Southern sea otters, the species that has historically inhabited California waters, have been protected under the Endangered Species Act since the ‘70s but have yet to re-establish an enduring population in the waters off Marin County. A new study published in August by scientists primarily from the University of California, Davis, and California’s Department of Fish and Wildlife delves into one of the less obvious causes of their high mortality rates: parasites. Melissa Miller, who specializes in otter deaths for Fish and Wildlife and was a contributing author for the study, explained that although white sharks pose the biggest threat for otters—often biting them and then letting them go for want of a satisfactory meal—nearly a third of otter deaths are linked to Toxoplasma gondii, which infects both wild and domestic cats. Cats become infected after eating rodents or birds infected with Toxoplasma gondii, and then shed millions of oocysts, or parasite egg cells, in their feces. Rainfall washes the feces into the water system, where it slips past most treatment plants and eventually enters coastal waters. The parasite is widespread in otters, and though many survive, others die from brain infections or overall weakened immune systems. The new study shows that Toxoplasma gondii was either the primary cause or a contributing cause of death for 29 percent of southern sea otters examined between 1998 and 2001. (Infection from the parasite can also make otters susceptible to other parasites, such as Sarcocystis neurona, which is less prevalent.) Ms. Miller, herself a cat person, said she takes precautions, keeping her cats indoors and throwing their poop into the garbage in plastic bags. “The bigger picture here is that this is an example of pollution spreading from land to the sea,” she said. The current range of southern sea otters begins in Half Moon Bay, extending south to Point Conception in Santa Barbara. Males—which wander, exploring new territory and inviting the females to join them should they find good habitat—have been spotted further north, including off the coast of Point Reyes. Although the southern sea otter population overall increased from 1,270 in the 1980s to 3,500 in 2016, it has plateaued at just under 3,000 in the last three years. The fact that the otters are concentrated in one part of California is a concern for conservationists, Ms. Miller said, because a disaster like an oil spill could wipe out the entire species. The next research Ms. Miller plans to publish concerns the effect of domoic acid, a biotoxin produced by algae blooms and another, lesser known cause of death for otters.