Life finds a toehold just about anywhere, and the hulking edifices of offshore oil- and gas-drilling platforms are no exception. But these structures, many of which are decades old, are starting to be decommissioned.
Researchers have now calculated the ecological impact of losing these human-made habitats. Over 95% of fish biomass around a platform would be lost if it were removed completely, the team found. Another option—severing platforms at a water depth of 26 meters and removing only the uppermost part of the structure—would deplete only about 10% of fish biomass, they concluded.
Ghost Ships off California
Twenty-seven oil- and gas-drilling platforms dot the coastline of California, most of them in the Santa Barbara Channel. They were built from the 1960s through 1980s, and they’re showing their age—several are in the early stages of decommissioning. (The Los Angeles Times called one of those platforms, Holly, a “ghost ship.”)
There’s ongoing discussion about what to do with these structures as they’re decommissioned, said Erin Meyer-Gutbrod, a marine ecologist at the University of California, Santa Barbara. “That’s going to mean plugging and abandoning the wells. And then conversations will start about what to do with the infrastructure, whether to remove everything or leave some of it in place for an artificial reef.”
A “rigs to reefs” approach—turning decommissioned structures into habitats for marine creatures—has been applied extensively in the Gulf of Mexico. Such artificial reefs have been shown to be extremely productive fish habitats.
For 24 of California’s oil- and gas-drilling platforms, Meyer-Gutbrod and her colleagues analyzed the ecological impacts of three decommissioning scenarios: leaving the platform in place, removing the portion of the platform in water shallower than 26 meters, and removing the entire platform. The team relied on observations collected by divers, crewed submersibles, and bottom trawl surveys between 1995 and 2013.
Donna Schroeder, a marine ecologist at the Bureau of Ocean Energy Management in Camarillo, Calif., was instrumental in collecting some of those data. She rode to the seafloor aboard the Delta, a petite submersible painted bright yellow. “You had to [lie] down horizontally in the front of the submersible and look out the portholes,” said Schroeder. Dives could last for several hours, so drinking a lot of liquids beforehand was ill-advised, she said. “There’s not much privacy and not many options.”
Schroeder and other researchers manually counted fish around the platforms. They used estimates of the animals’ sizes and spatial density to calculate the amount of fish biomass present near different parts of each platform.
Meyer-Gutbrod and her collaborators found that blacksmith were common in the upper water column. Rockfish inhabited the lower reaches of most platforms, and assorted small fish darted among piles of mussel shells that littered the platforms’ bases. Mussels cling to oil- and gas-drilling platforms but can be dislodged by waves or intentional cleaning efforts, said Meyer-Gutbrod. “They rain down and pile up into a big mound of shells beneath the platform.” That creates new habitat, she said. “The spaces amongst the mussels make good hidey holes for the smallest fish.”
The scientists calculated that removing the platforms in their entirety would result in 83%–99% losses in fish biomass. But cutting off just the top portion of the platforms—leaving structures below a water depth of 26 meters intact—would deplete, on average, just 10% of fish biomass. That significant difference makes sense, said Meyer-Gutbrod, because there’s “all of this added habitat” from a platform’s A-frame-shaped underwater structure.
“This work is on point,” said Claire Paris-Limouzy, a biological oceanographer at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science in Virginia Key, Fla., not involved in the research. The findings make sense, she said, because the submerged portion of a platform attracts new fish. “It’s a completely different community,” said Paris-Limouzy, so removing that habitat is bound to decimate fish populations.
These results were presented in February at Ocean Sciences Meeting 2020 in San Diego. They were also shared at a public forum in Long Beach, Calif., in January. It’s important to disseminate these findings to the public, said Meyer-Gutbrod, because many of the platforms—and the marine habitats they sustain—lie very close to the shore. “We can see Holly right from the beach that we walk to from our house.”
—Katherine Kornei (@KatherineKornei), Freelance Science Journalist