A luminous little fish that smells like cucumbers and has the power to slow the flow of water to thirsty California cities and farms is swimming closer to extinction.

Only six delta smelt—the lowest number ever found—were netted in a survey by state biologists last month. In previous years, as many as several hundred had been caught in spring surveys.

The population of the three-to-four-inch (eight to ten centimeters) silvery blue fish, which lives only in the San Francisco Bay and the Sacramento-San Joaquin Delta, has been falling for decades as huge volumes of freshwater are diverted through hundreds of miles of aqueducts and canals.

The new data, gathered in the fourth year of a record-breaking, severe drought, alarmed Peter Moyle, a University of California, Davis biologist, enough to warn state officials that the delta smelt is likely to soon be extinct in the wild.

Delta smelt nears extinction The survival of a tiny fish, the delta smelt, is threatened by a lack of freshwater flows to its habitat. With California in its fourth year of drought, the fish species' chance of survival looks grim. Historic Range The delta smelt, which is endemic only to California, spawns in the Sacramento-San Joaquin Delta and feeds in the San Francisco Bay. Delta smelt's historic range Sacramento-San Joaquin Delta AREA ENLARGED CALIF. Davis Sacramento Napa Vallejo San Pablo Bay Martinez Oakley Berkeley San Francisco Tracy San Francisco Bay 10 mi 10 km Effects of the drought Scientists predict that the smelt could go extinct in the near future, mainly due to drought and water diversions. Non-native aquatic species and pollutants are also playing a role in its decline. Water transfers Seventy percent of California's water originates in the northern part of the state, but the south accounts for over 70 percent of demand. In 2014, diversions, storage, and drought decreased freshwater flows to the San Francisco Bay by 50 percent. Major aqueduct or canal Major reservoir, sized by normal capacity Agricultural area Sacramento San Francisco CALIFORNIA Los Angeles San Diego 100 mi 100 km Northern Sierra/Trinity Central Sierra Southern Sierra Declining Snowpack The snowpack in California has reached critically low levels. The April 1, 2015, measurement found only five percent of the average amount for that date. This translates to an average snow-water equivalent of about 1.5 inches (3.8 cm). Statewide, the same measurement last year was 9.4 inches (23.8 cm). Ten years ago, in 2005, it was 39 inches (99 cm). Percent of April 1 normal snowpack in the Sierra Nevada Above average Below average Northern Sierra/Trinity 2005 124% 2014 24% 2015 5% Central Sierra 2005 135% 2014 40% 2015 5% Southern Sierra 2005 158% 2014 31% 2015 5% Delta smelt survey Only six smelts were counted during this year's March survey, a new low in 13 years of recordkeeping. In March 2014, 88 fish were counted. January February March LAUREN TIERNEY, KELSEY NOWAKOWSKI, CHIQUI ESTEBAN, AND GINNY MASON, NG STAFFSOURCES: California Department of Fish and Wildlife; San Francisco Estuary Institute; J. E. Merz, et al. California Fish and Game; CALIFORNIA DEPARTMENT OF WATER RESOURCES; BUREAU OF RECLAMATION

"The probability of the delta smelt surviving in the next three years is relatively low. The chances of its bouncing back from where it is today seem very unlikely," said Moyle, who has studied the fish for several decades. "There are a lot of things going on simultaneously. Everything that stresses that poor fish is out there. The drought is the final straw."

The Fish That Everyone Loves or Hates

Protected by the Endangered Species Act, the delta smelt is a powerful and divisive symbol of the troubles endured by the whole bay-delta ecosystem that is spread across northern and central California. Because smelt live only one year, they immediately reflect the ecological problems created by a massive system of dams, reservoirs, and aqueducts that export water south.

A century and a half ago, water from two great rivers—fed by Sierra Nevada snowmelt and carving out California's expansive Central Valley—flowed to a vibrant delta and bay, and then ran to the ocean. The flows of the Sacramento and San Joaquin Rivers kept saltwater from intruding into the fresher delta, and the smelt spawned in the delta and fed in the bay. The water also replenished Chinook salmon spawning grounds and freshened bay habitat for Dungeness crab, waterfowl, and thousands of other plants and animals in the vast network of sloughs, wetlands, and mudflats.

View Images Water in the Sacramento-San Joaquin Delta, fed by snowmelt from the Sierra Nevada, has been diverted to California's farms and cities for decades. Photograph by Robert Burrell, Reuters

But now most of that water is exported: 70 percent of California's water originates in the north, while more than 70 percent of the demand for it lies in the south. The flow quenches the thirst of two-thirds of the state's residents and three million acres of irrigated crops such as vegetables, citrus, grapes, and grains.

On Wednesday, Governor Jerry Brown ordered the first statewide mandatory restrictions on water usage in California history, aimed at a 25 percent cut. But that is unlikely to have any effect on the delta smelt's fate.

The fish used to be so plentiful that it was caught and sold commercially. Over the course of about three decades, "the delta smelt went from being the most common fish to one of the most rare," said Marty Gingras, a California Fish and Wildlife Department biologist who manages year-round smelt surveys.

In particular, the last two years of California's severe drought have contributed to the smelt's steep decline.

"It's a combination of Mother Nature, poor planning, and mismanaging of the water supply. Now everybody's hurting," said fish biologist Tina Swanson of the Natural Resources Defense Council.

Even in wetter periods, water managers for the last 15 years have created drought conditions in the bay by exporting the rivers' freshwater, Swanson said.

"It's not just the delta smelt. Nearly every single native and non-native fish species is showing the same pattern. To me, that is an excruciatingly clear indication that our management of the environment in which they live—the delta and upper bay—is insufficiently protective," she said.

View Images In a March survey, only six delta smelt were caught by state biologists, compared with as many as several hundred in previous years. The fish used to be so plentiful that they were caught and sold commercially. Photograph by René Reyes

Fish or Folly?

Water providers and farm groups have long argued that depriving growers and urban water users for the sake of the delta smelt is folly.

Jason Peltier, deputy general manager of the Central Valley's Westlands Water District, the nation's largest irrigation district, called the latest survey results "further proof that redirecting water from human use to environmental use in the name of helping the fish is not working." Half of that district's 600,000 acres of cropland went fallow last year when none of the water allocated under long-term contracts was delivered, he said.

The smelt's status as a federal and state threatened species gives it the clout to force officials to reduce deliveries of water when the pumps harm it or when its habitat is too salty.

Federal and state laws require minimum flows to protect fish and the environment, but the state can declare emergency waivers and continue to export water—which it did last year.

Water contractors, agricultural irrigation districts, and the Metropolitan Water District of Southern California, which serves Los Angeles, San Diego, and other cities, sued federal agencies in 2009, arguing that pumping restrictions to aid the smelt were "arbitrary and capricious." Last year, the Court of Appeals for the Ninth Circuit ruled that the water projects jeopardize the fish and its habitat.

"The issue in the delta has always been balancing the amount of freshwater outflow through the delta versus exports and upstream water diversions. Depending on your perspective, it's either wasted water or it's a beneficial use to the environment," said Carl Wilcox, a biologist with the state Fish and Wildlife Department.

Four Females and Two Males

The remaining delta smelt—no one knows the actual population—are scattered in such small numbers that scientists fear that the fish can no longer reproduce.

In March, the number counted fell to a record low of six: four females and two males. When the netting of spawning adults began, in 2002, biologists found 238 in March of that year. After a wet 2011, 295 smelt were netted in March 2012. The March numbers dropped to 65 and 88 in the dry years of 2013 and 2014.

Another state survey, which goes back nearly half a century and counts adults before they spawn, found nine smelt last year compared with 1,673 at their peak in 1970.

NRDC's Swanson said one of the best solutions to save the smelt is to better manage water for continuous dry years.

"Part of why we're in such desperate trouble is because in 2012 we acted as though it wasn't a dry year. Large volumes were delivered, and they drained the reservoirs. We can't plan as if next year is going to be wet. We have to plan as if the next year is going to be dry," Swanson said.

The fish also suffers from encroaching salinity, predators invading its territory, and discharges from agriculture and sewage treatment plants.

Moyle doesn't want to see the delta smelt go the way of spring-run salmon in the San Joaquin River. The once abundant commercial salmon fishery was wiped out when a dam blocked the river's flow.

The smelt "is a beautiful fish," he said. "It's the will of the American people that we can't let any species go extinct. I believe we have a moral obligation to keep the delta smelt around."