In 2017, a pregnant female nutria was captured in a private wetland near Gustine in Merced County, California. The California Department of Fish and Wildlife says nutria had been declared eradicated from the state by 1978, with no reported detections between then and their rediscovery. During the summer of 2017, officials captured 19 additional nutria in the Grasslands Ecological Area.
Ahrens also reported that the research team had developed a global genetic reference database to help other scientists compare nutria populations and investigate invasion routes. The findings suggest a recent introduction followed by rapid spread, although the genetic evidence cannot establish the precise route by which the animals reached California . To trace the population’s origins, researcher Kristen Ahrens contacted museums and universities across California to obtain historical specimens. One unusual contribution came from the Butte County Weights and Measures Department, which provided a taxidermied nutria. The project received funding from the California Department of Water Resources in cooperation with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service.
A 2025 study published in Evolutionary Applications found that California’s current nutria population was genetically most similar to invasive nutria in central Oregon.
The invasion threatens wetlands and agricultural land in the Central Valley, where nutria can damage vegetation, undermine streambanks and levees, and contribute to soil erosion. They compared the results with genetic data from populations in Oregon, Washington, Louisiana, Texas, Maryland, and Virginia. The California nutria formed a distinct genetic cluster, but their closest relatives came from central Oregon, while nutria from northern Oregon were genetically linked to Washington. Scientists also examined maternally inherited DNA and found that all California nutria shared the same marker, which also appeared in nutria from Oregon and Washington. However, this genetic overlap does not establish that the present population descended from animals that survived undetected in the state. The authors considered a recent introduction followed by rapid spread more likely, given California’s favorable climate and the animals’ reproductive potential. The study also offers guidance for eradication efforts. River distance appeared to matter more than straight-line distance in explaining gene flow, suggesting that waterways help nutria move through the landscape.
To investigate the animals’ origins, researchers analyzed 6,809 genetic markers from 267 California nutria sampled between 2020 and 2022. Because the historical and current populations shared a maternal marker, the evidence alone cannot determine whether the earlier population persisted or whether nutria were introduced again. Because researchers found little genetic differentiation across the invaded range, they argued that managers could treat the Central Valley as a single eradication unit. Because much of the invaded range is privately owned, and some wetlands cannot be trapped during hunting and nesting seasons, access to affected areas also poses a challenge.
The study also examined historical museum specimens to assess whether the current population might descend from earlier nutria populations in California.
