Use ‘soft start’ procedures to deter marine and freshwater mammals to reduce noise exposure
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 3
Background information and definitions
Activities that produce large amounts of underwater noise, such as seismic airgun surveys, pile driving and sonar, may disturb or cause auditory injury to marine and freshwater mammals (Gordon et al. 2003, Bailey et al. 2010). ‘Soft start’ (or ‘ramp up’) procedures may be used when commencing such activities to gradually increase the sound intensity over a period of time. The aim is to deter marine and freshwater mammals from the area before the full volume is reached so that noise exposure and the risk of injury are reduced (Von Benda-Beckmann et al. 2014). For example, seismic surveys may commence with a single airgun, with additional airguns activated over a period of time until the full array is operational. However, this relies on mammals having an avoidance response. Some mammals may be attracted to initially weak sounds and thus exposed to potentially harmful levels as the sound intensity increases (Compton et al. 2008). ‘Soft-start’ procedures may also prolong the total duration of operations, possibly increasing the total amount of acoustic energy that is transmitted into the environment.
Bailey H., Senior B., Simmons D., Rusin J., Picken G. & Thompson P.M. (2010) Assessing underwater noise levels during pile-driving at an offshore windfarm and its potential effects on marine mammals. Marine Pollution Bulletin, 60, 888–897.
Von Benda-Beckmann A.M., Wensveen P.J., Kvadsheim P.H., Lam F.-P.A., Miller P.J.O., Tyack P.L. & Ainslie M.A. (2014) Modeling effectiveness of gradual increases in source level to mitigate effects of sonar on marine mammals. Conservation Biology, 28, 119–128.
Compton R., Goodwin L., Handy R. & Abbott V. (2008) A critical examination of worldwide guidelines for minimising the disturbance to marine mammals during seismic surveys. Marine Policy, 32, 255–262.
Gordon J., Gillespie D., Potter J., Frantzis A., Simmonds M.P., Swift R. & Thompson D. (2003) A review of the effects of seismic surveys on marine mammals. Marine Technology Society Journal, 37, 16–34.
Supporting evidence from individual studies
A study in 2008 in a pelagic area in the South Atlantic Ocean, Gabon (Weir 2008) found that during a ‘soft start’ procedure using seismic airguns, a pod of short-finned whales Globicephala macrorhynchus changed course and travelled in the opposite direction to the seismic vessel for several minutes before milling at the water surface or travelling parallel to the vessel. Prior to the ‘soft start’ procedure, a pod of 15 short-finned whales was observed travelling steadily northeast for 24 minutes towards the seismic vessel. Nine minutes after the ‘soft start’ procedure commenced, the whales changed course by 180° and travelled southeast away from the vessel. Three minutes later, the whales were observed milling at the water surface or travelling parallel to the vessel as it passed their location within 900 m. In March 2008, a seismic survey was conducted using a single airgun array (consisting of six airgun strings) towed at a depth of 8.5 m and a speed of 4–5 knots y a 90-m vessel. An automated ‘soft start’ procedure was used with additional airgun signals added every 51 seconds during a 30-minute period. The whale pod was located 900 m away when the ‘soft start’ commenced. An observer on board the survey vessel recorded the position and behaviour of the 15 whales for 24 minutes before and 30 minutes during the ‘soft start’ procedure.Study and other actions tested
A study in 2011 of a pelagic area in the South Pacific Ocean off the east coast of Australia (Dunlop et al. 2016) found that during ‘soft start’ procedures using seismic airguns, migrating humpback whale Megaptera novaeangliae groups slowed their speed but did not significantly alter their course. Overall, migrating humpback whale groups swam more slowly as they approached a vessel during ‘soft start’ procedures compared to before ‘soft starts’ (data reported as statistical model results). However, whale groups did not significantly alter their course during ‘soft starts’. The authors reported a similar response by whale groups to vessels without airguns firing (see original paper for details). During each of 22 trials, a ‘soft start’ procedure was carried out by a 28-m vessel towing a small experimental airgun array (six 20–150 cubic inch air guns) at a speed of 7.4 km/h across a humpback whale migration path. Airguns were progressively activated (at 2,000 psi) during four stages, in which the sound exposure level was increased in steps of 6 dB. Migrating whale groups (1–3 whales) were tracked with a theodolite from two land-based stations and observed from three small research vessels for 1-h before and 30-minutes during the ‘soft start’ procedure during each of the 22 trials in September–October 2011.Study and other actions tested
A study in 1994–2010 of multiple pelagic areas around the UK (Stone et al. 2017) found that during ‘soft start’ procedures, a greater proportion of cetaceans avoided or moved away from survey vessels compared to when airguns were not firing. A greater proportion of cetaceans (including whales, dolphins, and porpoises) avoided or moved away from survey vessels during ‘soft start’ procedures (200 of 975; 21%) than with no airguns firing (98 of 975; 10%). The same was true when the data were analysed separately for dolphins (Delphinidae) (‘soft start’: 92 of 484, 19%; not firing: 39 of 484, 8%) and Lagenorhynchus spp. only (‘soft start’: 46 of 186, 25%; not firing: 15 of 186, 8%). Data were extracted from reports made by Marine Mammal Observers on board seismic survey vessels in 1994–2010. Observations were made of marine mammals during ‘soft start’ procedures with large airgun arrays (≥50 cubic inch total volume) and during periods when airguns were not firing.Study and other actions tested