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Davis, California

Tuesday, April 23, 2024

Pollution legacy slashing survival of San Francisco Estuary fish

A toxic mix of pesticides and industrial chemicals polluting the San Francisco Estuary is contributing to a downward spiral in fish populations by affecting the earliest stages of growth and development, according to a recent UC Davis study published in the journal Proceedings of the National Academy of Sciences.


The San Francisco Estuary connects the San Joaquin and Sacramento Rivers to the Pacific Ocean and includes both the Delta and San Francisco Bay.


Although several estuary fish species populations have been declining since the 1970s, record lows in recent years fly in the face of weather and river conditions that typically reverse these losses, according to the latest CALFED report on the subject.


The intensified decline reflects the estuary ecosystem’s cumulative response to various stresses including water flow diversions, invasive species and pollution, said lead study author David Ostrach, a research scientist at the UC Davis Center for Watershed Sciences.


Ostrach’s study showed that the offspring of striped bass – a model indicator of overall estuary health – are exposed to dangerous levels of contaminants passed on from the mother through eggs.


Past studies showed that the vast majority of river-collected progeny die shortly after hatching, while fish that survive to adulthood with contaminant exposure have higher incidences of disease, Ostrach said.


“The focus on effects in early life stages is appropriate because early development is highly sensitive to mutagenic pollutants,” said Ben Greenfield, an environmental scientist at the San Francisco Estuary Institute who was not involved in the study.


Researchers found significantly higher levels of contaminants in eggs from river bass compared to those from bass that were raised and bred in a hatchery with clean water. The list of elevated chemicals includes both discontinued and currently used pesticides, as well as PBCs and PBDEs, widely used organic compounds that have been shown to disrupt hormone balance and function and to cause permanent behavioral defects in mice.


To follow possible exposure effects on developing bass hatchlings (larvae), Ostrach used a novel approach to examine growth, organ formation and signs of health. Compared to their hatchery counterparts, river-raised larvae were deformed, grew slower, had stunted brain development and dysfunctional livers, and lacked enough energy stores to fuel their search for food.


“The majority of larvae would not make it to first feeding due to the developmental problems caused by the contaminants the mother fish is passing on to them,” Ostrach said.


In contrast to studies showing effects of a single to a few chemicals on fish development, our study clearly shows that a real world mix of contaminants is causing developmental problems, he said.


“These types of studies of conditions in the field are more difficult to perform than controlled laboratory experiments,” Greenfield said. “But they … often yield the most important results for managers.”


Further studies will examine the overall health of the striped bass population at all life stages and how problems might relate to other declining fish species in the Estuary, Ostrach said. The goal will be to provide conclusive data so that actions can be taken to mitigate some of the problems.


“These fish are being exposed to a lot of things that are a result of our human environment,” Ostrach said. “So to use them as a parallel model [regarding human health implications] is not unreasonable.”


PBDE levels in San Francisco Bay Area women’s breast milk are 100 times higher than elsewhere, indicating that humans are similarly accumulating the compounds and passing them on to children.


“In my opinion, more work by agencies, research scientists and the public needs to be addressing these contaminant issues … to seriously look at what is being [produced and] dumped into the environment … and to try and curtail that as well as to clean up the environment,” Ostrach said.


ELAINE HSIA can be reached at features@theaggie.org.



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