The biologists working in a fish hatchery near Shasta Dam grew increasingly concerned last year when newly hatched salmon fry began to act strangely — swimming around and around, in tight, corkscrewing motions, before spiraling to their deaths at the bottom of the tanks.

Certain runs of chinook salmon in California are imperiled; the hatcheries and the fry raised there are the federal government’s last-ditch effort to sustain these ecologically and economically vital fish populations.