Acoustic deterrents and attractors

Using sound to discourage or distract bycatch species from interacting with fishing gear. This includes passive acoustic devices.

Acoustic deterrents are not generally considered useful in reducing bycatch of seabirds, turtles and sharks, except in limited circumstances. In the main, this is because the feasibility and long-term effectiveness of an acoustic deterrent is affected by habituation (Southwood et al., 2008). Acoustic deterrents are used with some success for marine mammals. There is evidence to support the contention that pingers are one of the best technical measures available to mitigate bycatch of some cetacean species, predominantly in gillnet fisheries, albeit with strong caveats (FAO 2021). More detail is provided by taxon group, below.

Seabirds

Bull (2006) describes an acoustic deterrent as any noise used to deter birds from a vessel. Methods used include firing a shotgun, canons, hitting the steel hull, or commercial devices that emit high frequency and loud noises or distress calls (Brothers et al., 1999). However, loud noises frighten seabirds only briefly and at close range (Brothers et al., 1999). Bull (2006) notes that anecdotal observations have reported acoustic deterrents as being effective at temporarily scaring birds away (Crysell 2002). Brothers et al. (1999) suggest that the more often a frightening sound is produced, the less effect it has due to rapid habituation, as with gas guns in agricultural situations. Other commercially available acoustic bird-scaring devices emit high frequency and loud noises or distress calls. These may be effective if used sparingly to avoid habituation (Brothers et al., 1999).

The apparent futility of developing auditory deterrents for seabirds was demonstrated by Brothers et al. (1999) during a trial in which seabirds at a breeding colony were subjected to high-frequency and loud noise as well as distress calls. No detectable response to the sounds was observed.

Marine Turtles

Southwood et al. (2008) best sum up the potential for auditory techniques to deter sea turtles from interactions with longliners. They found that sea turtles and longline target species are hearing generalists that detect sounds within a similar range**, so any sound generated to prevent sea turtles from interacting with gear would also be detected by, and potentially have a deterrent effect on, target species.

Evidence for marine turtle habituation to auditory deterrents is less clear. Moein et al. (1994) exposed juvenile loggerhead turtles to repeated air gun blasts. Although the loggerheads initially avoided the region where the noise source was located, over a short period of repeated exposure the avoidance response rapidly waned. This change in behaviour may have been due to habituation or to hearing impairment caused by repeated exposure to high intensity sounds.

Barton and Ketten (in Brill et al., 2004) suggested that sea turtles may be attracted to the sound produced by longline floats. They proposed studies to determine the sound spectrum and sound pressure levels produced by both hard and soft floats used in longline fishing.

**Sea turtles and yellowfin tuna are low frequency specialists (Brill et al., 2004), detecting the same low frequency and high energy (i.e. loud) sounds (Swimmer and Brill, 2006).

Sharks

Little research has been conducted in the area of auditory deterrents and sharks; however, habituation has been demonstrated in sharks, as for seabirds and sea turtles. Using audio attractors to lure sharks away from fishing vessels, prior to setting purse seine nets on fish aggregating devices (FADs), has shown some promise but has not been successfully field tested.

Southwood et al. (2008) in their synthesis of knowledge of acoustic attraction in ocean-dwelling shark species summarised Myrberg (2001, Myrberg et al., 1972, 1976, 1978), as follows: It is generally concluded that sharks, like pelagic teleosts, are low frequency specialists. Silky sharks Carcharinus falciformis and oceanic white-tip sharks Carcharinus longimanis are attracted to low frequency sound within the range of 25 to 1000 Hz, with attractiveness increasing as sound frequency decreases. Irregularly pulsed sounds, such as might be generated by struggling prey, are more attractive than regularly pulsed sounds. Sudden transmission of high intensity sound at close range prompts an immediate and rapid withdrawal in both silky sharks and oceanic white-tip sharks. However, both species rapidly habituate to such signals.

The potential for using audio attractors to lead sharks away from fishing vessels prior to setting purse seine nets on FADs was discussed at a 2009 meeting of the International Seafood Sustainability Foundation (ISSF). Research to determine the feasibility of the different attractors was proposed and issues affecting their success identified, e.g., noisy environment near vessels that may mask attractors and the short period available prior setting to attract sharks away from the FADs. Other research questions include whether different attractors are needed for different shark species, their effect on target species, the type of sound and which frequency/ beat or range to use, the platform to distribute the sound and how to use attractors in conjunction with other techniques (e.g. Bait stations). In their review of technical mitigation measures for sharks and rays, Poisson et al (2016) found that there was a lack of evidence to support adopting the use of acoustic attractors for sharks

Marine Mammals

In 2021, FAO published "Fishing operations. Guidelines to prevent and reduce bycatch of marine mammals in capture fisheries". The following text is an extract from this report. The full text for Section 3.2 Acoustic alerting or deterrent devices can be downloaded here.

Acoustic alerting or deterrent devices (primarily pingers), can serve as an effective bycatch reduction measure in certain situations. In some fisheries, data from field research as well as those from fisheries observer monitoring marine mammal bycatch have shown that pingers can exclude certain species of marine mammal within the range of the sound field (Kraus et al., 1997). However, an opposite effect can also occur, whereby some marine mammals become attracted to the devices, while others can suffer serious injury from the use of deterrents with high sound outputs (Dawson et al., 2013).

Acoustic deterrents consist of a range of devices that either emit sounds, using electrical or mechanical means, or acoustically reflect those emitted by echolocating cetaceans. These devices may be deployed on or near fishing gear and include categories referred to as pingers, acoustic harassment devices (including seal-scarer devices), and acoustic alerting devices. Their intended use is to enhance detection of fishing gear by those cetaceans that echolocate for prey detection and other reasons: to do so, they may create an alert or unappealing sound that causes animals to avoid the sound source, or associate it with an obstacle to avoid. The units that actively produce sound span a range of power outputs that are measured in decibels (dB), audio frequency (Hz), sound duration, and the periodicity of sound emission – its duty cycle, which may be regular, random, or triggered by sounds such as those emitted by echolocating cetaceans. Separating these devices into different categories is somewhat arbitrary, although it helps in understanding of how different units are designed to function.

Pingers tend to be relatively small, cylindrical units roughly the size of a soda can. They produce sound at different frequencies, although generally in the 3–70 kHz range, and lower than 180 dB (re 1 pPa @ 1 m). Some devices operate at random frequencies, such as the Dolphin Deterrence Devices produced by STM Products, which has a range of 5–500 kHz. Pingers are most commonly used to avoid the bycatch of small cetaceans in gillnets, harbour porpoise in particular.

Acoustic Harassment Devices (AHDs) are intended to deter animals from approaching fish traps or aquaculture cages and sea pens, using higher sound outputs that typically inflict pain or discomfort. Devices of 180 dB or higher are sometimes classified as AHDs to distinguish them from pingers (Long et al., 2015). Seal-scarers are a type of AHD intended to keep seals and sea lions from preying on fish raised in aquaculture cages and sea pens.

Passive acoustic devices use air-filled or metallic components incorporated into fishing gear to increase their detection by echolocating cetaceans. The logic for using this approach is that marine mammals will avoid gear that they can detect acoustically.

Predator sounds mainly include the playback of killer whale calls, with the aim of prompting marine mammal prey species to flee or avoid the area the sound is being emitted from.

The most critical consideration is whether or not these deterrents elicit a behavioural response in a particular species such that bycatch is prevented or substantially reduced. Evidence shows that acoustic deterrents do not necessarily elicit a behavioural response that reduces bycatch for every marine mammal species. In controlled experiments comparing nets with and without pingers, and multi-year monitoring of bycatch levels, pingers have been shown to be effective in reducing bycatch or causing area avoidance for at least the following 7 species (although possibly as many as 12):

•  harbour porpoise
• striped dolphin (Stenella coeruleoalba)
• franciscana dolphin (Pontoporia blainvillei)
• several beaked whales (Ziphiidae family) – Cuvier’s, Hubb’s, Stejneger’s and Baird’s beaked whale (see reviews in Dawson et al., 2013; FAO, 2018).

A pinger trial involving Burmeister’s porpoise (Phocoena spinipinnis) suggested that pingers might also help reduce bycatch of this species (Clay et al., 2019), yet 19 acoustic deterrents appear ineffective with dugong (Dugong dugon) (Hodgson et al., 2007). Similarly, while some North Atlantic right whales (Nowacek, 2004) showed a behavioural response to high frequency sound exposure – just as humpback whales (Megaptera novaeangliae) did to pinger sounds (Lien, 1992; Harcourt et al., 2014; Pirotta et al., 2016) – there is no evidence that the type of response will help prevent entanglements in fishing gear. Some species, such as bottlenose dolphin (Tursiops truncatus), are attracted to the sound of pingers, presumably because they associate the sound with easy-to-catch fish caught in gillnets (Cox et al., 2004; Leeney et al., 2007). As such, there is no indication that pingers deter bottlenose dolphins from entering trawl nets (Allen et al., 2014). The interactions of both California (Zalophus californianus) and South American (Otaria flavescens) sea lions with gillnets appear to increase when acoustic deterrents are used; this has been termed the “dinner bell effect” (Barlow and Cameron, 2003; Bordino et al., 2002; Carretta and Barlow, 2011). Increasing the frequency to make pingers less audible to pinnipeds may eliminate this undesirable outcome. A trial in Argentina using a pinger with a higher frequency of 70 kHz, instead of 10 kHz, showed a similar reduction in franciscana dolphin bycatch without increasing the attraction of sea lions (Bordino et al., 2004).

Playbacks of predator calls have shown some potential for deterring particular marine mammal species (Werner et al., 2015), but they can also affect the behaviour of target fish, leading to a reduced target catch (Doksæter et al., 2009).

Passive acoustic devices with enhanced reflecting materials have shown to be effective in some studies but not others (Trippel et al., 2003; Bordino et al., 2013), and would be limited to echolocating marine mammals. Given the insufficient evidence of a bycatch prevention effect with louder devices (AHDs), predator playbacks or passive acoustic deterrents, it can be concluded that of all the devices available pingers are the most appropriate ones to use where they are effective.

In addition to species-specific differences, the effectiveness of acoustic deterrents is also dependent upon their experimental design, the fishery in which they are tested, the sound they create, the ambient noise level, gear type and fishing practices. Tests of the devices should therefore be carried out in local fisheries before widespread implementation. Monitoring the use of pingers is also critical to ensure that bycatch reduction targets are being met, even when they have been shown to reduce bycatch experimentally, as results reported from experiments often show greater reductions than when implemented in a fishery (Dawson et al., 2013).

Introducing unnatural sounds into the environment is far from straightforward. Many variables influence how they are propagated, as well as how the sounds are received by animals, which in turn affects the degree of bycatch deterrence. A partial list of physical factors that influence sound propagation includes depth, bathymetry, temperature, turbulence, density of particulate matter, and refraction (Erbe et al., 2018). Furthermore, acoustic deterrents vary in the strength of their signal and the directionality of sound waves. Pingers also have a range of duty cycles (i.e. the periodicity and duration of signal output, including how it is activated). The spacing of multiple units and whether or not they are all in working condition can also affect how effective they may act as a deterrent, with different sound frequencies attenuating at different distances from the source. Some guidelines for deploying pingers are provided in Box 2.

The use of acoustic deterrents without a carefully considered plan of deployment and appropriate monitoring can cause more harm than good. The improper or unmanaged uses of acoustic deterrents can create an assumption that the marine mammal bycatch problem has been solved when this is not the case, with potentially negative consequences for fishers, marine mammals and the environment. These may include habitat exclusion (if the units are deployed in a dense fishery that is also a major critical habitat for marine mammals), excessive sonification (saturating an area with an introduced source of sound), habituation, physical harm (such as long-term hearing impairment when using AHDs), and operational safety concerns. Encouragingly, habituation has not been reported from fisheries on the east- (multi-species gillnet) and west-coast (driftnet) fisheries of the United States of America, which have long-term monitoring data (FAO, 2018). Nevertheless, all of the concerns mentioned above need to be considered prior to implementing acoustic deterrents in a fishery. The pros and cons of using acoustic deterrents in gillnet (and possible trawl) fisheries are presented in Table 2.

In summary, there is much evidence to support the contention that pingers are one of the best technical measures available to mitigate bycatch of some species, predominantly in gillnet fisheries. However, many factors can influence their effectiveness, suitability and/or practicality as a deterrent. They therefore require scientific evaluation within a fishery prior to their widespread implementation, and their use should be subject to ongoing monitoring.