There’s no doubt San Joaquin County harbors its share of invasive species.
Their origin stories, however, raise a few more questions.
Golden mussels presumably found their way into the marshes of the Delta after attaching themselves to the bottoms of ships arriving from Asia, where they are native. They were first detected in California near the Port of Stockton in 2024.
Airborne pests like mosquitoes that carry West Nile Virus may have simply flown in, scientists say. The disease was first reported in Southern California in 2023 and has since spread to all 58 counties, including San Joaquin.
But what about nutrias, the giant amphibious rodents that have found a home in swampy sections of San Joaquin County and elsewhere in the Delta? Measuring up to 3 feet in length and weighing more than 30 pounds, they didn’t exactly thumb a ride to the Central Valley.
Or did they?
New research points to the distinct possibility the critters were deliberately brought to the Delta by people who may or may not have known better.
The study comes from the California Department of Fish and Wildlife, which sought to understand how a species that had been eradicated from the Delta in 1978 suddenly started showing up again in 2017.
Researchers took DNA from nutria captured in the Delta and compared it to samples from the same species in other states. They found that DNA from Delta nutrias is most closely linked to specimens in animals found in Oregon — not Texas, Louisiana, Maryland or Virginia, some of the other states where they are known to populate.
The surprising conclusion suggests there may have been “intentional introduction.”
The agency’s wildlife genetics research lead, Mike Buchalski, told Stocktonia that he stands by the finding.
“I think it’s highly likely,” he said. “There’s basically no way that they could have made it to that region of California naturally.”
Still, it’s not like the animals haven’t been here before. At one time, nutrias were raised for fur. Some of the mammals could have escaped or were released into the wild, which would explain how they kept showing up in the Delta until 1978.
In more recent times, if they were indeed reintroduced by humans, whoever brought them might have had the notion of reviving the fur trade. Or, Buchalski said, it’s possible someone may have thought they’d make a cute pet. Or they could have been released deliberately for the benefit of hunters.
Whatever the reason, nutrias are certainly creating headaches for the department. They chomp through sensitive Delta vegetation at a great rate and multiply prolifically, making them environmental troublemakers.
The state’s wildlife department spends about $5 million a year trying to eliminate nutria. Since 2017, they’ve humanely trapped 7,841, but there seems to be no end in sight. Federal wildlife officials have gone so far as to suggest eating the invasive species in order to cull the species.
As for how they got to California, whether their entry was accidental or deliberate, one thing is clear: It’s unlikely the creatures could have trotted down Interstate 5.