STANLEY, Idaho (AP) - More than 500 endangered sockeye salmon have arrived at a central Idaho fish hatchery, the most in more than two decades.

The arrival of the sockeye, listed as endangered under federal law in 1991, has started to slow in recent weeks, but state fish biologists said 507 arrived at fish traps near the Sawtooth Fish Hatchery near Stanley as of this week.

To reach central Idaho, the sockeye travel about 900 river miles, gain 6,500 feet in elevation and pass eight dams on the Columbia and Snake rivers.

Biologists said the returning fish are from 180,000 smolts released in the valley's lakes in 2006 for the journey down the Salmon, Snake and Columbia rivers to the Pacific Ocean. Some of the returning fish were artificially spawned at the Eagle Fish Hatchery in southwestern Idaho as part of a program to help boost the sockeye returns.

As many as 35,000 sockeye once returned naturally to spawn in Redfish, Pettit, Alturas and other lakes around the Sawtooth Mountains, but the numbers have dwindled severely, a trend groups including Idaho Rivers United blame mostly on four dams along the lower Snake River in Eastern Washington.

Between 1991 and 1998, only 16 wild sockeye returned to central Idaho.

After 257 sockeye returned in 2000, the numbers fell to single digits in the last five years.