More than 250 dead California sea lion pups have been observed at Año Nuevo Island, a small island off the coast of Northern California between San Francisco and Santa Cruz.

Patrick W. Robinson, a director at Año Nuevo Reserve, in an email to USA TODAY Thursday, said he first "observed an unusual increase in the number of aborted California sea lion fetuses at Año Nuevo Island about two weeks ago."

Approximately 50 dead sea lion pups were observed on May 7, with the number rising to 288 on May 14, based on an aerial survey, said Robinson, who is also a lecturer at UC Santa Cruz.

Editor's note: Some of these images may be graphic.

On May 16, UC Santa Cruz campus veterinarian Megan Moriarty and a local marine mammal stranding network, along with the California Department of Fish and Wildlife conducted necropsies on several still-born pups, but "no obvious cause of death" was noted, Robinson said.

"This is definitely an unusual event, but we don't yet know the cause," Robinson said, adding that other researchers who work in the Channel Islands and Mexico have made similar observations, making it bigger than a local event.

Pups born in mid-May 'usually do not survive': Robinson

Robinson said that the Año Nuevo Island "lies at the edge of the breeding range" for sea lions and is a relatively new colony compared to the main ones in the Southern California Channel Islands. Peak pupping season for the species is mid-June, as per Robinson, and pups born in mid-May "usually do not survive."

In recent years, approximately 700 to 800 pups were born on the island, with a couple dozen stillbirths, said Robison. The wildlife expert explained that sea lion fathers are not very invested with their offspring, while mothers, who give birth to dead pups, "depart shortly after the event."

The National Oceanic and Atmospheric Administration (NOAA) that deals with marine mammals did not immediately respond to USA TODAY's request for a comment on the incident.

Is this event related to the increase in sea lions in San Francisco?

On whether this event has anything to do with the influx of sea lions at a pier in San Francisco, Robison said that "several anomalous events happening right now, and it is not clear if they are connected."

"Many pelicans have been in distress, cormorant nesting is delayed, [the] sea lions' stillbirths, and the influx of sea lions in San Francisco's Pier," Robison said.

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Saman Shafiq is a trending news reporter for USA TODAY. Reach her at [email protected] and follow her on X @saman_shafiq7.