Warming Waters Pave the Way for Jellyfish Blooms in the Pacific Ocean

By Abby Meyers, ESRM ‘27

This past March, large masses of small, translucent jellyfish washed up along the Pacific coast, spanning from Northern Washington to Central California. These organisms, called velella velella, are often referred to as “by-the-wind sailors” because of their fragility and susceptibility to being blown ashore by coastal winds. Velella velella skim the water using a translucent sail that extends an impressive inch above the water’s surface. They also have fragile tentacles that extend below to capture zooplankton and small larval fish for sustenance.

COASST, or Coastal Observation and Seabird Survey Team, has been at the forefront of Velella velella research. COASST is a system of trained citizen scientists that record data on a specific portion of the Pacific coast on a monthly basis, based at UW’s School of Aquatic and Fishery Sciences. Velella velella are not the focus of the program, nor is reporting them a requirement, but citizen scientists have noted enough opportunistic sightings for researchers to investigate why they have been washing up.

As it turns out, the period of wash ups correspond with massive die-offs of common murres seabirds, Cassin’s auklets, sea lions, and baleen whales. These are not isolated events. Die offs of Velella velella have occurred on the Pacific coast after multiple anomalously warm winters, caused by a combination of warming sea temperatures and El Nino Southern Oscillation weather patterns. Warmer than average ocean conditions are typically calmer, which provides a more habitable environment for these fragile organisms. Scientists have hypothesized that marine heat waves have caused spikes in northern anchovy, which became an additional food source for Velella velella. When Northern prevailing winds shift East during the spring months, the jellyfish are blown ashore.

Velella velella are unique in that they may be benefiting, at least in a small degree, from global warming. But the story is not that simple. Accumulation of Velella velella during the winter just means larger jellyfish die-offs in the spring. More broadly, changes in Velella velella populations are just another sign that the baselines of marine ecosystems are shifting. The implications of changing jellyfish populations are yet unknown for marine food webs, but are sure to manifest in the near future.