Beachgoers across Northern California were stunned this week as millions of delicate, jellyfish-like sea creatures washed ashore — part of a phenomenon scientists say could intensify in the coming weeks.
The translucent blue animals, known as Velella velella or "by-the-wind sailors," were spotted covering stretches of sand from Marin County's Bolinas Beach to Pacifica State Beach in San Mateo County and Rodeo Beach earlier this week. The National Park Service also confirmed sightings at Stinson Beach in mid-March.
"They are at the mercy of the winds and the currents," said Jennifer Stock, an education specialist with NOAA's Greater Farallones and Cordell Bank National Marine Sanctuaries. "Their appearance is usually fairly brief, so it's hard to say how much longer they will be around."
The mass beachings — known as "strandings" — are being driven by strong seasonal upwelling along the California coast, which was delayed this year by 2024's strong El Niño conditions but has returned with greater force than usual, scientists say.
Upwelling occurs when offshore winds push warm surface water away from the coast, allowing colder, nutrient-rich water from the deep ocean to rise. Now in full swing, this cycle brings a feast of microscopic phytoplankton and zooplankton to the surface — a prime buffet for Velella velella, which feed on these nutrients through their dangling tentacles.
NOAA Fisheries reported last month that this year's upwelling has been "even more strongly and consistently" observed than usual, due in part to La Niña and colder-than-average ocean temperatures.
Julian Espinoza, a National Park Service spokesperson, said the creatures formed a thick layer on Bolinas Beach over the weekend and blanketed other beaches soon after. He noted that while their arrival is often sudden, they usually disappear just as quickly.
They typically appear somewhere along the coast "every spring during upwelling season when the ocean shifts," Stock added. That churn introduces phytoplankton blooms that form the basis for an entire ecosystem of animals — from fish to marine mammals to seabirds. "Upwelling is what makes the California coast so abundant with ocean wildlife."
Still, where the Velella will go next remains uncertain. Lacking any means of steering, their small, triangular sails catch the wind, drifting them wherever the breeze may take them.
A 20-year study by the University of Washington's School of Aquatic and Fishery Sciences found that mass strandings were reported along the coast from Northern California to Washington in 14 of the 20 years between 2000 and 2019 — almost always from mid-March through April.
This year's influx could be larger than usual. So if you're heading to the beach, don't be surprised if the sand is carpeted with the gleaming, otherworldly sailors of the sea.
Originally published on HNGN
