No one knows why these tropical sea creatures are appearing on the West Coast

pyrosomes Matt Stabler/CBC

For months now, fishermen on the coast of British Columbia have been pulling up their nets with a strange catch. Millions of translucent, tropical creatures have invaded the area, clogging nets and scattering beaches.

These creatures are called pyrosomes; shaped like a tube, they can grow up to 10 metres long. These gelatinous, glow-in-the-dark animals are usually found in warm tropical waters, but have been spotted as far north as B.C. in the past. Now large colonies have stretched from Oregon to the Gulf of Alaska.

“They got here and have been flourishing—just super abundant,” Jennifer Fisher, a research assistant at the Hatfield Marine Science Center in Oregon, told National Geographic. “But that’s the weird thing: Why here? Why now?”

In 2014 and 2015, sea temperatures were up and unexpected visitors appeared in the eastern Pacific, including warm-water sharks and tuna found in Alaska. When the waters cooled, things started going back to normal—almost. Instead of disappearing, the pyrosomes began to multiply. This spring, they have dominated the top several hundred feet of sea, putting a strain on commercial fishing along the coast.

“For something that’s never really been here before, the densities are just mind-boggling,” says Laurie Weitkamp, a biologist with the Northwest Fisheries Science Center in Oregon, told National Geographic. “We’re just scratching our heads.”