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.”