The parasites are called trematodes. Two species of the flatworms were discovered in California’s freshwater fish, according to a study published Tuesday in the Journal of Infectious Diseases. The tiny, flattened and sluglike creatures can cause gastrointestinal problems, weight loss and lethargy when a person eats an infected fish.

More than 90% of popular freshwater game fish in Southern California contained an introduced parasite capable of infecting humans, according to a new study from researchers at UC San Diego’s Scripps Institution of Oceanography.

Nearly all freshwater game fish at popular recreational fishing areas in Southern California have been exposed to invasive parasites with the ability to infect humans, a new study published in the