Seismic testing, a controversial method used to map the ocean floor, has been shown to impact to marine mammals. But how does it effect fish? Some local scientist set out to discover how fish react to testing.
It sounds like an explosion, but it’s actually an underwater recording of a seismic airgun. They can be heard more than a thousand miles from the source and it’s known to impact marine mammals. The question is, does it effect fish? Seismic surveying is a process used to map the geologic structure of the seafloor to find oil and natural gas deposits. It’s also used to determine the best location to install offshore renewable energy infrastructure and where to extract sand for beach nourishment. The method has drawn criticism from environmentalists and conservationist concerned the pulses, comparable to the intensity of exploding dynamite, can negatively impact sea life. Associate Professor at the Duke Marine Lab in Beaufort Douglas Nowacek.
“We do know from several studies that what we see in some of the toothed whales, some of the eco-locaters, so the sperm whales, the pilot whales, sperm whales for sure from some experiments in the Gulf of Mexico, they reduce their feeding rate when exposed to seismic surveys.”
Past studies have focused on how seismic testing affects whales and other marine mammals’ ability to communicate, hunt for food and reproduce. However, little is known on how fish species are affected, until now. Marine Ecologist and PhD candidate at the University of North Carolina’s Institute of Marine Sciences Avery Paxton was the lead author of a two year study observing how fish respond to seismic testing.
"And in the fall of 2014, I was studying reefs off of our coast when we had a really rare opportunity. A seismic survey was going to be conducted off our coastline near this reef I was studying. And we were able to monitor it.”
In September 2014, seismic testing was conducted off our coast to better understand earthquake potential. Usually seismic testing occurs in deep water, but this survey was going to occur about 30 miles out of Beaufort Inlet at a site known as 210 Rock. The tropical looking reefs are composed of a natural, rocky hard bottom about six feet high and submerged in about 100 feet of water. Federally designated as Essential Fish Habitat, colorful corals, sponges and other invertebrates cover the surface.
“This particular reef is a fabulous place for fish to call their home. The fish use it for a nursery ground. They also use it as a place to forage, they eat lots of plants and animals that are associated with or growing on the reef. And they also use the reef likely as places to reproduce and to live, to escape from predators.”
Three days before seismic testing began, divers placed underwater microphones, called hydrophones, as well as video cameras to record sounds and what fish on the reefs were doing. A hydrophone was placed less than a half a mile away from the closest approach of the surveying vessel’s path on a natural, rocky reef, another hydrophone four miles away on an artificial reef, and a GoPro video camera about five miles away on a natural reef. Since divers aren’t allowed in the water during the survey, the camera was set up to record 10 second videos every 20 minutes. The hydrophones capturing continuous audio.
This is the reef before seismic surveying, the sound of fish and shrimp feeding. Paxton and a team of researchers with the Duke Marine Lab and the National Oceanic and Atmospheric Administration’s National Ocean Service were able to use the information collected at the reef to count and identify fish.
“Three days before the seismic survey, the fish exhibited a pattern that was repeated. And that pattern was that in the morning and early afternoon hours, the number of fish stayed about constant. And then during the evening, their numbers peaked, up into about 300 fish.”
Paxton calls the congregation of fish “happy hour.”
“And so what happened on the day when the seismic noises were present, is that the abundance was lower throughout the day and never peaked in evening hour.”
With cameras rolling and audio recording as seismic testing started, Paxton says they observed many of the fish leave the area. Reef abundance declined by 78%.
“What was really interesting about this decrease is that it happened during the evening hours when before the seismic survey was in the area, the fish were most abundant on the reef. So phrased another way, four out of every five fish were no longer accounted for on the reef.”
According to the study, seismic blasts recorded by the hydrophone, less than a half a mile from the surveying area, exceeded 170 dB. In comparison, 150 dB is the noise level of a jet taking off at 25 meters. In humans, exposure at this level can cause eardrum rupture and permanent hearing loss. Dr. Jill Lewendowski is the Environmental Assessment Chief in the Office of Environmental Programs Bureau of Ocean Energy Management. After reviewing the report submitted to BOEM, she says the levels recorded at the reef site were very basic, arguing more advanced techniques would be preferred.
“You know, not all microphones are created alike. So I would say the same is for hydrophones. You have to… very carefully determine for the purposes of what you’re doing which kinds of hydrophones you use, how they’re placed in your study sites, what methods after that you’re using to interpret your data.”
Dr. Lewendowski has read the report and watched the videos that show how the fish vacate the reef during seismic testing.
“There clearly is a response by the fish. The problem, or the challenge we have is, mostly in a study, you would want more than one sample size. So this is sort of one point in time. And you would want to have a good before, during and after data amount of data so you can understand what was happening before, what was happening during and what was happening after.”
Due to an equipment malfunction, Marine Ecologist and lead author of the study Avery Paxton says they weren’t able to document if and when fish returned to the reefs.
“We don’t know the duration of this decrease in fish and how long it lasted. We suspect that the fish came back to the reef probably pretty quickly, but we don’t again know what time scale that was on. We think that the fish likely hid in nooks and crannies on the reef or moved from the noisy reef to a nearby quieter reef.”
Despite the absence of this data, Paxton says the study provides evidence of a significant reduction in fish due to seismic testing, representing a disruption to daily pattern. This, she says, means further studies are needed to understand where the fish are going, how long they leave a particular area and why they leave in such high numbers.
“I think in this instance, we have a snapshot of something. And what we really need is more information to sort of fill that picture out.”
Dr. Lewendowski says BOEM will continue to work with their partners, including the Institute of Marine Sciences, Duke Marine Lab, and NOAA’s National Ocean Service. She cites an upcoming survey with NOAA that seeks to better understand auditory thresholds for commercially important fish such as black sea bass. BOEM has funded more than $50 million in surveys, including Paxton’s study, to explore the affects sound has on marine mammals, sea turtles, fish and invertebrates. Currently, there are no authorized air gun surveys in the Atlantic Ocean due to a decision by the Obama administration. However, that could change.
Hydrophone sample before seismic testing. (Credit: Avery Paxton)
Hydrophone sample during seismic testing. (Credit: Avery Paxton)
Reef video before seismic testing. (Credit: Avery Paxton)
Reef video after seismic testing. At 3 seconds, a seismic airgun can be heard from about five miles away. (Credit: Avery Paxton)