A team of scientists analyzed 14 survey programs carried out by state and federal agencies, as well as UC Davis, and concluded that employing such a diverse variety of long-term surveys is essential for accurately tracking and assessing the overall health of San Francisco Estuary’s ecosystem and its resident fishes. The research is described in the June issue of San Francisco Estuary and Watershed Science. Lead author Dylan Stompe, of UC Davis, explains that the research arose from concerns that too many people – reporters included – have relied on the results of one annual survey, the California Department of Fish and Wildlife’s Fall Midwater Trawl, to make sweeping conclusions about the health and abundance of several fish species. The trawl, which samples 122 sites in San Pablo Bay and the Delta, has captured fewer and fewer individuals of several species almost every year. This has raised legitimate concern that these fishes, which include Delta smelt and longfin smelt, are nearing extinction. These species have undoubtedly declined dramatically, Stompe says, but when data from multiple surveys are analyzed together, “the story gets more complicated. We still see a long-term decline [in several fish species] but it’s much more muted,” he says. Stompe poses an explanation: The Fall Midwater Trawl uses geographically fixed sampling sites in a sediment-based ecosystem that is shifting constantly. Over time, this could produce data trends that reflect habitat changes, not necessarily trends in fish populations. Stompe and his coauthors concluded that overlaying the results of multiple surveying efforts produces more accurate summaries of significant population trends and ecological changes. Unfortunately, this doesn’t change the prognosis much for the endangered Delta smelt: “They’re definitely in bad shape,” Stompe says. “All the surveys are now having trouble catching them.”

Pearls in the ocean of information that our reporters didn’t want you to miss
Trawl survey in the North Delta. Photo courtesy of Dylan Stompe
 

By looking solely at the results of a single annual fish-counting survey, Californians may be seeing an incomplete reflection of Bay-Delta fish population trends.

A team of scientists analyzed 14 survey programs carried out by state and federal agencies, as well as UC Davis, and concluded that employing such a diverse variety of long-term surveys is essential for accurately tracking and assessing the overall health of San Francisco Estuary’s ecosystem and its resident fishes. The research is described in the June issue of San Francisco Estuary and Watershed Science. Lead author Dylan Stompe, of UC Davis, explains that the research arose from concerns that too many people – reporters included – have relied on the results of one annual survey, the California Department of Fish and Wildlife’s Fall Midwater Trawl, to make sweeping conclusions about the health and abundance of several fish species. The trawl, which samples 122 sites in San Pablo Bay and the Delta, has captured fewer and fewer individuals of several species almost every year. This has raised legitimate concern that these fishes, which include Delta smelt and longfin smelt, are nearing extinction. These species have undoubtedly declined dramatically, Stompe says, but when data from multiple surveys are analyzed together, “the story gets more complicated. We still see a long-term decline [in several fish species] but it’s much more muted,” he says. Stompe poses an explanation: The Fall Midwater Trawl uses geographically fixed sampling sites in a sediment-based ecosystem that is shifting constantly. Over time, this could produce data trends that reflect habitat changes, not necessarily trends in fish populations. Stompe and his coauthors concluded that overlaying the results of multiple surveying efforts produces more accurate summaries of significant population trends and ecological changes. Unfortunately, this doesn’t change the prognosis much for the endangered Delta smelt: “They’re definitely in bad shape,” Stompe says. “All the surveys are now having trouble catching them.”

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.

Related Posts

A large pulse of water sent through the Yolo Bypass in summer 2016 boosted phytoplankton biomass and food web conditions in Cache Slough and the lower Sacramento River, a new report confirms.

 “Our goal was to improve estuarine habitat by increasing net flows through the Cache Slough Complex to enhance downstream transport of lower trophic-level resources, an important driver for fish such as the endangered Delta Smelt,” write the authors—including esteemed (and recently retired) California Department of Water Resources lead scientist Ted...

New research indicates that survival of juvenile Chinook salmon in the Sacramento River system can be significantly boosted by achieving key thresholds for river flow.

The findings, published in the journal Ecosphere, add important context to the general scientific understanding that more water in the river improves fish survival. Previous studies, the authors explain in their paper, have demonstrated that more juvenile salmon migrating toward the sea complete their journey when the Sacramento River system...

Electrofishing is a powerful but underutilized tool for monitoring Delta fish, particularly species favoring “structured” habitats that are difficult to sample using more common methods like trawls and seines.

By analyzing fish catch data from past surveys, researchers Ryan McKenzie, of the US Fish and Wildlife Service, and Brian Mahardja, of the US Bureau of Reclamation, determined that electrofishing resulted in better detection rates for many native and non-native species than net-based surveys. Although electrofishing is currently restricted to...