Some restrictions set to avoid scare during spawn
Egil Dragsund, Det Norske Veritas
Det Norske Veritas (DNV) has prepared a report that concludes seismic activities on the Norwegian continental shelf have little effect on fish. Several Norwegian fishery and oil organizations asked DNV to compile a report based on more than 20 years of scientific studies into seismic activity’s impact on fish and the fisheries industry. The organizations are the Norwegian Fisheries Association, Norwegian Oil Industry Association (OLF), Fish Export Council of Norway, and Norwegian Seafood Federation.
Studies show there is negligible direct physical damage, but that there may be a behavioral change in the vicinity of the seismic source. Seismic surveys will have an impact on fish behavior, but the reported magnitude of an area covered by this impact is variable. To avoid scaring effects in spawning areas, seismic activity time restrictions have been introduced for and in areas where concentrated spawning migrations take place.
In seismic surveys, a sound pulse is emitted from a source towed behind the survey vessel. This pulse reflects off boundaries separating subsurface geological layers, and the reflected signals are recorded by hydrophones towed in a cable (2D) or several cables (3D) several kilometers long.
The report concludes that physical damage to fish caused by the sound from seismic sources only occurs within less than a few meters of the air guns. Adult fish will flee from the sound, but eggs and larvae within that distance may be affected by the signals. The effects on eggs and larvae leads to a mortality rate per day of less than 0.03%, insignificant compared to the natural mortality rate of 5–15 % per day for most species at these early life stages.
Public concern
Seismic surveys and their possible effects on fish and the fisheries industry have been given a great deal of media coverage lately in Norway. A number of stakeholders are claiming that the sea is being carpet bombed and that seismic surveys seriously affect fish stocks.
Such public concerns are expected in such a complex matter as oil and gas exploration, and that is why the organizations wanted an objective report on seismic impact.
Seismic surveys are the initial tool used to locate hydrocarbon reservoirs under the surface of the earth. The data from seismic surveys can be used to map the reservoirs and define possible drilling. Different types of seismic data are needed at the various stages of the activities, from the early exploration phase to the development and production of potential reserves in a field. It may, therefore, be necessary to repeat seismic data collection several times in the same area, but with different geographic coverage and time periods. The individual surveys may take from a few days to several weeks, depending on the purpose of the survey and size of the sea area to be surveyed.
May frighten fish
Several studies have documented that adult fish seem to be frightened by the sound waves from seismic activity, and pelagic fish seem to be the most sensitive. The scare effect has been demonstrated in a radius from 1 km (0.6 mi) and up to more than 30 km (18.6 mi) from the sound source. One study observing cod, pollock, coalfish, and whiting on a bank in sheltered, shallow waters, found no systematic migration away from either the airgun or the bank. Another study conducted in Australia in 1996-1999 found scare effects out to distances of 1-2 km (0.6-1.2 mi) from the seismic vessel.
In a 1984 Norwegian study on Gullfaks field in the North Sea, it was observed that the quantity of fish — mainly cod and pollock — was reduced by 36% during seismic activity. The study could not conclude if this was caused by fish migrating out of the seismic area or by fish moving so close to the bottom that they could not be observed using echo sounders. Another Norwegian study also observed that herring and blue whiting moved to lower depths in areas where 3D seismic data was collected.
Sketch of a Bolt-PAR airgun cross-section before firing (left) and after firing (right). Figure courtesy www.bolt-technology.com
A large-scale Norwegian study in 1992 on the North Cape bank in the Barents Sea mapped the extent and duration of the effects of seismic survey activities on fish quantities in the area and on the catch rates for commercial species. The seismic shooting took place over a period of five days within an area of 3 x 10 nautical mi (5.5 x 18.5 km). In addition to trawling, catches were made using lines before, during, and after shooting, and fish quantities were measured using acoustic mapping in the same periods.
The catch rates during the shooting were found to decrease over a radius of 18 nautical mi (33 km) from the seismic area. The acoustic measurements that showed changes in the quantity of fish were found to decline in the same ratio as the reductions in catches.
There are insufficient data to determine when fish that have been frightened by airguns firing return to an area they have abandoned, or again become available for catching. The effects are considered to be limited geographically, while local catch reductions seem significant.
Effects on farmed fish
A study of the effects of seismic surveys on rainbow trout in fish farms reported no significant behavioral changes during seismic activities. The fish consumed normal amounts of food during the entire experiment. The sound pressure level from the sound source was approximately 229 dB at 1 m (3.3 ft) from the airguns. The sound waves were measured at two locations: 142 dB at a distance of 4,000 m (13,123 ft) at the fish farm and 186 dB at a distance of 150 m (492 ft) from the airguns. The fish in the fish farm were monitored using video cameras and appeared to remain calm throughout the experiment.
Seismic sound source
In seismic surveys, a strong sound pulse is emitted from the sound source towed behind the survey vessel. This pulse is reflected from the boundaries separating the geological layers in the subsurface, and the reflected signals are recorded by hydrophones towed in a cable (2D) or several cables (3D) several kilometers long.
Today, airguns are the predominant sound source. Air at 140 atm (14 MPa), the most commonly used supply pressure, is released quickly into the water through the gun portals to create a pressure wave in the water.
Sound pressure is commonly expressed in decibels (dB), which is a logarithmical ratio in relation to a reference level. Due to different reference levels in air and water, dB values cannot be directly compared, but in general, 160 dB in air (threshold for direct damage) corresponds to 222 dB in water. In water, a change of 6 dB is equivalent to a doubling or halving of the sound pressure, while a change of 20 dB indicates that the pressure changes by a factor of 10. A typical seismic pulse has a maximum spectral pressure amplitude of 210 dB 1 m (3.3 ft) from the source.
Fish hearing and seismic sound
Sound has a dualistic nature and may be described as fluctuations in pressure or particle fluctuations in a medium. The relevant stimulus parameter regarding perception of sound by organisms varies from species to species. While marine mammals are sensitive to sound pressure, the inner ear in fish is sensitive to particle movement, and the relevant stimulus parameter is particle acceleration. Fish with swim bladders, e.g. herring and cod, can, however, also detect sound pressure since the swim bladder acts as a converter between pressure and movement. Fish without swim bladders, e.g. flounder and mackerel, are not sensitive to sound pressure.
Far from the sound source (25 m [82 ft] or more), there is a constant ratio between the pressure component and kinetic component of the sound. Closer to the sound source, the kinetic component is relatively stronger. Therefore it is likely that many of the harmful effects observed on organisms close to the sound source are due to particle acceleration and not due to sound pressure. It is important to be aware that sound with the same sound pressure may be more harmful at short distances than longer ones.
In a workshop in Halifax in 1985, it was concluded that mortality can occur when the sound pressure pulse has a peak pressure exceeding 229 dB and the rise and decay time of the sound wave is less than 1 msec. Detonations of chemical explosives are needed to reach such rise and decay times. Airguns have a much slower rise time, and therefore are not as dangerous with regard to potentially lethal damage.
The environmental impact of seismic surveys has been studied for many years by a range of institutes and organizations. Several of these have analyzed the conditions leading to direct damage to fish from high-level sound.
An Australian study of pink snapper found serious damage to hearing sensor cells. The fish were kept in cages and the seismic vessel towed the airguns from start-up at 400-800 m (1,312-2,624 ft) away to 5-15 m (16-49 ft) at closest approach to the cage. Some of the observations were confusing. After the second exposure using the same fish, no effects were found. One of the researchers did similar studies on three other species and found no permanent effects on the hearing and no damage.
About the Author
Egil Dragsund is a marine biology graduate from the University of Oslo, Norway. He is working in DNV’s energy section as a principal consultant, mainly within environmental impact and risk assessments.
