Photo courtesy of ICF.

FEATURE: Dam Releases Attempt to Boost Delta Smelt Habitat

By Jacoba Charles, Estuary News Group

What is three inches long, nearly transparent, and holds sway over much of northern California’s water politics? The answer, of course, is the diminutive Delta smelt—a secretive, federally threatened fish that was once abundant and now is on the verge of extinction. As a result, much effort goes into finding new ways to bolster the fish’s habitat and population, with mixed results according to new analysis.

Carefully timing bursts of water into the Delta via changes to reservoir releases or to exported water from the South Delta is one method that scientists theorized might temporarily increase some of San Francisco Estuary habitat that has been lost to the fish over the last century.

“The water releases are hypothesized to affect a number of habitat variables that were seen to benefit the smelt,” says Calvin Lee, a researcher with the consulting firm ICF, and lead author of a study examining the efficacy of such releases, recently published in the journal San Francisco Estuary and Watershed Science. The project compared wetter years in which what was officially dubbed “flow augmentation” was conducted, to drier years when it was not.

Map of sampling locations across four years; red lines and values represent the location of the low salinity zone known as X2,which is used as a proxy for Delta smelt habitat.

The results—likely due to the complexity of the hurdles the fish face to survive in the modern estuary—were mixed, showing that the habitat and food sources did expand as predicted, but, without any corresponding increase in smelt population.

“There are other factors that might be interacting to affect them,” Lee says. “What about food availability during the summer? What about temperatures when they hatch, and when they are larvae? These actions address a certain time period in their lifecycle but not any other issues that might pop up earlier.”

In other words, each effort to save the fish is a shot in the dark—and whether (or which) other steps might need to be taken in synchrony with a given effort is unknown. This is yet one more way the deck is stacked against these fish.

The Delta smelt’s needs are both specific and complex. They are semi-anadromous, hatching in the fresh waters of the Delta, then generally migrating downstream into brackish (but not entirely salty) waters. There they live from summer until early winter, maturing and fattening up in the estuaries where their preferred meals of zooplankton were historically abundant,  before returning upstream to spawn in freshwater. While some subpopulations of the fish have varying habitat preferences, remaining in either freshwater or brackish waters for their entire lives, the largest group tends to migrate.

The augmented flows were aimed at boosting the autumn, brackish water feeding habitat of the smelt. By sending a pulse of fresh water down from the reservoir releases, the smelt’s ideal zone of salinity—termed X2 by scientists—was pushed downstream, to encompass the Suisun Marsh and Suisun Delta, which historically were key foraging areas and the location where majority of the fish were found.

Simultaneously, the increased freshwater shifted the type of zooplankton found in these areas toward a species that the smelt prefer to eat. The releases tilted other factors in favor of the smelt as well—such as increased sediment and nutrients. More habitat and more prey—it would seem a win-win, and yet, the smelt did not bounce back.

“The last time [before this study] that there was high water outflow was in 2011, and the population responded quite noticeably,” Lee said, explaining that response is largely why the increased flow in this study was also expected to boost Delta smelt numbers. But as noted, only part of the team’s hoped-for results materialized, as myriad complex, human-induced changes to the Delta have ravaged the smelt populations.

Found only in the San Francisco Bay estuary, these fish have lost dramatic amounts of habitat due to water exports from the watershed to other parts of the state, which has led to a greatly reduced amount of water in the Delta. At the same time, the fish also face challenges such as pollution, predation by and competition with invasive species, decreases in water quality and increases in water clarity, habitat loss due to wetland conversion, and drought.

During the early 1980s, the fish populations plummeted by 80%, leading to their listing as a special status species. Numbers fluctuated through the early 2000s, at which point they again began to drop precipitously. In 2023, the US Fish and Wildlife Enhanced Delta Smelt Monitoring Report shows most sampled sites yielding zero smelt catches; when smelt were found, the numbers were in single digits in all but one location.

Meanwhile, smelt have been reared in since the 1990s as a safeguard—or, at least, a stopgap—against imminent extinction. The hatchery program, run by the UC Davis Fish Conservation and Culture Laboratory, has historically been bolstered by up to 100 wild-caught smelt each year, but in recent years reaching that quota has not been possible.

Beginning in 2022, the system began to work in reverse: the hatchery raised thousands of smelt that were released into the wild as part of a program run by a consortium of partners including the California Department of Fish and Wildlife, the California Department of Water Resources, UC Davis, the USGS, and more.

As with all aspects of Delta smelt recovery, the program continues to evolve: this year, rather than abruptly releasing the hatchery fish from the steel drums in which they were transported, a multi-day “soft release” program was developed to acclimate the fish to their new surroundings.

And what surroundings they are! Whether wild or hatchery-reared, the modern Delta is a challenging environment for these smelt. Boats zoom through the channels, the waters are clearer and warmer, wetlands are fewer and predators are numerous, and the giant pumps of the Central Valley and State Water Projects continue to slurp smelt up. Some of the Delta’s myriad invasive species, such as the overbite clam, that aren’t interested in eating the smelt instead devour their food. And the smelts’ “preferred” food is not even their historic food—the native zooplankton that they evolved to eat has itself been extirpated and replaced by an invasive, Lee said.

So it is in the midst of these challenges that humans continue to investigate what steps might successfully pull the Delta smelt back from the brink.

“There are a lot of things that could be done—but can they be done? Should they be done? What gets us the most bang for the buck? That’s the kind of thing that’s being evaluated,” Lee said.

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