Reefs De Rigueur

Fish earstones may offer a verdict on the environmental value of oil rig reefs.

Published May 26, 2015

Life swarms to an oil rig the moment its massive steel legs plunge into the ocean. Algae, barnacles, anemones, sponges, and other less-mobile creatures latch onto the hard metal structures. Darting fish soon join the fray. But petroleum wells dry up after a couple of decades, and traditionally this has spelled the end of a rig: the steel legs, and the habitat they create, are decommissioned, dismantled, and hauled away.

In 1979, the United States introduced the concept of the “rig-to-reef,” wherein an oil platform’s legs are left in the water post-decommissioning to retain the constructed ecosystem. Soon after the first rig reefs were built along Florida’s coast, these artificial ecosystems began popping up everywhere. And soon after that, the protests started—the most famous being in 1995 when Greenpeace occupied Shell’s Brent Spar platform in the North Sea for nearly a month.

There are a number of points of tension over oil rig reefs. Some are ideological or political—“All offshore oil leases were granted on the reassurance that the seafloor would be returned to as near natural conditions as possible,” says The Ocean Foundation’s Richard Charter—while some challenge the ecological value of the reefs themselves.

The central scientific dispute over rigs-to-reefs is one that has dominated the debate for 40 years: do the structures actually encourage growth of reef-dwelling species or do they merely attract marine life that’s passing by?

Without a clear understanding of rig reefs’ effects, the international response has been mixed. Following Greenpeace’s 1995 protest, plans for rig reefs in the North Sea stalled. Elsewhere, oil rig reefs blossomed. In Texas, more than 140 rigs have added to the state’s artificial reefs since 1990. Over the next 10 years more than 6,500 oil rigs are due for decommissioning. How many will be turned into reefs is still up for debate.

For their part, oil companies like rig reefs because it saves them money—to the tune of tens of millions of dollars in saved decommissioning costs—as do US state governments, which often get a kickback for some of the money saved. Environmentalists are torn.

A novel scientific technique, however, may offer a way to resolve this long-running controversy by finally giving scientists a way to determine whether reef fish treat these rigs like a home or like a hotel.

In 2008, fish ecologist Ash Fowler and his team ventured onto the water off northwestern Australia. There, four separate oil rigs were being decommissioned. When the steel structures were hoisted onto the ship’s deck, dozens of red-belted anthias spilled forth. (This seafloor-dwelling species is often found hiding in the drilling wellhead.) The team collected the fish and returned to shore, where they chemically examined each fish’s earstones, or otoliths.

Much like a tree’s trunk, otoliths grow annual rings. As these rings grow, they absorb chemicals from the surrounding environment. By firing a laser at the otoliths, Fowler and his colleagues learned that each oil rig gave the fishes’ earstones a distinctive chemical composition. The overall shape of the earstone honed the geographical marker, allowing the otolith to serve as a home address.

“Otoliths are used to identify fish stocks over regional scales, but with our technique, we could identify home [oil rig] structures over distances as short as 10 kilometers,” says Fowler.

This geolocating technique can be used for almost any artificial structure, says Fowler, because the chemical distinctions between sites are based on common elements found in seawater (rather than, for instance, the pollution that might seep from a drilling site). As such, the technique can also distinguish oil rig reef fish from those living on natural reefs nearby. If combined with a genetic screen for lineage, Fowler’s otolith technique may reveal whether fish live around the same rigs for generations, or are just passing through.

Separating oil rig fish from natural reef fish has been a major challenge for the handful of scientists who have tried to compare the ecology of rigs-to-reefs with natural environments.

In a 2003 report, researchers from the US Minerals Management Service found that a single natural reef supported more than two million fish, which would be comparable to the number found in 1,000 oil rig reefs. More recently, a 2014 study argued that California’s oil rigs have become a fertile ground for juvenile fish, making the rigs one of the most productive fish habitats in the world. But without the ability to separate permanent resident fish from visitors, properly interpreting these studies becomes difficult.

Milton Love, an ecologist who worked on the 2014 study, doubts that answering the scientific question will actually end the debate over rigs-to-reefs. Even if the rigs are responsible for creating a bountiful ecosystem, he says, it’s an artificial creation that carries the risk of introducing invasive species, such as orange cup coral or Australian spotted jellyfish into new environments. In the Gulf of Mexico, commercial fishermen love the artificial reefs because they attract red snapper and other profitable catch, but the steel legs sticking up from the seafloor strip shrimpers of their ability to trawl the seafloor. Some conservationists want the rigs removed, but that would mean the animals that live stuck to the metal frame would die and wind up as waste dumped into the sea or on shore.

“In the end,” says Love, “the decision has nothing to do with anything except your moral compass.”