Ocean Overrun With Gentle Gelatinous Salps

What looks like a jellyfish but is closely related to humans? The answer is an oceanic animal called a salp, and right now the waters off California are teeming with unprecedented numbers of these creatures.

Salps are essentially transparent jet-propelled tubes. Their life cycle alternates between solitary swimmers, each smaller than your hand, and aggregated colonies that can grow longer than a bus.

Shimada 2012 Pyrosoma, Carinaria, and salps — The pink tubes in the upper left are pyrosomes, a close relative of salps that have also increased in abundance. To the right of the pyrosomes is a transparent sea snail that preys on salps--these sea snails were unusually large this year. The rest of the tray is full of Salpa fusiformis, the "brown-eyed salp." Photo by {link url=http://student.uncw.edu/mab1828/Matthew_Birk/Home.html}Matthew Birk{/link}.

As individuals, salps are innocuous. They don't sting. They don't hunt. They're gentle plankton eaters. But as populous blooms, salps can wreak havoc. So far this year their sheer gelatinous mass has shut down one nuclear power plant and destroyed two fishing nets.

The NOAA Southwest Fisheries Science Center in Santa Cruz has surveyed the central California coast every spring since 1983, gathering information on individual fished species and on the ecosystem in general. The 2012 survey took the ship Bell M. Shimada from San Diego to Newport between May 6 and June 17.

"We've had wild salp catches over the last few weeks," wrote research fishery biologist John Field in a message from the boat to colleagues back on shore. "No one from the survey has ever seen anything like it."

Each time the scientists dragged a trawl net through the water, they caught an average of 30,000 salps. Prior to 2012, the maximum salp catch for a single trawl was 235 in 1999. This year, some hauls held over half a million.

Shimada 2012 midwater trawl net covered in salps — Trawl net covered in salps - photo by {link url=http://student.uncw.edu/mab1828/Matthew_Birk/Home.html}Matthew Birk{/link}

And that was when they could even use the nets. "For the first time in the history of the survey," wrote research fishery biologist Keith Sakuma, "both the primary and backup midwater trawl nets were ripped apart due to the sheer number of salps in the water." Often they had to cancel the trawls because there were simply too many salps.

So what does such a freak salp bloom mean? It's tempting to dredge up the "jellyfish gone wild" hypothesis--the idea that gelatinous creatures are taking over the oceans because they can adapt to human pollution. But as MBARI scientist Steven Haddock has pointed out, historical data for most species isn't good enough to be certain of an increase in numbers or a significant tolerance for pollution. Furthermore, in the case of California salps, there's reason to believe shifts in abundance are driven by natural oceanwide changes.

Over the course of decades, the Pacific Ocean alternates between "warm" and "cool" phases. During a warm phase from 1977-1998, salps declined in abundance; the trend reversed after 1998 with a shift to a cool phase. Yet none of the years since 1998 have shown salp numbers even close to the banner year of 2012. Scientists will surely be seeking further explanations and waiting to see how long the bloom continues.