Te 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 shoal the sound pressure level was 140 dB, which is well above the hearing threshold of cod but still below their threshold for behavioural change. Te highest sound level measured at
the hydrophone rig 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
200 190 180 170 160 150 140 130 120
21:00 00:00 03:00 06:00 Time
catch of haddock showed a sound pressure level of maximum 155 dB @ 10 km. See Figure 3.89b. 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. Ten 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. Tis 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.
3.12.4 Effect of Seismic on Fisheries
Te survey clearly indicated that fish react to the sound from the seismic guns by changing their behaviour, resulting in increased catches for some species and smaller catches for others. It appears that pollock 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. Te most probable explanation for both increased and
reduced catches for the different species and types of fishing gear is that the sound from the airguns put the fish under some stress, causing increased swimming activity. Tis 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),
144
Figure 3.89b: Sound pressure level (peak value) at a hydrophone rig deployed at 73m depth as function of airgun distance. The closest distance is around 10.1 km when the maximum sound pressure level was 155 dB re 1 µPa.
the seismic acquisition activity was concentrated within a smaller area, 81 km2 one 5,012 in3
. Te vessel was equipped with two streamers and seismic source. Tere 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 Barents Sea study. In terms of the number of shots per square kilometre per 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 60
years. Te 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. Tis is positive for fish and fisheries. In addition, the sensor systems in the seismic streamers are gradually being
Figure 3.90: Firemore Bay on the coast of north-west Scotland was the site of a 2001 study on the impact of seismic shooting on fish.
09:00 12:00
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15:00
Pressure amplitude (dB rel. 1µPa)
Løkkeborg et al (2010) Distance from source (km)
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