The Science of Pingers

Some authors suggest that pingers may not be successful bycatch mitigation devices for all cetacean and pinniped species (Dawson et al. 2013, Hamilton & Baker 2019).

For example, they have been described as acting as ‘dinner bells’ for California sea lions (Zalophus californianus), South American sea lion (Otaria flavescens) and common bottlenose dolphins (Tursiops truncatus), with individuals of these species having been observed depredating nets equipped with pingers (e.g. Bordino et al. 2002, Read et al. 2003, Carretta & Barlow 2011, Snape et al. 2018).

That said, early studies that reported pinniped depredation all deployed low frequency (10kHz) pingers – presumably as this was the frequency mandated for use in US fisheries and the acoustic consensus for pinger use at that time. The audiograms for California sea lion and other pinnipeds show optimal hearing at 3-20kHz (Mulsow 2012, Erbe et al. 2015), meaning easy detection of low frequency pingers attached to nets and the subsequent “dinner bell” association.

However, pingers are changing and recent trials with higher frequency “seal safe” pingers (50-120kHz) that are tailored to transmit outside the optimal hearing thresholds of pinnipeds eliminated the dinner bell effect noted earlier by Bordino et al. 2002 when low frequency pingers were dismissed as a mitigation measure for preventing bycatch of the Vulnerable Franciscana dolphin (Pontoporia blainvillei) in coastal set net fisheries off Brazil (Lopez et al. in press).

Furthermore, recent trials in Norway using 10kHz pingers found that the bycatch of harbour seal (Phoca vitulina) in set net fisheries there was three times higher than using no pingers or nets equipped with 50-120kHz pingers (Øien & Haug 2017). If the pings can no longer be detected by seals and the catch remains safe in the nets, this excuse for not using pingers is no longer valid.

There has been concern that the initial aversive reaction of cetaceans to the sound of pingers may wear off over time, leading to a reduction in their effectiveness – commonly referred to as habituation (e.g. Cox et al. 2004, Cox et al. 2001 , Amano et al. 2017). Whilst these concerns are often cited by managers as a reason to prevent pinger uptake, the evidence base from the two longest pinger trials globally don’t show any evidence of habitation (Koschinski and Culik 1997, Palka et al. 2008, Carretta and Barlow 2011).

Research using our Banana Pingers supports these findings; Swedish (2017) and UK (2013) trials both experienced 99.8% reduction in rates of cetaceans around nets equipped with our Banana Pingers and zero habituation over prolonged periods of time. The figure below shows acoustic evidence from the Swedish Baltic that the detectable clicks per minute (DPM) of cetacean species remained consistently low in the presence of the Fishtek Banana Pinger throughout the study period demonstrating empirically that habituation wasn’t occurring. These results support the long-term observations from fisheries mandated to use pingers.

In 2019 our own primary research with harbour porpoise in the UK found that the effect of the pinger was consistent across the period of study at all sites, and concluded that there was no habituation that would lead either to harbour porpoise ignoring the pinger and becoming more prevalent, or being significantly deterred by the pinger and avoiding the area longer term.

One difficulty with potential habituation is that there are two different fishery/cetacean interactions, and they are often conflated. Habituation to pingers by cetaceans who depredate (take fish out of fishermen’s nets) has been documented more often than when pingers are used to prevent incidental entanglement of passing cetaceans in fishing nets (Dawson et al. 2013). This is because cetaceans who are depredating nets benefit from an easy meal at the end of the interaction, they have an incentive to override their instinctive aversive reactions. The findings of all studies where accidental dolphin or porpoise entanglement is the issue show that pingers remain extremely effective bycatch mitigation.

A well-publicised paper by Palka et al. (2008) researched whether bycatch reduction using pingers was as effective in a commercial fishing setting as it appeared to be in scientifically controlled trials. It found “that pingers appear to have reduced the bycatch rate, particularly when the required number of pingers were used and in nets using mesh sizes of 15cm or less”.

However, some fisheries managers around the world seem to be mis-citing the paper, believing that it states that incorrect use by fishermen (e.g. trying to reduce costs by increasing the spacing between pingers, or inadvertently increasing spacing by not maintaining and confirming functionality of each pinger) made bycatch worse than not using pingers at all. This interpretation of the findings is causing management concern over pinger use around the world. In the UK, this management uncertainty has directly resulted in a lack of conservation action and continued bycatch in high risk fisheries.

Fishtek Marine spoke to Dr Palka directly about her findings, how they are portrayed in the paper, and how they are interpreted. She agreed that for many of the years of the study period, the analysis was undertaken without accounting for pinger failure rates, which is an important variable which should be considered. Although this limitation is explicitly stated in the discussion section, overwhelmingly (and disappointingly) the impression readers are left with is that incorrect use of pingers triples bycatch. 

Looking at the data it is clear that when pinger functionality was high (see 2006 data from figure below) bycatch in hauls with an incomplete set of pingers was actually three times less than hauls without pingers, but as expected, not as effective as hauls where full pinger compliance was observed. Palka et al. (2008) only looks at 2006 data where pinger functionality is high, but we believe that if other years where high pinger functionality was confirmed were looked at, the same observation would be observed – that is, incomplete pinger use is better than no pingers but not as good as full compliance (see figure below) which is opposite to the take home message of the paper.

This is a concern that by using pingers, fishermen are adding noise to the marine environment which could exclude cetaceans from areas of sea. The worry is that widespread deployment of pingers could prevent animals being able to access their important supporting habitats.

There is no evidence to substantiate these predictions. In 2000 Larsen and Hansen looked at potential ensonification in the Danish gillnet fishery and estimated that, even if all Danish gillnets were equipped with pingers, less than 1% of the North Sea would be affected. They also noted that the use of pingers in a Danish trial had not prevented porpoises from getting entangled either in nets set without pingers nearby, or in nets set soon afterwards, suggesting a relatively limited (temporal and spatial) degree of exclusion.

Northridge et al. (2011) had similar results when they modelled different pingers, spacing, and fishing sectors and found that, even when fishing activity is at its most intensive, only between 0.04% and 11% of the total area of the Celtic Sea and Western Channel (SW England) could potentially be denied to cetaceans.

From our own research with Banana Pingers and harbour porpoise we have found that when pingers are active, harbour porpoise still remain detectable and are therefore not deterred from the fishing grounds. Empirical evidence is consistent with the anecdotal evidence from multiple fishers using pingers who state that deterrence is localised and temporary. Fishermen report that the harbour porpoise appear to stay back from the point source of the pinger. The deterrent effect is less pronounced with distance from the sound source and has no lasting effect as the harbour porpoise immediately return to the area when the nets are removed. 

Although the results of trials and feedback from fishermen show that ensonification is not a problem for some species, it would not be correct to assume that it couldn’t occur for other species with different reactions to pingers and different ranges, especially if nets were set intensively in their important habitats. Carlström et al. 2002 concluded that a displacement effect by pingers is likely to be more prominent in coastal waters where access to bodies of water is limited and the consequence may be serious if the area is critical to the survival of the porpoise population.