Trillions of sailing jellyfish wash ashore after warm winters

(Credit: Jake/Flickr)

New research uncovers patterns that explain why huge numbers of “by-the-wind sailor” jellyfish wash ashore and strand on beaches around the world, including up and down the US West Coast.

While these mass stranding events are hard to miss, researchers actually know very little about how or why they happen.

As their name suggests, by-the-wind sailor jellyfish know how to catch a breeze. Using a stiff, translucent sail propped an inch above the surface of the ocean, these teacup-sized organisms skim along the water dangling a fringe of delicate purple tentacles just below the surface to capture zooplankton and larval fish as they travel.

Thanks to 20 years of observations from thousands of citizen scientists, researchers have now discovered distinct patterns in the mass strandings of by-the-wind sailors, also called Velella velella. Specifically, large strandings happened simultaneously from the northwest tip of Washington south to the Mendocino coast in California, and in years when winters were warmer than usual. The results appear in the journal Marine Ecology Progress Series.

A researcher holds up a measuring stick next to the by-the-wind sailor jelly, showing it's a centimeter or two tall
A by-the-wind sailor that washed ashore at Moolack Beach, Oregon, in 2018. A citizen-science volunteer measured the organism for this photograph. (Credit: COASST)

“Citizen scientists have collected the largest and longest dataset on mass strandings of this jelly in the world,” says senior author Julia Parrish, a professor in the University of Washington School of Aquatic and Fishery Sciences and executive director of the Coastal Observation and Seabird Survey Team, known as COASST.

“This paper contributes to fundamental scientific knowledge of this organism in a way that traditional ‘mainstream’ marine science has been unable to do. Thousands of trained, dedicated observers are better than any satellite because they know their beach and can alert us if something is weird or unusual.”

A by-the-wind sailor jellyfish washed up on shore, covered in sea foam, with it's translucent "sail" pointed upwards
Velella velella, also called “by-the-wind sailor” jellies, that washed ashore at Moolack Beach, Oregon, in 2018. (Credit: COASST)

By-the-wind sailor jellyfish data mining

COASST’s citizen scientists are trained to search for and identify carcasses of marine birds that have washed ashore at sites from northern California to the Arctic Circle. Participants are asked to record and submit photos of anything strange or different they see on their stretch of beach.

In 2019, program managers received an email from COASSTers in Oregon who had expected to see Velella on their beach based on past observations, but hadn’t. That prompted COASST scientists to comb the database—23,265 surveys in total—to see if others had taken note of these by-the-wind jellyfish over the years. This “data-mining” returned 465 reports of Velella littering 293 beaches, often in more than one year.

A beach covered in jellies, which all look white and dried out
A mass stranding of by-the-wind sailor jellies at South Jetty Beach, California, in 2014. When they wash ashore, these jellies quickly dry to the consistency of potato chips and their brilliant colors fade to white. (Credit: COASST)

“On the water, Velella are beautiful, fragile creatures. When they wash ashore, these jellies quickly dry to the consistency of potato chips. During a mass stranding it’s like walking on a crunchy carpet,” Parrish says. “So of course, COASSTers reported in. Suddenly, we realized we had the largest dataset about Velella velella anywhere in the world.”

Winners and losers

In analyzing the citizen science observations, researchers discovered that most by-the-wind sailors wash ashore on West Coast beaches during the spring, when the winds shift and push the organisms to shore.

However, their analysis also revealed truly massive stranding events in 2003-2005 and again in 2015-2019. During the later years, jelly carcasses covered more than 620 miles (1,000 kilometers) of continuous coastline, all within a single two-week window between mid-March to mid-April.

The second period corresponds with the timing of the long-lasting marine heat wave known as “the blob“—also to blame for the largest seabird die-off of common murres, as well as mass die-offs of Cassin’s auklets, sea lions, and baleen whales.

The researchers hypothesized that warmer winters during these years allowed for populations of by-the-wind sailors to spike in the open ocean. Then, when the winds shifted in the spring, massive numbers of the jellies were swept to shore and stranded.

Put another way—and though many ultimately end up dying on beaches—the jellies appear to be “winners” during warmer periods, because they can amass more numbers in the ocean. There’s some evidence that warmer-than-average winters are also calmer and less wavy in the open ocean, allowing increasingly large Velella aggregations to persist, Parrish explains.

“This paper and our data really do suggest that in a warming world, we’re going to have more of these organisms—that is, the ecosystem itself is tipping in the direction of these jellies because they win in warmer conditions,” Parrish says. “A changing climate creates new winners and losers in every ecosystem. What’s scary is that we’re actually documenting that change.”

As warmer winters are expected to increase with climate change, these findings could have clear implications for this jelly population, as well as for the fish they eat and the beaches where they strand and die.

Washington Sea Grant and the National Science Foundation funded the work.

Source: University of Washington