SACRAMENTO, Calif. — The fragile little fish that swim around in dark tanks in a lab are the last hope for their kind. The lab director who oversees their care, Tien-Chieh Hung, explained that when the Delta smelt captives are young, scientists at the UC Davis Fish Conservation and Culture Laboratory put snails in the black interiors of the tanks as a slow-moving mollusk cleaning crew. The snails must do the work because the fish are so delicate, they could get tangled in a human cleaner’s arm hair and perish. The whole project of painstakingly raising these temperamental endangered creatures,...