Diverse sampling programs highlight pulses of Velella velella along the US West Coast

Sporadic mass strandings of the hydrozoan, Velella velella, along the US West Coast fascinate beachcombers and perplex oceanographers. Velella often arrive to the coast in the spring concurrent with a shift in onshore winds and after winters with warmer water temperatures. Understanding the factors that bring Velella to coastal waters provides new information about the ecological impacts of local and basin-scale environmental variability in the northeast Pacific. We summarize several compelling, non-exclusive hypotheses that could explain the recent increase in Velella, including increased population size, larger surface patches and enhanced coastward transport. We compiled a comprehensive dataset spanning planktonic larval to colonial adult life stages from 10 sources covering ~1900-2025. We highlight pulses of Velella and an unprecedented number of strandings and at-sea observations since 2014. We found that wind patterns in the central-east Pacific favor transport toward the coast during years of Velella presence. However, we found no clear relationship between surface frontal features and Velella abundance, nor a consistent association with El Niño. We also identified phalarope abundance as a potential seabird proxy for increased Velella abundance at-sea. Finally, we provide recommendations for future monitoring efforts to improve long-term assessments, given the current inability to clearly distinguish among hypotheses.