El Niño Impacts Black Abalone, but Some Show Signs of Recovery

Fall 2016

Unusual warming of the Pacific Ocean by the “Warm Blob” and El Niño in recent years worried scientists studying the highly endangered black abalone. They knew that earlier El Niños had taken a heavy toll on the elusive shellfish, fueling outbreaks of a fatal abalone disease called withering syndrome.

“During past strong El Niño events we saw mass mortality of black abalone because of the relationship between warm ocean temperatures and a bacterial disease known as withering syndrome,” said Pete Raimondi, Research Professor at the University of California Santa Cruz. “We expected the same results this time, but the impact was somewhat mixed.”

In short, there is good news and bad news, depending on where the abalone are located.

black abalone with withering syndrome

A black abalone from coastal California displaying a shrunken foot caused by Withering Syndrome disease. Photo: John Steinbeck, Tenera Environmental Consulting

The bacteria that causes withering syndrome exists in all wild black abalone, but does not always cause disease. Warm water conditions cause the bacteria to multiply, weakening the abalone’s muscular foot and causing the animals to lose their grip on their rocky reef home and eventually die.

NOAA Fisheries listed black abalone as endangered in 2009, citing the effects of overharvesting and withering syndrome.

John Steinbeck, a researcher with the consulting firm Tenera Environmental, surveys black abalone along an 8- to 10-mile stretch of coastline in San Luis Obispo County, south of Morro Bay. He reported increased mortality of black abalone from withering syndrome over the last two years.

“We saw large black abalone adults lying dead next to where they have lived in the intertidal zone for decades now. We thought that maybe these individuals were somehow immune to withering syndrome. It was very sad,” said Steinbeck.

Strong El Niño conditions followed by a withering syndrome epidemic reduced black abalone populations on the mainland coast by up to 99 percent in some areas between 1988 and 1997, and their recovery has been very slow. So few abalone remain in some areas that they cannot reproduce successfully. Abalone are “broadcast spawners,” which means they send their eggs and sperm into the water in the hope that ocean currents bring the two together. 

In contrast to San Luis Obispo County, black abalone in the California Channel Islands and other mainland sites have shown more promising trends. Raimondi has observed recent settlement of young abalone, which is a sign of successful reproduction.

healthy abalone

Healthy black abalone in the intertidal zone on San Nicolas Island. Photo: David Witting, NOAA

“After studying this population for more than 30 years, it looks like we are starting to see natural recovery of black abalone even though we did have some mortality due to withering syndrome. The recent reproductive success of some populations may have helped offset the impacts,” says Raimondi.

At San Nicolas Island, which is owned by the U.S. Navy and is one of the most remote Channel Islands, black abalone are recovering faster than anywhere else. Scientists believe that some black abalone individuals may be showing immunity to the disease.

Carolyn Friedman from the University of Washington has found a virus in some black abalone that appears to reduce the impacts of withering syndrome. Her work suggests that another factor, termed “evolved resistance,” may be improving the survival rates of black abalone. If so, scientists suspect that black abalone with the virus would be more likely to survive and then pass on this resistance to their progeny.

Black abalone on San Nicolas Island were not completely immune to impacts of withering syndrome during the recent 2015/16 El Niño event.  Biologists did observe a roughly 25 percent decline in the population overall from a combination of withering syndrome and heavy sedimentation brought on by winter storms.

They also observed the disease at sites where black abalone had the highest rates of recovery and appeared to show resistance to the disease during previous outbreaks.

Given this variation in responses, it remains unclear why the latest El Niño event proved less catastrophic in some areas for black abalone than previous El Niños. Biologists want to know why some individuals survived while others within the same tidepool did not. They also want to know if the recent signs of natural recovery will continue.

NOAA Fisheries is incorporating the latest scientific findings into a draft black abalone recovery plan that will be released within the next year. The plan, which is required for endangered species, identifies the key recovery components necessary to boost black abalone populations back to sustainable levels.

Homepage photo of healthy black abalone in the intertidal zone on San Nicolas Island by David Witting, NOAA