Male fish dominate at polluted beach near San Francisco

"Exposure pathways are different, of course, but what we see happening with fish is indicative of potential issues that could cause problems with human health. We're not swimming around in a soup of hormones and pesticides, but we're exposed to those things through food or in the air," says Susanne Brander. (Credit: Susanne M. Brander)

Chemicals that mimic hormones may help explain why there are more male silverside fish swimming at an urban beach near San Francisco Bay.

Researchers collected Mississippi silversides (Menidia audens) at two beaches in Suisun Marsh: an urban beach and one near a cattle ranch. They measured the effects of endocrine-disrupting chemicals, which mimic hormones, on the fish.

Endocrine disrupting chemicals are known to cause physiological and behavioral abnormalities in fish. They come from a variety of sources, such as agricultural, urban and residential run-off. They’re also found in wastewater effluent, which includes pharmaceuticals such as birth control, hormone replacement therapy, and some anti-inflammatory medications known to contain endocrine disruptors.

Larger males


The ranch beach was less polluted than the urban beach, with ranch run-off being the primary source of pollution.

The study, published in the journal PLOS ONE, shows that male fish at the ranch beach had a higher expression of genes normally only expressed in females, compared to their male counterparts at the urban site. Yet, this did not appear to affect the sex ratio or gonadal health of fish at the ranch site, which was roughly 50-50 male and female.

However, males dominated at the urban beach, which was exposed to several sources of pollution—a nearby wastewater treatment plant, the surrounding urban community, as well as ranch lands. The males had smaller, less healthy gonads relative to their size, indicating that they might produce less sperm than male fish at the ranch site.

Fish at the urban site also grew more slowly than the ranch fish. And while female silverside fish are typically larger than males, males at the urban beach were larger than both the urban females and the ranch males.

“The DNA sequence for a hormone receptor in a fish isn’t that different from the DNA sequence for a hormone receptor in a human,” says lead author Susanne Brander, who was a doctoral student at the UC Davis Bodega Marine Laboratory when the study was conducted. She is currently an assistant professor at the University of North Carolina, Wilmington.

“Exposure pathways are different, of course, but what we see happening with fish is indicative of potential issues that could cause problems with human health. We’re not swimming around in a soup of hormones and pesticides, but we’re exposed to those things through food or in the air.”

The researchers analyzed the fishes’ response to endocrine disruptors at the molecular, cellular, organism, and population levels to better predict the ecological impacts of such exposure. For example, would an abnormality at the cellular level indicate abnormalities for the whole population?

Are androgens to blame?

A potential explanation for the differences seen at the beaches, the researchers hypothesize, is that exposure to androgens—hormones like testosterone that control the development of male characteristics—reduced the expression of estrogen-dependent genes, which the females need to develop and reproduce successfully.

Androgen exposure from endocrine disruptors may also be causing some of the fish that would have been female to become male, Branden says, adding that ongoing research is working toward investigating that hypothesis.

The study also found that the Mississippi silverside appears to be a good indicator species for studying the effects of endocrine disruptors. Researching the effects of these chemicals in the San Francisco Bay estuary has been challenging for scientists because many of the region’s highly impacted native species cannot be collected in large enough numbers to study.

However, the Mississippi silverside, introduced in the early 1970s to the estuary, shares similar habitat, diet, and lifespans with some endangered fishes.

“A lot of endocrine disruptor work has been done with fish such as Japanese medaka and zebrafish, but you can’t go out and catch those in the San Francisco Bay,” Brander says. “The Mississippi silverside is a fish that can be used both for studying ecotoxicity in the wild and translating that to what’s happening in the lab.”

Scientists from the University of California, Davis; the United States Geological Survey; the University of North Carolina, Wilmington; and the Swiss Centre for Applied Ecotoxicology contributed to the study. The Delta Science, the National Science Foundation, Sacramento Regional County Sanitation District, Interagency Ecological Program, National Institutes of Environmental Health Sciences, and the California Department of Fish and Wildlife funded the work.

Source: UC Davis