Rat poison from marijuana farms appears to be sickening and possibly killing owls, fouling wildlife habitat and allowing rodents to spread contamination through large swaths of Northern California’s most famous cannabis-growing region, University of California researchers said in a report published Thursday.

Rodenticide was found in seven of 10 northern spotted owls and 34 of 84 barred owls tested between 2006 and 2016 near marijuana plantations in Humboldt, Mendocino and Del Norte counties. Both species are protected under the Migratory Bird Treaty Act.

The study, by UC Davis and the California Academy of Sciences, is the first account of deadly rodenticide in northern spotted owls, a redwood tree-loving bird that is listed as a threatened species under the federal and state Endangered Species acts.

Large quantities of rat poison were previously found in the weasel-like creatures known as Pacific fishers, which also live in the Emerald Triangle marijuana-growing region.

“We’re deeply concerned that there aren’t sufficient conservation protective measures in place,” said Mourad Gabriel, a researcher on the faculty of the UC Davis School of Veterinary Medicine and the lead author of the study. “If no one is investigating the level at which private marijuana cultivators are placing chemicals out there, the fragmented forest landscapes created by these sites can serve as source points of exposure for owls and other wildlife.”

The study, published Thursday in the journal Avian Conservation and Ecology, is a clear indication that rats poisoned on illegal farms on private property are spreading the toxins to the animals that eat them, a problem that until now had only been documented in wilderness areas and public forests where drug cartels had planted secret crops.

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The poisons, known as second-generation anticoagulants, interfere with blood clotting, resulting in uncontrollable bleeding and a slow, agonizing death for any animal that captures a poisoned rodent or scavenges on its contaminated carcass. Although some products are being phased out, they are still available despite widespread pressure from federal regulators, wildlife officials and environmentalists to remove them from store shelves.

Gabriel said the study areas all contained critical owl habitat surrounded by private property or recently purchased timberland where people had planted marijuana, presumably to cash in on a growing market that experts believe will be worth billions of dollars in the next few years.

The owls, which were either found dead in the field or were provided by researchers conducting unrelated studies in the area, are crucial indicators of ecosystem health.

“Northern spotted owls were exposed to rodenticide and exposed throughout the study area,” said Gabriel, who is also executive director of the nonprofit Integral Ecology Research Center, in Humboldt County. “The chemicals were likely being placed ubiquitously through the study area.”

The spotted owl competes with the barred owl, a native of the east that has expanded its range west. Rodents make up 70 to 80 percent of the spotted owl diet and 30 to 50 percent of the barred owl diet. That probably explains why 70 percent of the native owls compared to 40 percent of the eastern invaders had toxins in their bodies, Gabriel said.

He said spotted owls feed along forest edges, which cannabis growers create when they clear areas to plant their crops.

The study, funded by the U.S. Fish and Wildlife Service, is a follow-up on previous studies by Gabriel in 2012, 2013 and 2015 linking rat poison from marijuana farming to wildlife deaths, including Pacific fishers, thick-coated carnivores that feed on rodents in the remote forests of California and the Pacific Northwest.

Gabriel is concerned that California’s legalized recreational sales of marijuana, which began Jan. 1, will result in more large unpermitted cultivation sites, particularly in Humboldt County, and, consequently, more rat poison in the ecosystem.

Only 2,300 of the 15,000 or so growers in Humboldt County have applied for permits to cultivate marijuana under the state’s system, county officials say.

“It’s another reason why it is so important that we draw more growers into the regulated market,” said Hezekiah Allen, executive director of the California Growers Association, an advocacy group for more than 1,000 medical marijuana farmers and business owners and a longtime supporter of a rodenticide ban. “If we did have the bulk of the growers inside the system, law enforcement could focus on the outliers.”

Gabriel said regulations likely will have to be stiffened and more law enforcement deployed in the rural, forested regions of California if the problem is going to be solved.

“Our results clearly indicate that environmental contamination is happening in northern spotted owl habitat,” he said. “There needs to be sufficient regulatory oversight to ensure that individuals partaking in this industry are also stewards of conservation and California’s natural resources.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite