Cold winter reduces invasive quagga mussel numbers in Baltic Sea

A sharp decline in the invasive quagga mussel population has been observed in the Baltic Sea following this year’s cold winter, according to researchers from the Technical University of Denmark (DTU).

The species, also known as the “killer mussel,” has plagued the Baltic for two decades, threatening native fish populations by consuming cod eggs. However, recent surveys aboard DTU’s research vessel Dana reveal a significant drop in numbers, expedition leader Marie Storr-Paulsen told Danish broadcaster DR.

“After years of rising populations, we’ve seen an incredible reduction this time,” Storr-Paulsen said. “The cold winter has had a clear positive effect—quagga mussels have decreased dramatically.”

Originally from the U.S. East Coast, the mussel spread to Europe via ship ballast water. Unlike in its native habitat, it has no natural predators in the Baltic, allowing rapid growth under favorable conditions. The species is hermaphroditic, capable of self-fertilization, and can multiply from one individual to 10,000 in just four weeks.

Mild winters in recent years accelerated its spread, with record-high populations reported in 2023. But this year’s freeze mirrors a similar collapse seen after the cold winter of 2011–12, when researchers found almost no specimens. DTU first documented the quagga mussel in the Baltic in 2007.