Plastic-Filled Seabirds And Fish Found In Monterey Bay

Microparticles recovered from the guts of some seabirds living in Monterey Bay exhibited estrogen-like effects that have the potential to disrupt normal hormone functions

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Microplastic pollution is widespread in the waters of California’s Monterey Bay, according to a recent study, and is found in the digestive tracts of northern anchovies, Engraulis mordax, which are small marine fish that feed on plankton, and that mistake these tiny plastic particles for breakfast. After ingestion by anchovies, these microplastics work their way up the food web and become more concentrated along the way. According to the study’s findings, microplastics were found in the digestive tracts of all common murres examined, and almost one-quarter of those particles showed estrogen-like activity. Common murres, Uria aalge, are a common black-and-white seabird that feeds almost exclusively on anchovies.

The study was conducted under the leadership of conservation biologist Sami Michishita, who was working on a Master’s Degree when this work was completed, and who now works as a California Sea Grant Fellow with the State Water Resources Control Board. The overall goal of the study was to provide information about the prevalence, composition, and estrogenic activity of microplastics in Monterey Bay because it is a highly productive and economically valuable ecosystem. For example, this is an important refueling and resting stop on the Pacific Flyway for tens of millions of migrating birds.

Microparticles were collected from the intestinal tracts of donated anchovies that had been locally fished, and from murres killed by oil spills. To identify the molecules in the microparticles, the researchers used a non-destructive method that is common in chemistry to provide a structural fingerprint to identify molecules.

“These tiny plastic particles are leaching substances that have the potential for hormonal disruption that can have cascading effects on reproductive and immune functions”, the study’s senior author, ecotoxicologist Myra Finkelstein, an adjunct professor of environmental toxicology at the University of California, Santa Cruz, said in a statement. Professor Finkelstein’s research primarily focuses on understanding when and why individual estrogenic effects have population-wide ramifications – a key area that has not been well studied and where there are still more questions than answers.

Previous research has established that most plastics release estrogenic chemicals (ref). Additionally, we know that estrogenic activity can potentially harm the health of birds and mammals (yes, including humans), especially at extremely low doses (picomolar to nanomolar) during fetal through juvenile stages of development (ref).

But were all these tiny fibers found in the guts of anchovies and murres actually made of plastic?

“When you’re looking at tiny fibers under the microscope, you can’t always tell if it’s cotton or polyester, so we took that next step to determine what it was, and then took the further step of testing them for estrogenic activity”, Professor Finkelstein replied. Assessing estrogenic activity was conducted in a collaboration with the San Diego Zoo Wildlife Alliance.

The researchers found that 58% of anchovies and 100% of murres had microparticles (particles smaller than 5 millimeters) in their digestive tracts. Most of the particles (78%) were fibers, and more than half of the particles (57%) were identified as plastic (Figure 1). Of those plastic microparticles, almost a quarter (23%) exhibited estrogenic activity.

But what effects are these plastic microparticles on the health of these animals? That’s a much harder question to answer. Although it is known that estrogen-like chemicals released by plastics can mimic the physiological actions of natural estrogens and thereby disrupt normal biological functions, it is not known what, if any, effects are actually occurring in either the anchovies or the murres.

“The next step is to see how this may be affecting the birds,” Professor Finkelstein replied. “With microplastics, it seems we are finding them anywhere we look. But we need to do more work to find out what the biological impact is.”

Further, plastics often smell like a tasty seafood dinner so many seabirds are eating it with increasing regularity.

“One of the main problems with macroplastics is that they’re taking the place of food,” Professor Finkelstein said. “With microplastics, a major concern is the toxic compounds that may be leaching out of it.”


Sami Michishita, Corinne Gibble, Christopher Tubbs, Rachel Felton, Jenessa Gjeltema, Jackelyn Lang and Myra Finkelstein (2022). Microplastic in northern anchovies (Engraulis mordax) and common murres (Uria aalge) from the Monterey Bay, California USA – Insights into prevalence, composition, and estrogenic activity, Environmental Pollution 120548 | doi:10.1016/j.envpol.2022.120548

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