Day

November 18, 2020
18
Nov

As organizations and agencies scramble to preserve the Central Valley’s dwindling Chinook salmon runs, a group of scientists believes they may be overlooking a key factor in the decades-long decline of the fish: disease.

In a paper published in September’s issue of San Francisco Estuary and Watershed Science, a research team proposes that diseases—caused by viruses, bacteria and other microbes—could be suppressing juvenile salmon survival in a river system that once hosted millions of adult spawners each year. According to tracking studies, nearly all juvenile Chinook born from natural spawning die before they reach the Golden Gate Bridge; habitat enhancement efforts have failed to mitigate this mortality rate. Short-term studies of Central Valley salmon have indicated high rates of infectious diseases, which lead author Brendan Lehman of UC Santa Cruz says demonstrates the need for ongoing systemwide monitoring. This could involve sampling fish directly for infections or conducting DNA testing of water samples. A...
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18
Nov

Current estimates of young salmon lost to the south Delta pumps are based on a smattering of studies from the 1970s and should be updated, according to a new analysis.

“They don’t represent current operations,” says Ukiah-based consultant Andrew Jahn, lead author of the analysis reported in the September 2020 issue of San Francisco Estuary and Watershed Science. Current operations at the State Water Project (SWP) and the Central Valley Project (CVP) can reverse flows in the Old and Middle rivers, diverting salmon on their way to the ocean towards the projects. Existing salmon loss estimates also fail to account for a likely Old River hotspot for predators, drawn to the influx of salmon and other fish near the two water projects. Evidence of this hotspot lies in the numerous acoustic tags from young salmon to be found in the sediment, which were presumably defecated by striped bass, as well as...
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18
Nov

Fish passage structures can be improved for the benefit of multiple species, if they are designed to account for differences in behavior, physical ability and size, according to a new literature review.

Historically, most fish passages have been designed to help native salmon return to their upstream habitat and spawning grounds, with little consideration for other migrating species such as sturgeon and lampreys. “There is an assumption that if you just build a fish passage structure, fish will go thorough it, but that is not always the case,” says Department of Water Resources fisheries biologist Zoltan Matica, who conducted the review. “The challenge is to understand that this isn’t only a physical barrier, it can be also a mental barrier.” For example, some species that engage in schooling behavior, such as shad, may refuse to even enter a structure if it limits them to passing one at a time. According to the...
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18
Nov

Spawning and rearing habitat for important forage-fish species in San Francisco Bay apparently shifts geographically by many miles depending on how much freshwater is flowing into the Estuary.

In a recent study, a team of scientists found that in a dry year, Pacific herring and longfin smelt larvae occurred farther up the Estuary than in a wet year, when spawning and recruitment was pushed seaward. The results suggest that the fish have broader geographic ranges than previously believed, a finding that could inform efforts to manage and protect their habitat. Biologists have long assumed that longfin smelt, a protected species in steep decline for decades, spawn strictly in upper reaches of the Estuary. But by towing a weighted hoop net behind a research vessel during the dry year of 2016 and again in the following year of heavy rainfall, the authors of the new study, published in October’s...
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