Jim Tranquada

Oil platforms off the Southern California coast are some of the world’s most productive marine fish habitats, a new study has found – data that has important policy implications for the decommissioning and construction of oil platforms, wind farms and other offshore structures.

The study by marine biologists at Occidental College, UC Santa Barbara and the U.S. Bureau of Ocean Energy Management is the first to estimate rates of production for the entire community of fishes associated with oil platforms, which were then compared to previous research that made similar measurements in highly productive estuary, coastal lagoon and coral reef ecosystems.

The new study was published in the Proceedings of the National Academy of Sciences.

Using up to 15 years of annual visual surveys of sea life at 16 platforms from Orange County to Point Concepcion by Milton Love of UC Santa Barbara and BOEM’s Ann Bull, the Occidental researchers developed a model to estimate fish production at each site.

They found that the platforms tended to produce about 10 times more fish biomass (or weight)– chiefly various species of rockfish and lingcod – than other more conventional marine habitats studied in the Pacific and North Atlantic oceans, Mediterranean and North seas, the Gulf of Mexico and along the coasts of South Africa and Australia.

When compared to the fish production on natural rocky reefs at similar depths off the Southern California coast, the platforms, on average, produced more than 27 times as much fish, according to the study.

"We were surprised by how large the numbers were relative to the research done elsewhere," said Jeremy Claisse, adjunct assistant professor of biology at Occidental and study co-author. "On other hand, given the size of these structures – they range from about 150 to 650 feet tall -- and the staggering numbers of fish consistently observed there, it’s not that surprising."

Multiple lines of evidence also suggest that the offshore platforms are not simply drawing fish away from other natural habitats, but producing a net overall increase in the fish population.

The study could potentially inform decisions to be made about the inevitable decommissioning of the world’s roughly 7,500 oil and gas platforms. Rather than completely removing the structures, underwater portions could be left intact to provide habitat to supplement increasingly threatened fish populations on natural reefs.

"The most exciting thing for me is that this study could provide a basis to start thinking about how to modify new renewable energy-generating structures like wind farms or wave energy devices in ways more beneficial to marine conservation and fisheries," Claisse said. "From a biological standpoint, the fish don’t know if it’s an oil platform or the bottom of a wind turbine. Complex structures can provide a lot of different kinds of habitat to various species of fish at multiple points in their life cycle."

The researchers next hope to explore how individual platforms contribute to regional fish production in the Southern California Bight, and to better understand what makes one platform more productive than another – insights that could then be used to create fish-friendly designs for other offshore structures.

The current study was made possible by two factors: years of painstakingly gathered data on fish populations at Southern California platforms and detailed information about the biology of the individual species present. With documented rates of growth and mortality, researchers were able to estimate fish populations over time, expressed as grams per square meter per year.

"Most other studies haven’t covered such an extensive time period," Claisse said. "Having at least five and up to 15 years of data for each platform we examined was important, given how variable production rates can be year over year."

The marine biologists deliberately sought to keep their estimates on the conservative side. For example, their calculations included only those fish found within two meters of the platform structure. Fish in the substantial space between structural elements were not counted.

The study was funded with $100,000 in supplemental funding the U.S. Bureau of Ocean Energy Management, building on a previous project funded by the California Ocean Science Trust.