Distribution of Delta Smelt in San Francisco Bay and Estuary (Source: CA DFG, Dec 2008)
The Case of the Shrinking Smelt:
Diminutive fish tells a story of survival…but for how much longer?
Delta smelt are fascinating. Not just because they have been listed as threatened for fifteen years, or because they are found only in the San Francisco Estuary, or because their status prompted a federal judge to reduce the amount of water sent south by the state and federal water projects. Delta smelt are fascinating because they have survived into this century, despite major changes in their habitat and food supply. They have managed to persist despite being picked off by introduced predators, such as striped bass and the hungry pumps that can siphon fish into their waterworks from miles away. In spite of all the introduced threats, the elusive smelt can still be found in the Delta's open water, seeking the salty margins between estuary tides and fresh rivers of Sierra snow melt. In the few semi-salty regions that remain, a few juveniles are beating the odds for the 150-200 days they need to mature.
Most Delta smelt live about a year, which provides them with advantages and disadvantages for survival. Each new generation presents another opportunity for the smelt to adapt a bit more to its changing environment. On the other hand, a short life means that an abbreviated spawning seasons or peaks in mortality can have a devastating effect on the ability of spawning adults to rebound in subsequent years. The few smelt that survive two years may be part of an evolutionary insurance policy to keep the species viable.
Delta Smelt, Hypomesus transpacificus (Source: CA DFG, Dec 2008)
Bill Bennett, Ph.D., researcher at the Center for Watershed Sciences, John Muir Institute of the Environment, has conducted studies that have increased our understanding of what makes smelt populations tick. Bennett came to UC Davis from Boston to complete his doctorate with renowned fish expert Peter Moyle. He investigated the effects of food supply and pesticides on striped bass, another open-water (pelagic) species. It differs from smelt in that it grows much larger, has a lifespan of twenty years or more, and survives in a wider range of habitats, from the ocean to fresh water. Compared to bass, smelt have more specific temperature and salinity requirements and a smaller habitat volume. They live mainly in the Delta and Suisun Bay and let the river and tidal currents carry them to suitable places to feed and grow. While their more frequent, short-lived generations put them at greater risk of population crashes than striped bass, this aspect of their life cycle also presents opportunities to collect data on how smelt populations respond to changing conditions in the estuary, and life-time population dynamics. A crucial process is the recruitment of juvenile fish into the adult population of spawners.
"In the dynamic ecosystems of the Delta, recruitment during drought conditions presents different challenges than during flood conditions, and each year brings a new scenario in which to observe the smelt adapting to the extensively managed Delta conditions. Since 1999, the most consistent finding is that the smelt decline a bit more each year, in drought years and in flood years," explained Bennett. "I want to explain why this happens, so future water resource policies can prevent fish extinction while continuing to supply water. If we can keep the smelt healthy, we can apply the lessons learned to keep striped bass and salmon healthy as well."
As fish screens narrow, small fish are forced into the state Skinner Fish Salvage facility. Large fish congregate in the Clifton Court Forebay where predation is high. (Photo courtesy of Suanne Klahorst, UC Davis)
When Bennett began his research the early 1990s, the San Francisco Estuary was already in serious trouble. Drought conditions reduced the primary productivity--the growth of plankton that feeds the entire food web. Water exports in drought years reduced fresh water flows to the San Francisco Bay, decreasing nutrients carried by these flows. Lower flows of fresh runoff failed to "flush" pollutants discharged from cities and farms out of the Delta waters. Major declines in phytoplankton (including algae) were attributed to grazing by the overbite clam, first reported in the Delta in 1986. The invasive Asian freshwater clam entered the Delta's inland waters about the same time. The clams, which carpet the bottom in many places, filter plankton from the water, reducing the pelagic food supply. New species of zooplankton, tiny animals called copepods, predominated in Suisun Bay, replacing the established copepods that served as prey for young fish during summers in the Suisun Marsh. Some of the new species of copepods are not easy prey, nor are they thought to be nutritionally equivalent to the smelt's former diet. These changes and the invasion of Brazilian waterweed, all contributed to the long-term decline of three of the Delta's key pelagic fish: delta smelt, longfin smelt and striped bass. The thick mats of waterweed reduce the open water that smelt require while changing water quality in ways that don't favor the smelt.
The role of the export pumps during low water conditions has been a concern for decades. Drifting larval fish are not protected from pumps by the fish screens and are exported with the water. Larger fish are trapped before they reach the pumps. They are counted and transported by tanker truck to safer regions in the San Francisco Bay Estuary. But it is unclear how many smelt survive the journey--Bennett suspects that few do.
Otolith, ear bone, of a fish.
While scientists were grappling with the ecological mysteries of the transitory food web, salinity variations, increases in waterborne toxins, loss of habitat, and fish salvage stress, Bennett was systematically collecting smelt samples to freeze. After publishing on starvation in striped bass, Bennett and his collaborator Swee Teh, School of Veterinary Medicine, began to apply their methods to Delta smelt. They developed precise methods for using structures from fish ears called "otoliths" which literally translates to ear-stones. Smelt accumulate layers of calcium carbonate on their otoliths that resemble tree rings, one for each day of life. The otoliths provide a daily history of how the fish was growing and where it was living in the estuary.
Bennett suspected that the smelt decline had more complicated reasons than just losses from pumping. Like his colleagues, he didn't believe that the decline of the once abundant Delta smelt was due to a single cause. He compares his investigation of Delta smelt to the work of a forensic scientist studying a crime scene, gaining clues and insights into how smelt adapt to change and whether trends in water management will help or hasten their disappearance.
In 2006, Bennett presented a new theory to colleagues in the California's Department of Water Resources Interagency Ecological Program. His hypothesis incorporated a research concept called the "Big Momma theory," a term coined by Kenny Rose, Louisiana State University. As the name implies, the theory proposes that large females are the most fit, with more eggs of better quality. These females are first to spawn, and may even spawn twice in a season. Bennett notes that the pumps pulled higher flows in the winter and pumped less in the spring, ironically to protect the smelt during spawning. Because of this diversion regime, he speculated that the smelt's big mommas were "fished out" early by the pumps. The pumps had become agents of evolution, selecting for smaller females that spawn later. This unintended selection mechanism could account for two observed changes in the survivors, a later spawning season and a 20 percent reduction in average length. Bennett's data from smelt sampled a decade ago support the theory that larvae from large females are more robust.
His data was taken from a fish population that might be changing as fast as scientists can study it. Bennett is passionate about using knowledge gained from Delta smelt to improve the management of all threatened fish. He is encouraged by the increase in funding for science-based evidence, "We are closer than we have ever been to understanding the Delta's ecosystem. I think we are ready to answer some very hard questions. Without a better understanding of the outcomes of past management decisions, we can't make sound investments in the future." In 2006, he published the definitive paper on Delta smelt life history in the journal San Francisco Estuary and Watershed Science, in which he first explored his ideas of what regulates smelt populations. He plans to publish his new findings next year while continuing his search for clues in the case of all the declining pelagic fish.