Measuring responses of harbour seals to potential aversive acoustic mitigation signals using controlled exposure behavioural response studies

Abstract

1. Some anthropogenic activities pose acute risks for marine species. For example, pile driving could damage the hearing of marine mammals while underwater explosions can also result in physical damage or death. Effective mitigation is required to reduce these risks, but the exclusion zones specified in regulations can extend over hundreds or thousands of metres and seals pose particular problems because they are difficult to detect at sea.2. Aversive sound mitigation aims to exclude animals from high-risk areas before dangerous activities take place by broadcasting specific acoustic signals. Field research is needed to identify signals that might be effective in eliciting short-term avoidance by marine species such as harbour seals (Phoca vitulina). A series of controlled-exposure experiments (CEEs) were undertaken to measure seal movements in response to acoustic deterrent devices (ADD) and predator calls, and to assess the effectiveness of candidate signals for aversive sound mitigation.3. Seals were fitted with UHF/GPS transmitters providing continuous high-resolution tracks and real-time transmissions of their locations. A tracking/playback vessel located seals at sea and transmitted either ADD signals or orca (Orcinus orca) calls over a range of distances while seals were foraging or moving between sites.Behaviour before, during and after exposure was analysed to assess responses. 4. One-hundred and ten CEEs were assessed as being of at least 'adequate' quality.Of the 71 adequate trials with the Lofitech ADD, all 38 at ranges of <1 km (predicted received level 134.6 dB RMS re 1 μPa) elicited a response. The maximum response range was 3123 m (predicted RL: 111 dB RMS re 1 μPa). However, the responses observed did not always result in substantial movements away from the source, especially for seals that were travelling at the time of the exposures.More work is needed to better understand how exposure risks would be reduced in different scenarios.5. The mean net speed of horizontal movements for seals responding to aversive sounds (1.15 m s −1 ) was only 7% higher than their mean travel speed.6. Responses to broadcasts of orca calls were highly variable.

Simulated seal scarer sounds scare porpoises, but not seals: species-specific responses to 12 kHz deterrence sounds

“…The lack of seal deterrence observed here is, however, in line with results from the few other studies, where effects of AHD have been tested on wild seals in contexts not related to fishery or foraging sites. Gordon et al [ 17 ] found that all seals they observed out to a distance of approximately 1000 m from a Lofitech AHD responded to the playback. However, not all animals reacted by moving directly away from the sound source and some individuals were often seen at closer ranges.…”

“…However, not all animals reacted by moving directly away from the sound source and some individuals were often seen at closer ranges. The absolute deterrence range of all animals was therefore estimated by Gordon et al [ 17 ] to be 225 m with a predicted received sound pressure level ( L eq ) of 145 dB re 1 µPa. Graham et al [ 28 ] tested the effectiveness of the Lofitech for excluding seals from Atlantic salmon rivers in Scotland in 2006 and found closest approaches of seals to be within 200 m of the AHD.…”

“…This study was conducted close to a haul out site, as in Jacobs & Terhune [ 15 ] and Götz & Janik [ 16 ], where seals may react differently than in more open water, as in the study by Gordon et al . [ 17 ]. Habituation may also influence the responsiveness.…”

“…Indeed, a few studies suggest large differences in deterrence distances of AHDs. Harbour seals and grey seals ( Halichoerus grypus ) have been seen to react at ranges from less than 100 m up to about 1 km, depending on characteristics of the device [ 15 – 17 ], whereas porpoises have been shown to react to similar AHDs at ranges of several kilometres [ 8 , 9 , 18 , 19 ]. Such differences in response ranges may constitute very large, but unintended impacts on harbour porpoises in fishery-related context where the AHD solely targets seals in order to reduce depredation.…”

Balancing risks of injury and disturbance to marine mammals when pile driving at offshore windfarms

“…ADDs appear more equivocal (Gordon et al, 2019;Mikkelsen et al, 2017). (Figure 3) or the identification of a behavioural response to ADD playbacks ( Figure 7).…”