During our recent Northern California Current ecosystem research cruise aboard the NOAA ship Bell Shimada, the talk centered around pyrosomes, pyrosomes, pyrosomes! Throughout the 10 days at sea, dozens of these odd creatures were caught in almost every net tow, and video cameras deployed to depths of up to 100 m show thousands floating eerily. This cruise came just three months after a similar cruise in February where high pyrosome densities were observed, but they seem to be even more abundant and larger this spring.
Since 2015, more and more pyrosome sightings have been reported off of the Oregon coast, but not in the numbers we have been seeing lately. Pyrosomes have become the center of attention with reports of them washing up on beaches, clogging up fishing and research gear, and causing a general sense of befuddlement for people who have been frequenting these waters for many years, but to our knowledge, have never seen these creatures in such high densities off of the Oregon coast before.
So…what exactly is a pyrosome and why are we suddenly seeing so many of them?
Pyrosoma atlanticum, are colonial pelagic tunicates made up of individual zooids that come together to form a hollow, tube-like structure. Pyrosomes filter feed on plankton and undergo diel vertical migration from depths of hundreds of meters, where they are found during the day, up to the surface waters at night. Very little is known about their feeding behavior and impacts to food webs, especially when they are in such high abundance. Known predators of pyrosomes include various fishes, dolphins and whales.
Since so little is known about these organisms, during our research cruise, we began to count and measure the catches to try to get a better sense of their relative abundance, size, and distribution in relation to environmental variables. We observed some of the highest abundances approximately 40-150 miles off shore, with the largest colony size at a whopping 78 cm.
More research is needed to understand why we are seeing so many pyrosomes in recent years, but one hypothesis is that they are being delivered to coastal waters from farther offshore, and that warmer ocean conditions over the past three years are creating an ideal environment for them to thrive. Current research is striving to better understand the relationship between recent environmental conditions and pyrosome populations, as well as their role in food web dynamics and impacts to fishing and recreation in the Northern California Current.
For example, will the cold upwelled water that we are now observing off of the Oregon coast drive the pyrosomes away? We may only discover what made them flourish once they are gone, or perhaps they will stick around!