In a new study published in the March 2021 issue of San Francisco Estuary and Watershed Science, researchers Lenny Grimaldo, William Smith, and Matthew Nobriga used advanced statistical approaches to understand what factors best predict Delta smelt entrainment at the pumps. The paper builds upon research that Grimaldo conducted in the 2000s, which provided the basis for regulations established in the 2008 Delta smelt Biological Opinion. “This study reinforces previous work that adult Delta smelt salvage is largely explained by hydrodynamics, water clarity (turbidity), precipitation, and sub-adult abundance,” the authors write. Historically, when large numbers of smelt began to appear in the salvage screens of the Delta pumps, pumping would be curtailed. Today, Grimaldo says, the fish are so rare that it is almost impossible to rely on salvaged fish as indicators of total entrainment losses. “But we found that turbidity can be used as an important factor to help manage real-time entrainment losses,” he says. The theory behind this, he explains, is that when the water becomes turbid, the fish—which prefer murky water to clear—migrate upstream and into the pumps. The new findings will help resource agencies better manage Delta exports to help protect the rapidly vanishing species.

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During the 2015 drought, DWR scientists tracked the distribution of turbid water in the Delta to protect endangered fish and water supply. Photo: Florence Low, DWR
 

Water turbidity in the Sacramento-San Joaquin Delta can be used as a reliable indicator of smelt entrainment rates in the fish screens of the export pumps at the southern edge of the Estuary.

In a new study published in the March 2021 issue of San Francisco Estuary and Watershed Science, researchers Lenny Grimaldo, William Smith, and Matthew Nobriga used advanced statistical approaches to understand what factors best predict Delta smelt entrainment at the pumps. The paper builds upon research that Grimaldo conducted in the 2000s, which provided the basis for regulations established in the 2008 Delta smelt Biological Opinion. “This study reinforces previous work that adult Delta smelt salvage is largely explained by hydrodynamics, water clarity (turbidity), precipitation, and sub-adult abundance,” the authors write. Historically, when large numbers of smelt began to appear in the salvage screens of the Delta pumps, pumping would be curtailed. Today, Grimaldo says, the fish are so rare that it is almost impossible to rely on salvaged fish as indicators of total entrainment losses. “But we found that turbidity can be used as an important factor to help manage real-time entrainment losses,” he says. The theory behind this, he explains, is that when the water becomes turbid, the fish—which prefer murky water to clear—migrate upstream and into the pumps. The new findings will help resource agencies better manage Delta exports to help protect the rapidly vanishing species.

About the author

A native to San Francisco, Alastair Bland is a freelance journalist who writes about water policy in California, rivers and salmon, marine conservation and climate change. His work has appeared at NPR.org, Smithsonian.com, Yale Environment 360 and News Deeply, among many other outlets. When he isn't writing, Alastair is likely riding his bicycle uphill as fast as he can.

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