These fish can get killed directly by the water pumps, Rypel said, if they get sucked in or stuck on the intake filter. More importantly, decades of pumping and diverting water have changed the levels, temperature, and salinity of the water, dramatically altering the habitat to which smelt have adapted.

“We are putting the ecosystem in a perpetual state of drought through water extraction, and increasingly also with climate change,” said Rypel. “The droughts are longer and more sustained. The smelt just can’t handle that.”

Their populations have mostly perished.

In the ’90s, the delta smelt was listed as threatened under the federal Endangered Species Act and under state law. In 2009, California upgraded the fish’s status to endangered, and their numbers appear to have only dwindled since then. While these fish can be difficult to detect, wildlife surveys in the estuary have failed to turn up a single delta smelt in recent years. That’s why experts like Rypel describe them as “functionally extinct” — meaning they’re so rare they effectively provide no function in their ecosystem, such as feeding predators. Rypel guesses there are less than 100 of them left in the wild. (As a last-ditch effort to save the species, UC Davis breeds the fish in captivity on land.)

Why Trump cares — or rather, doesn’t — about the delta smelt

In an effort to stave off extinction, the delta smelt is protected under both state and federal law. Those protections limit to a small extent when and how much water........

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