Seismic Fish

Scientists have the first evidence of how seismic testing disrupts fish colonies on reefs.

MOREHEAD CITY — Seismic surveying emits underwater noise to detect oil and gas deposits beneath the sea floor. Scientists know the noise affects whales and dolphins, but now there is new evidence showing how it disrupts fish using the reefs. UNC-TV Science's Frank Graff talks with Avery Paxton, marine ecologist and Ph.D. candidate with the UNC Institute of Marine Sciences, to learn more. Read the interview below.

The graphic below shows researchers' images of the reef before and after a seismic test.

Reef Before and After

Avery Paxton: The reefs off the coast of North Carolina come in so many different shapes and sizes. Some are rocky reefs that are taller than you and I. Others are shipwrecks or structures that are purposely sunk to form artificial reefs. And all of these reefs form incredible habitats that are home to fish. Fish use them to find food sources, they hide from predators, they use them as nursery grounds. They are really valuable.

This particular reef that we studied is about 30 miles out of Beaufort Inlet, so 30 miles off the middle part of the coast and it is absolutely incredible underwater. When you dive down 100 feet to this reef you’re first met with this rocky structure. It’s taller than I am in places and there’s lots of overhang, and lots of crevices and it’s usually alive with marine life. It has schools of silvery fish, usually crowned by big predators. And there’s colorful critters growing on the reef itself.

Frank Graff: So along comes seismic testing. Explain that: Why are they doing it? What are they looking for?

Paxton: Seismic surveying is a method that’s used for geological research as well as to detect oil and gas beneath the surface. And this particular survey was for geological purposes. What it does is emit loud sounds under water. You can think of them as grenade explosions. So underwater explosions and these sounds are loud. And we wanted to know what happens to the fish when they are exposed to this noise. When you typically think of seismic surveying you think of how it can effect marine mammals, such as whales and dolphins. But we always wondered, what about the fish?

Graff: So what happened? How did you do the research?

Paxton: Our divers couldn’t go underneath the water because of the noise so beforehand we put underwater microphones (we call them hydrophones) and cameras on the reef. And what we found when we picked these instruments up after the survey was done was that four out of five fish were no longer accounted for on the reef.

We don’t know what happened to the fish, we don’t have any after data. But what we hypothesize is that the fish moved from the noisy reef to a nearby quieter reef or perhaps they hid in cracks and crevices in the reef until the survey vessel moved by.

Graff:  So I’ve heard that it’s been documented that where this kind of testing is being done it’s several days before the fish come back. That sounds possible from what you’re saying.

Paxton: During the seismic surveying there’s a 78 percent decline in the number of fish on the reef. And what was interesting was that for each of three days before the survey, the number of fish was relatively low in the morning and afternoon and the number of fish peaked in the evening. So the fish were all congregating on the reef. 

But we saw a different pattern on days with seismic surveying. We saw that the fish stayed low throughout the morning and afternoon and did not peak in the evening like they did before. As so that’s where we saw the greatest decrease in numbers on the reef.

Graff: So why is that a concern, or is that a concern that the reefs are empty for a period of time?

Paxton: The fact that the fish left the reef in such high numbers is really concerning. It raises conservation concerns because these fish are so important for the coastal citizens of North Carolina. Fishermen and fisherwomen, divers and the tourism industry depend on it. And again we don’t have the after data so we don’t know how long this effect of lower fish numbers on the reef lasted. We assume that the fish came back but we just don’t know. So in the future we definitely want to follow up on this research and find out where the fish went, why they left and when they came back. 

Graff: Because noise travels so well underwater, any sense of how far the boom is heard?

Paxton: On our reefs there was one reef that was 0.7 kilometers away. On that particular reef the noise level was over 170 decibels underwater. To compare that, that’s a similar level to a rock band or being near a plane engine. But the levels aren’t easily comparable between air and water because it’s a different medium so it’s difficult to pinpoint exactly what that sounds like on our land. 

Graff: What’s the takeaway from this? Sounds like we have to keep in mind the effects on fish.

Paxton: Certainly. From our research our main finding is that when we move forward with seismic surveys and consider allowing different surveys to occur we need to remember the fish. We need to not ignore them and think about how they might be influenced by these loud noises. 


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