Seal Scarers Research Articles

Effectiveness of Targeted Acoustic Startle Technology on seal depredation in an inshore gillnet fishery

Fisheries interactions are recognised as the dominant threat to pinnipeds (bycatch) worldwide and can cause substantial economic losses to coastal fisheries. Conventional anti-depredation acoustic deterrent devices (ADDs, ‘seal scarers’) for mitigating seal depredation generally operate at much higher source levels than ‘gillnet pingers’ (for odontocete bycatch reduction) and these devices can pose a risk of causing hearing damage and habitat exclusion in non-target species. Non-lethal deterrents for depredation reduction in commercial sea fisheries are therefore limited. Furthermore, seals have shown habituation to conventional acoustic deterrents particularly where food motivation is involved. This study tested the effectiveness of the Genuswave acoustic startle device (ASD) in reducing seal-fishery interactions in an inshore gillnet fishery for mackerel. The Genuswave ASD uses the Targeted Acoustic Startle Technology (TAST) which has been shown to avoid habituation by eliciting an autonomous reflex in seals with no adverse effects occurring on non-target species (such as harbour porpoise). This is achieved by using short, isolated startle-eliciting signals emitted at a low duty cycle (0.6–1.0 % per unit) in a frequency band where seals are more sensitive than odontocetes (the species group with most sensitive underwater hearing). Trials were undertaken by two commercial fishing vessels and represent the first time the device has been deployed fully submersed in a static net fishery. Total catch weight, landed (saleable) catch weight and numbers of damaged and depredated fish were recorded for ‘control’ nets that were fished normally without any ASD units, and ‘test’ nets with one or more units deployed. We used a Generalised Linear Mixed Models framework to explore the effectiveness of the ASD at reducing seal depredation on the test nets compared with the control nets. The models showed that the deployment of the ASD units increased total catch weight in the test net by 74 %, or a factor of 1.74 (eβ:1.74, CI [1.054, 2.887], p=0.03). Similarly, the analysis model for landed (saleable) catch indicated a 72 % increase in saleable catch in the test net compared to the control net (eβ: 1.724, CI [1.052, 2.826], p=0.031). In addition, we found evidence that efficacy can be further increased by adding additional units to the net and/or increasing duty cycle per unit. This reflects better coverage along the length of the net while giving seals less time to forage on the net between emitted signals. Deployment of the Genuswave ASD demonstrated its potential for mitigating seal depredation in the inshore gillnet fishery in Torbay. An adjustment of the duty cycle and the number of deployed units per net should allow this positive effect in catch to be increased further. Further improvements and testing are required for the ASD to be considered a viable non-lethal seal deterrent for wild capture fisheries; however, this study has demonstrated that it shows promise and further development should be explored.

Response of Eurasian otters (Lutra lutra) to underwater acoustic harassment device sounds

Seal scarers (or acoustic harassment devices, AHDs) are designed to deter seals from fishing gear and aquaculture operations, as well as to prevent seals from entering rivers to avoid predation on valuable fish. Our study investigated the potential effects of AHDs on non-target species, specifically the Eurasian otters (Lutra lutra), by testing the reaction of two rehabilitated otters to simulated AHDs sounds at 1 and 14 kHz, with a received sound intensity of 105–145 dB re 1 µPa rms. The 1 kHz sounds were used to investigate alternative frequencies for scaring seals without scaring otters. The otters reacted to both 1 and 14 kHz tonal signals when retrieving fish from a feeding station 0.8 m below the surface. Their diving behaviour and time to extract food progressively increased as sound intensity increased for all tested sound levels. Notably, the sound levels used in our tests were significantly lower (40–80 dB) than the source levels from commercial AHDs. These findings highlight the importance of caution when using AHDs in river and sea habitats inhabited by otters, as AHDs can change their behaviour and potentially result in habitat exclusion.

Response of harbor porpoises (Phocoena phocoena) to different types of acoustic harassment devices and subsequent piling during the construction of offshore wind farms

Before piling of offshore wind farm foundations, acoustic harassment devices (AHDs) are used to drive harbor porpoises out of the area where they could suffer injuries. Until 2017, a combination of pingers and seal scarer devices (usually SPL = 174-193 dB re 1 μPa (rms) @ 1m at 1 to 20 kHz depending on the device) was prescribed for mitigation purposes in Germany. However, seal scarers led to decreased porpoise detection rates in much larger distances than intended, when 750 m is usually rendered sufficient to avoid injuries. Therefore, devices specifically designed for mitigation purposes were developed and are prescribed since then. These acoustic porpoise deterrents (APDs; e.g. FaunaGuard Porpoise Module; SPL = 172 dB re 1 μPa (rms) @ 1m at 60 to 150 kHz) aim to keep the animals away from offshore construction sites but should not lead to large-scale disturbance as caused by a seal scarer. Although project-specific evaluations indicated that APDs are effective, a cross-project analysis and a comparison with data from previous piling procedures employing seal scarers were still pending. The present study aimed to fill this gap. Between March 2018 and April 2019, harbor porpoise detection rates were monitored acoustically in four offshore wind farm projects using CPODs before, during and after piling at different distances up to 10 km from piling. APD operation led to a significant decrease in detection rates in the vicinity of the device, indicating the displacement of the animals from a small-scale area. Depending on the wind farm, detection rates during APD operation decreased by 30 to 100% at 750 m distance compared to 6 hours before APD operation. Furthermore, reduced detection rates during APD operation were only observed up to about 2.5 km distance even when the APD was switched on for over 40 minutes. Given that the extent of disturbance to harbor porpoises is lower when using an acoustic porpoise deterrent compared to the seal scarer, we consider that preferential use of an acoustic porpoise deterrent is an improvement to mitigation strategies and an important step forward to a less harmful piling procedure.

Minke whales Balaenoptera acutorostrata avoid a 15 kHz acoustic deterrent device (ADD)

Underwater noise has increased globally, yet our understanding of how baleen whales may be affected is limited. A controlled exposure experiment was used to determine whether an acoustic deterrent device (ADD) would modify the behaviour of minke whalesBalaenoptera acutorostratain Iceland. The Lofitech Seal Scarer in this study emitted a 14.6 kHz tone with a source level of 198 dB re 1 μPa re 1 m (rms). Video-range tracking was used to monitor behaviour during pre-treatment, control, treatment and post-treatment phases, with the ADD deployed at 338-1562 m during treatment (n = 10). Dive duration, swimming speed, reoxygenation rate and measures of path predictability were used to quantify responses. In all deployments, the ADD caused focal animals to move away immediately, typically increasing horizontal speed and directness away from the source and extending dives. Whales exhibited sustained movement away after deactivation. Deployment distance had the largest effect in a multiple regression model; closer deployments caused whales to move away faster. However, increases in dive duration and horizontal speed were apparently caused by the signal alone, rather than its magnitude. A noise propagation model suggested whales received maximum sound pressures of 150 dB re 1 μPa (rms) for the closest deployment (338 m). When modelling cumulative exposure over 24 h, deploying the ADD 25 m or more from individuals moving away at the average speed of 3.4 km h-1did not risk injury; temporary threshold shifts may be expected if deployed any closer. This study provides evidence that minke whales clearly react to signals at the likely upper limit of their hearing sensitivity.

The use of seal scarers as a protective mitigation measure can induce hearing impairment in harbour porpoises.

Acoustic deterrent devices (ADDs) are used to deter seals from aquacultures but exposure of harbour porpoises (Phocoena phocoena) occurs as a side-effect. At construction sites, by contrast, ADDs are used to deter harbour porpoises from the zone in which pile driving noise can induce temporary threshold shifts (TTSs). ADDs emit such high pressure levels that there is concern that ADDs themselves may induce a TTS. A harbour porpoise in human care was exposed to an artificial ADD signal with a peak frequency of 14 kHz. A significant TTS was found, measured by auditory evoked potentials, with an onset of 142 dB re 1 μPa2s at 20 kHz and 147 dB re 1 μPa2s at 28 kHz. The authors therefore strongly recommend to gradually increase and down regulate source levels of ADDs to the desired deterrence range. However, further research is needed to develop a reliable relationship between received levels and deterrence.

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

Acoustic harassment devices (AHD) or ‘seal scarers’ are used extensively, not only to deter seals from fisheries, but also as mitigation tools to deter marine mammals from potentially harmful sound sources, such as offshore pile driving. To test the effectiveness of AHDs, we conducted two studies with similar experimental set-ups on two key species: harbour porpoises and harbour seals. We exposed animals to 500 ms tone bursts at 12 kHz simulating that of an AHD (Lofitech), but with reduced output levels (source peak-to-peak level of 165 dB re 1 µPa). Animals were localized with a theodolite before, during and after sound exposures. In total, 12 sound exposures were conducted to porpoises and 13 exposures to seals. Porpoises were found to exhibit avoidance reactions out to ranges of 525 m from the sound source. Contrary to this, seal observations increased during sound exposure within 100 m of the loudspeaker. We thereby demonstrate that porpoises and seals respond very differently to AHD sounds. This has important implications for application of AHDs in multi-species habitats, as sound levels required to deter less sensitive species (seals) can lead to excessive and unwanted large deterrence ranges on more sensitive species (porpoises).

Noise Exposure Criteria for Harbor Porpoises.

Despite a major research effort, no generally accepted exposure limits are available for harbor porpoises. Recent studies of the temporary threshold shift (TTS) in porpoises indicate that the sound exposure levels (SELs) required to induce low levels of TTS depend on stimulus frequency and roughly parallel the shape of the audiogram. A number of studies on behavioral avoidance reactions (negative phonotaxis) to pingers, seal scarers, and pile driving show a similar dependence on stimulus frequency. Both TTS and behavioral data suggest that weighting sound pressure levels with a filter function resembling the inverted audiogram would be appropriate.

Hearing thresholds of harbor seals (Phoca vitulina) for playbacks of seal scarer signals, and effects of the signals on behavior

Acoustic mitigation devices (AMDs) are used to deter marine mammals from construction sites to prevent hearing injury by offshore pile-driving noise. In order to quantify the distance at which AMDs designed as ‘seal scarers’ are detected by seals, the 50% hearing thresholds for playbacks of their sounds were determined. The broadband hearing threshold sound pressure levels (SPLs) of two harbor seals for signals from two AMDs were similar (63–69 dB re 1 μPa, rms). Effects of the AMDs on the seals’ behavior were quantified at three SPLs: one which just did not cause behavioral changes, one which caused one of the seals to haul out occasionally, and one which caused one of the seals to haul out more than 10% of the time. The corresponding mean received SPLs were, respectively, Ace Aquatec: 109, 124, and 134 dB re 1 μPa, rms; Lofitech: 128, 133, and 138 dB re 1 μPa, rms. Thus at similar received levels, the Ace Aquatec seems more effective than the Lofitech in deterring harbor seals. Detection and behavioral response distances of the AMD sounds for harbor seals at sea can be estimated by combining the results from the present study with data on AMD source levels, propagation conditions, and background noise levels near construction sites.

Hearing thresholds of a harbor porpoise (Phocoena phocoena) for playbacks of seal scarer signals, and effects of the signals on behavior

Acoustic Mitigation Devices (AMDs) are used to deter marine mammals from construction sites, in order to prevent hearing injury by offshore pile driving noise. To estimate the distance at which two AMDs designed as ‘seal scarers’ (Ace Aquatec and Lofitech) are detected by harbor porpoises, the 50% hearing detection thresholds for playbacks of recordings of the AMD sounds were assessed. Both became audible at a received broadband sound pressure level (SPL) of 55 dB re 1 μPa. The effect of the AMDs on porpoise behavior was quantified at three SPLs determined during a pre-test: one which just did not cause a behavioral change, one which caused a small change in surfacing and swimming pattern, and one which caused the harbor porpoise to swim away from the transducers. The corresponding mean received SPLs in the pool were respectively: Ace Aquatec: 77, 117, and 139 dB re 1 μPa; and: Lofitech: 91, 121, and 151 dB re 1 μPa. As the mean received SPL increased, greater displacement occurred, and higher numbers of surfacings and increased swimming speed occurred in test periods than in associated baseline periods. By combining these results with the source level, the current local propagation conditions and the background noise, the deterring distance at sea for the AMDs can be calculated

The effectiveness of a seal scarer at a wild salmon net fishery

Abstract During 2009 and 2010 an Acoustic Deterrent Device (ADD) was tested at a salmon bagnet. Treatment blocks when the ADD was either on or off were randomly assigned and observations were carried out from shore. During periods when the ADD was switched on, there were fewer seals observed and approximately a third more fish were landed per hour than when the ADD was switched off, these differences were both significant. Model results indicated that the higher fish landings when the ADD was operating were a direct result of the reduction in the number of seals in the vicinity of the net. Photo-identification of the seals resulted in ten grey seals and four harbour seals being identified, and showed that the majority of these were identified on one occasion, and that two grey seals made up 63% of the visits to the study area when individuals were identified. Seal-damaged fish were only found within the bagnet during off treatments. Overall, the ADD was found to be an effective seal deterrent. Refinement in the method of ADD deployment could further increase effectiveness.

Seal scarers as a tool to deter harbour porpoises from offshore construction sites

Seal scarers as a tool to deter harbour porpoises from offshore construction sites

Far‐reaching effects of a seal scarer on harbour porpoises, Phocoena phocoena

ABSTRACT Although seal scarers are widely used both to reduce economic losses at fish farms caused by seal predation and to reduce risks posed to marine mammals by offshore pile driving activities, the spatial extent of their deterrent effect on harbour porpoises is still largely unclear. However, this information is crucial to understanding the effects these devices have on the marine environment and to judge their potential as a mitigation measure. A study was conducted in the German North Sea, using passive acoustic monitoring and to some extent simultaneous aerial surveying to specifically study the spatial extent of the deterrence effects of a seal scarer on harbour porpoises. In order to link porpoise detections at various distances to actual sound levels, sound measurements of the seal scarer signal were carried out at several distances from the source. C‐POD recordings revealed a significant deterrence effect on harbour porpoises up to 7.5 km away (at about 113 dB re 1 μParms), much further than previously reported. During seal scarer operation the number of porpoise detections within 750 m of the C‐PODs decreased by between 52 and 95% of the value before the seal scarer was activated. An aerial survey revealed a significant decrease in porpoise density from 2.4 porpoises km‐2 before to 0.3 porpoises km‐2 during seal scarer operation within the 990 km2 study area, showing that the decrease in porpoise detections by passive acoustic monitoring was probably indeed the result of a decrease in porpoise abundance. These results may raise serious concerns about unwanted disturbance effects on harbour porpoises in the context of seal scarer use at fish farms and also highlight the need for caution when applied as a mitigation measure during offshore construction.Copyright © 2012 John Wiley & Sons, Ltd.

A survey of antipredator controls at marine salmon farms in Scotland

A questionnaire survey was sent to managers of marine salmon farm sites in Scotland to update knowledge on problems from predators and the use of antipredator controls. Of the 195 respondents, 5 had no antipredator controls with the remaining 190 having some level of control. Throughout Scotland, managers reported 12 species of predator. Seals were reported to be predators at 81% of sites. In total, 19 different types of antipredator control were used. Top nets were employed most widely, being used by 88% of the 195 respondents followed by seal scarers (52%) and shooting (49%). Shooting was considered to be very effective by 45 (62%) of the 73 site managers who expressed an opinion on its effectiveness. Views on the effectiveness of seal scarers varied markedly among site managers, and only 21 (23%) of 92 managers considered them to be very effective. There was also considerable variation in the perceived effectiveness of different types of antipredator nets. Comparison with previous studies showed that although there was a significant increase in the proportion of farms at which seals were reported to have caused loss or damage between 1985 and 1987, there has been no further increase. There were no significant changes over time in reported problems from other species of predator. The proportions of farms using nets and shooting to deter predators are remarkably consistent between the study periods. However, the use of seal scarers has increased significantly from 10% in 1985 to 52% in 2001. The survey suggests that the degrees of protection afforded by predator control methods vary appreciably between sites. There is a need for a range of antipredator devices to be tested under controlled conditions to determine under which circumstances each is most effective. This is particularly the case regarding seal scarers. © 2004 Published by Elsevier B.V.

Interaction between seals and marine fish‐farms in Tasmania, and management of the problem

Abstract Seals that attacked fish on 15 marine fish‐farms in south‐eastern Tasmania were large subadult and adult male Australian fur seals Arctocephalus pusillus doriferus. From July to November 1988 there were 235 attacks on fish‐farms where Atlantic salmon Salmo salar and rainbow trout S. gairdneri were held in pens. Single animals usually attacked pens at night, irrespective of the size and species of fish in the pens. The seals damaged both pens and fish, and sometimes fish escaped as a result of the attacks. The vulnerability of fish‐farms was influenced by their proximity to seal haul‐out sites. Proximity to fishing ports and size of the fish‐farm had little influence. The use of shooting as a protection method was inefficient and ineffective because seals usually entered fish‐farms at night and showed no fear of shooters. There were many attacks in the presence of underwater acoustic seal scarers. Deterrents such as pursuit with boats, lights, seal crackers and emetics helped reduce the number of seal attacks. The only way totally to prevent seals from attacking fish‐farms is to exclude them from the vicinity of the fish pens with physical barriers that they cannot penetrate. These are currently in use, and include perimeter fences and protection nets made of steel mesh set around individual pens.