Every year, on beaches around the world, colonies of sailor jellyfish (Velella velella) end up stranded on beaches by the thousands. They then dry up and die, becoming a “crunchy carpet” of dehydrated corpses covering the sand, Julia Parrish, a University of Washington professor and co-author of a new study on mass Velella strandings, said in a statement.
The jellyfish strandings are common when seasonal winds change course. But in rare cases, like a 2006 event on the west coast of New Zealand, the jellyfish corpses came up in millions instead of thousands. What caused that?
Parrish and her colleagues wanted to find out. In their new study (published March 18 in the journal Marine Ecology Progress Series) they looked into 20 years of Velella observations reported along the west coast of the United States.
The observations came from a program called the Coastal Observation and Seabird Survey Team, also known as COASST, which trains citizen scientists to search their local beaches for marine birds that have washed up on shore, as well as any other unusual wildlife sightings. The COASST program covers hundreds of beaches stretching from northern California to the Arctic Circle.
The researchers found almost 500 reports of Velella strandings in the COASST database. They were seen on nearly 300 beaches. According to the reports, the biggest strandings happened during spring months from 2015 to 2019. During that time, dead jellyfish covered more than 620 miles of coastline.
Interestingly, the jellyfish die-offs also coincided with a massive marine heat wave known as “the blob.” In 2013, surface waters off the Pacific coast began heating up to levels never recorded before. The intense warming continued through 2016, which resulted in mass die-offs of seabirds, baleen whales, sea lions, and other creatures. According to the new study, it’s likely that the blob caused the mass die-offs of by-the-wind sailor jellyfish reported during those years.
However, those warming ocean waters may have actually been good for the jellyfish the researchers said. As the blob increased ocean surface temperatures, certain fish (such as northern anchovies) benefited from longer spawning seasons. This provided more food for Velella jellyfish earlier in the year. This may have caused jellyfish populations to spike before the seasonal wind changes blew the creatures ashore.
“A changing climate creates new winners and losers in every ecosystem,” Parrish said in the statement. “What’s scary is that we’re actually documenting that change.”