The release of silty water from the bottom of the Bridgeport Reservoir to irrigate alfalfa has killed hundreds of fish in one of the last self-sustaining trout streams in the West, authorities said. The release turned a 9-mile stretch of the once-pristine East Walker River in the eastern Sierra Nevada into a silt-choked brown wash. Some of the trout asphyxiated when silt clogged their gills. The river was declared dead by California Trout, a sportsmen’s conservation group that opposed the water release. Farmers and ranchers said they had a legal right to the water. They said they’re struggling after two straight dry winters and that the water might save this year’s crop of alfalfa, used as livestock feed.