slowly dried up, he said.

Ninety-four percent of the fish lost were native species including shiners, suckers and chubs, and more than 20,000 were trout species, according to McGinn.

Fish entrapment in canals can be limited by installing screens at canal head gates, McGinn said. Some are being widely used in the Northwest.

"Fish screens are much less costly when designed into a new or rebuilt irrigation diversion structure," he said. "We need to be proactive and start thinking about installing fish screens into all new structures now, rather than waiting until a local fish species is listed (as endangered) and then be forced to do it anyway."