“I know that the human being and the fish can coexist peacefully.” George W. Bush, 43rd U.S. President (1946- ), Saginaw, Michigan; September 29, 2000
There is growing interest in understanding the effects of human-generated sound on fish and other aquatic organisms. The sources of anthropogenic sounds are many. Boats and ships are a major source of noise, and sonars, not only used by navies but also by the shipping and fishing industries and the oceanographic research community, are a significant sound source. Pile driving and seismic airgun arrays are also high-intensity sources.
When a seismic survey is conducted, fish in the area hear the sound of the air guns and will often react to it, but how they react will depend on many factors, and it is difficult to conduct unambiguous studies in order to map the reaction patterns. Here, we summarize the most recent and, to date, the largest study performed by the Norwegian Institute of Marine Research (IMR) on the effect of seismic surveying on fisheries.
Studies on Fish and Fisheries
Many studies have been conducted on the effect of seismic signals on fish in all stages of life. These include studies of possible direct damage in the very early stages (eggs and fry), but since adult fish can move away from the sound source if they are disturbed, the studies for this group mostly concern behavioral effects.
Organisms can be damaged when exposed to sound pulses with a rapid rise time (i.e. rapidly increasing sound pressure) and a peak value of 230 dB or more. Sound pulses from air guns will often have a relatively slow rise time, and for this reason organisms can tolerate a higher peak pressure from these than from, for instance, underwater explosions. Sound pressures with a peak pressure of more than 230 dB will only occur in the immediate vicinity of the air guns, within a radius of just a few meters.
Dalen et al (1996) concluded that there is such a small amount of eggs and fry present within the danger zone that damage caused by air guns will have no negative consequences for the fish stocks. They calculated that the mortality caused by air guns might amount to an average of 0.0012% a day. In comparison to the natural mortality rate of 5–15% a day, the effects of seismic-induced damage seem insignificant.
The interaction between petroleum exploration and fish is a central issue in Norway, Australia and Brazil in particular. In Norway, the IMR executes an oil-fish programme with the objective of generating new knowledge about the acute and long-term effects on fish and other marine organisms of discharges of oil to the sea and chemicals used in drilling and production. It also seeks to obtain new knowledge about the effects of seismic shooting on fish and other marine organisms.
The Lofoten and Vesterålen Survey
From 29 June to 6 August 2009 the Norwegian Petroleum Directorate (NPD) carried out a 3D seismic survey (approximately 15 x 85 km2) outside Vesterålen in an area known as Nordland VII, about 250 km south-west of Tromsø in the Norwegian Sea. The period was chosen based on advice from the Directorate of Fisheries, the IMR and the fishermen’s organisations, with the aim of conducting the survey during a period when the seismic acquisition activity would cause as little inconvenience as possible to the fishing industry, and avoiding spawning periods.
The seismic vessel was equipped with eight streamers and two seismic sources, each with a total volume of 3,500 cu.in. (57 l). The sources were fired alternatively in a “flip-flop” configuration at 138 bar (2,000 psi) every 10s. A total of 41 lines, around 85 km long and 400m apart, were shot.
At the same time, the NPD initiated and funded a NOK 25 million research project, one of the largest ever conducted, with the task of examining the consequences of seismic data acquisition on the presence of the fish species normally caught in this area: Greenland halibut, redfish, ling, pollack, haddock and saithe (all hearing generalists). The institute commenced its studies 12 days prior to the start-up of the seismic data acquisition, which took 38 days, and continued for 25 days after the activities stopped. As the chartered gillnet and longline vessels were fishing, a research vessel worked with another chartered fishing vessel to map the occurrence of fish and plankton in the area using echo sounders and sonar. In addition, stomach specimens were taken from the catches, and recordings were made of the sound from the air guns on the seismic vessel.
Sound Levels Recorded
The airgun sound pressure was measured at the sea floor by deploying a hydrophone rig at various locations. The figure shows the sound pressure level from one seismic survey line relative to the distance from the airgun array. The hydrophone rig was at 184m depth. At the start of the line, about 46 km from the rig, the sound pressure level was measured at 140 dB re 1 μPa, and it kept almost constant until the vessel was at 30 km distance. As the vessel approached, the sound pressure level steadily increased up to 170 dB @ 6 km. Then the level increased more rapidly and the maximum at 191 dB was obtained when the vessel passed close at distance of 500m. The sound level turned out to be somewhat higher with distance after the vessel passed than that when it approached the rig, probably due to variation in water depth. (The shallower the water, the stronger will be the acoustic signal recorded in the water column).
The measurements show that the fish in the survey area were exposed to varying levels of sound pressures, depending on their distance from the seismic vessel. At 30 km distance from a fish field the sound pressure level was 140 dB, which is well above the hearing threshold of codfish but still below their threshold for behavioural change.
The highest sound level measured at the hydrophone rig at a depth of 184m was 191 dB, when the vessel passed at a distance of 500m. At this level, fish at this distance would be expected to react strongly if they have not already chosen to swim away, with known reactions including increased swimming activity, startle responses, changes in schooling behaviour, and vertical movement.
Sound measurements in the area of line catch of haddock showed a sound pressure level of maximum 155 dB @ 10 km. Fish can hear this level but it will probably not induce behavioural changes. For several weeks the seismic vessel operated many kilometres away from the haddock lines, so the fish were first exposed to low sound levels over a long time. Then the vessels approached the catch area, and the sound level gradually increased.
Fish are known to adjust to external influences. For instance, a novel sound in their environment, like seismic, may initially be distracting, but after becoming accustomed to it their response to it will diminish. This decrease in response to a stimulus after repeated presentations is called habituation.
Habituation may have led to the higher response levels for haddock. However, lower line catch rates for haddock as the seismic vessel approached the lines indicate that the haddock reacted to the seismic sound at closer distances.
Effect of Seismic on Fisheries
The survey clearly indicated that fish react to the sound from the seismic guns by changing their behavior, resulting in increased catches for some species and smaller catches for others. It appears that pollack and parts of the schools of saithe migrated out of the area, while other species seemed to remain. Analyses of the stomach contents in the fish caught did not reveal changes attributable to the seismic survey. Neither were any changes in the distribution of plankton proven during the seismic data acquisition.
The most probable explanation for both increased and reduced catches for the different species and types of fishing gear is that the sound from the air guns put the fish under some stress, causing increased swimming activity. This would, for example, explain why Greenland halibut, redfish and ling were more likely to go into the net, while long line catches of the same species declined.
However, the results of this study, showing few negative effects of seismic shooting, deviate from the results of previous studies, which demonstrated considerable reductions in the catch rates for trawl and line fishing. In research from the North Cape Bank in the Barents Sea in 1992, reported in Engås et al (1996, 2002), the seismic acquisition activity was concentrated within a smaller area, 81 km2. The vessel was equipped with two streamers and one 5,012 cu.in. seismic source. There were 36 sail lines, around 18.5 km long, with a separation of 125m, compared to 450m for the Vesterålen survey, entailing a stronger and more continuous sound impact on the fish than in the Vesterålen study. In terms of the number of shots per square kilometer and hour, the sound influence was approximately 19 times higher in the 1992 survey than in the Vesterålen survey (Løkkeborg et al, 2010).
Seismic has been acquired offshore Norway for almost 50 years. The technology has become more sophisticated since the 1990s, largely as a result of the number of streamers that seismic vessels can tow. A high number of streamers implies that the sail lines have larger spacing; thereby, the number of shots in the area is reduced. This is positive for fish and fisheries. In addition, the sensor systems in the seismic streamers are gradually being perfected, so that streamers can be towed at greater depths where ambient ocean noise is less, thereby perhaps accomplishing seismic shooting with lower decibels of sound. We will discuss the issue of seismic and fish further in a later edition of GEO ExPro.
J Dalen et al, 2008. Kunnskapsstatus og forskningsbehov med hensyn til skremmeeffekter og skadevirkninger av seismiske lydbølger på fisk og sjøpattedyr: Rapport til Oljedirektoratet, Fiskeridirektoratet og Statens Forurensningstilsyn fra spesielt nedsatt forskergruppe
A Engås and S Løkkeborg, 2002. Effects of seismic shooting and vesselgenerated noise on fish behaviour and catch rates: Bioacoustics 12, 313–15.
I Gausland, 2003. Seismic Surveys Impact on Fish and Fisheries: Norwegian Oil Industry Association (OLF)
S Løkkeborg et al, 2010. Effects of seismic surveys on fish distribution and catch rates of gillnets and longlines in Vesterålen in summer 2009: Fisken og havet no 2
A N Popper and M C Hastings, 2009. The effects of human-generated sound on fish: Integrative Zoology 4, 43-52