Seismic sensitivity periods.
In the sea, fish hear sounds from ships and towed gears long before they see them but tend only to avoid them at close range. In seismic surveys, exceptionally loud and repeated 'bangs' are added to the towing ship's sounds and even at several hundred metres range will create an uncomfortable sound stimulus. In some circumstances seismic surveys have been observed to cause fish to move away from and avoid the survey area. There are two concerns of seismic surveys from a fisheries standpoint.
Firstly, marine fish tend to migrate to particular areas to spawn and these chosen areas may have attributes that contribute to successful spawning. It is therefore considered reasonable to avoid seismic survey disturbance in identified spawning areas at times when it is known that spawning regularly takes place.
Secondly, seismic cables, sometimes comprising many synchronised air guns, are towed just below the sea surface. This is the zone where the eggs and larval stages of many fish species drift while they hatch and grow. Cables are towed at speeds of ca 3 metres per second (6 knots) and guns are fired frequently (10-20 seconds). The smaller larval fish and eggs, unlike adult stages, are unable to move aside from the track of the ship and at each explosion of the guns, those within 2 metres or so of each gun, will be unavoidably damaged. Studies show that at this range the very intense pressure impulse released by the air gun can cause internal rupture of tissues leading to bleeding and death. Each detonation of the multiple gun arrays may therefore kill a fresh sample of eggs and larvae and the accumulated effect where these are more densely concentrated in a breeding area may have stock consequences.
All fish will hear and may choose to avoid seismic sound. Most fish are additionally sensitive because they contain an internal air bladder, the swim bladder, which aids their buoyancy. Mackerel, flatfish, rays and sharks do not contain air bladders and are not thought to be as sensitive as other fishes. Shellfish also do not contain air bladders and commonly live on or in the sea bed. For the most part therefore, they too are not thought to be adversely affected by seismic surveys.
As a precautionary measure, regulators set exclusion windows to minimise seismic disturbance to individual species at or around the peak times and locations of spawning activity. A knowledge of sensitive spawning periods by block and by month will enable operators to plan their activities more efficiently. The accompanying monthly maps have been derived by simple area addition from the individual species maps shown earlier. There is less concern for species with very wide spawning distributions (eg pout, blue whiting, sprat and lemon sole) and they are not included in the restriction maps. Where exclusion windows cause exceptional operational difficulty there may be room for fine tuning and operators in this position should talk to the regulator or to fisheries advisors.
Many fishermen have traditional fishing grounds and local fishermen should always be consulted about seismic survey activity in order to explore possible operational and commercial sensitivities. The possibility of interaction with static gears is likely to be most important (see effort map on page 50). In addition, prior and timely consultation will allow commercial concerns related to the scaring away of nomadic and migratory fish to be properly discussed.
Production Operations Index
