9 General Conclusion
It is clear that acoustic deterrent devices have the potential to play an important role in marine mammal conservation and welfare, and in the management of interactions with fisheries, aquaculture and in certain areas of marine engineering. Much of this review has focused on the specific application of reducing pinniped depredation at salmon farms as this is the main current use for acoustic deterrent in Scotland, and is clearly a significant source of profit reduction within an increasingly important industry. The potential application of acoustic deterrence, however, is a long way from being fully realised and conservation, welfare and economic benefits are being lost as a result. A poor understanding of how animals respond to acoustics signals and how sound affects them being the main contributory factor. We are beginning to obtain a much clearer picture about how some ADDs work and what their impacts are on marine mammals, but there are still large areas of uncertainty.
Despite being used to protect Scottish aquaculture sites since the mid-1980's, it is striking that there is so little independent evidence of the effectiveness of ADDs in reducing seal depredation. What little research has been conducted overseas has shown little support for long-term, continuous use of ADDs, but the lack of formal evidence is critical. A paucity of research effort and regulation in this area has encouraged the development and widespread deployment of a number of devices without evidence of their effectiveness, some without any documentation of their acoustic characteristics.
Depredation at Scottish fish farms appears to have reduced over the last decade, but this is probably due to improvements in containment and husbandry practice rather than the widespread use of ADDs. Other methods for reduction of depredation, such as the use of anti-predator nets and lethal removal of seals, are not seen as 'complete' management strategies, but are worthy of increased research effort.
Acoustic deterrents have been found to be effective in reducing the bycatch rates of some species in capture fisheries, and this effect has not been seen to have decreased over time. The neophobic nature of some species, including harbour porpoise, is likely responsible for a substantial degree of this success.
Most sound sources can have negative as well as positive consequences and both must be understood in order to allow a balanced assessment of costs and benefits. A major concern is that powerful devices may have detrimental impacts on the auditory system, behaviour and ecology of both target and non-target species.
There is evidence to suggest that the criteria currently used to assess the likelihood of hearing damage may not be as precautionary as they were intended to be. In the case of aquaculture, for example, it seems entirely plausible that seals which spend extended period of time in the vicinity of fish farm ADDs may well be impacted by hearing impairment. More information to allow the development of realistic movement models for individual animals in the vicinity of devices is required before this risk can be assessed in finer detail. Similarly, there is considerable evidence that at least some of the devices being used may have significant impacts on the distribution and therefore the ecology of some cetaceans, notably harbour porpoises. Population level impacts of such displacement and hearing damage are very hard if not impossible to determine, but it is clear that if animals are highly dependent upon the detection of signals at or near their sensitivity threshold, then even small threshold shifts could have large impacts. The potential for hearing damage is a concern in respect to ADD effectiveness as well as animal welfare.
From a legislative perspective, deliberate disturbance of European Protected Species, such as cetaceans, contravenes the Habitats Directive (Conservation (Natural Habitats, &c.) Regulations 1994 as amended in Scotland). The question of whether and in what circumstances the use of ADDs will need to be licenced is still being deliberated by regulators such as Marine Scotland and SNH. If the use of ADD's in particular circumstances requires a licence, it will be important to demonstrate the efficacy of ADDs. At present there is evidence that ADDs can work, at least for short periods of time, in deterring some seals from the proximity of river mouths and from salmon trap-net fisheries and that the use of ADDs at salmon trap net fisheries can reduce the amount of damage to fish that are caught, but these results cannot necessarily be extrapolated to infer anything about long-term use of such devices at salmon farm sites. There is anecdotal evidence to suggest that ADDs can reduce seal depredation at aquaculture sites but there has as yet been no independent and objective assessment of how effective ADDs are in managing seal depredation at Scottish aquaculture sites. Such information would greatly facilitate an objective assessment of the costs and benefits of ADD use.
Apart from aquaculture, aversive sound mitigation has the potential to reduce risks to marine mammals associated with construction and energy installations. Suitable mitigation ranges will depend greatly on the species, pile-diameter and environmental conditions. There is also the potential for unintended consequences such as the separation of groups including mothers and calves, strandings, and widespread exclusion. A more complete understanding of likely effects is therefore required before these techniques can be broadly adopted.
Finally we consider the original questions posed for this project.
- What types of ADD are currently employed, or are in development, which are used to deter marine mammals in different scenarios, for example at fish farms, netting stations, rivers, and in/around areas of development ( e.g., oil and gas, renewables)?
The commercially available devices that are commonly used in Scotland are listed in Table 2. All of these devices have been used at fish farms to a greater or lesser extent. Netting stations and river fisheries have mostly used or tested Lofitech ADDs and more recently Airmar devices. A sixth device developed at the University of St. Andrews is currently under development (see section 2) but is not yet available for commercial purchase.
We have found no reliable sources of information on the types of acoustic deterrent devices used to keep marine mammals away from marine industrial development sites. Although the use of ADDs is part of the Joint Nature Conservation Committee ( JNCC) protocol for minimising the risk of injury to marine mammals from piling noise, no co-ordinated records are maintained on which types of device have been used where. This is an area which will be examined in detail under phase one (of project four) of the Offshore Renewable Joint Industry Programme, with the intention of developing a protocol for ADD use at development sites. Detailed studies of the Lofitech device for this application suggest that it has successfully been used abroad to some extent.
- Are these devices fit for purpose and appropriate for deterring marine mammals in a range of scenarios and often at a very local scale? For example, are some commercial devices more applicable for deterring seals in more constrained salmon rivers, while others are more appropriate for deployment in coastal or offshore waters? Will some devices be more appropriate for long-term deployment as opposed to short-term?
Because so few trials have been conducted, and there is such a poor understanding of how and under what circumstances these devices actually achieve their apparent deterrent effect, it is impossible to determine the relative effectiveness of the currently available candidate devices. All have different acoustic characteristics and different logistical constraints (power supply, robustness) which on a practical level may make them more or less suitable for different applications. There is little objective evidence as yet to suggest the relative merits or superiority of any one device compared to the others for a specific context (see section 2.2.4). It is very likely, given the range of target and non-target species which will be encountered in different applications, that multiple signal types will be required, dependent on environmental context as well as industrial requirement. Work toward development of such signal types is currently in its infancy.
- Are certain devices more appropriate to a particular species? Are there different requirements for seals, toothed cetaceans, and baleen whales (dependent on the purpose of deterrence)?
A complete list of devices is provided in Table 1. Across the range of these acoustic deterrent devices, several different types of pinger, primarily used in set net fisheries, have proved effective in displacing porpoises. However, such devices remain unproven with respect to most other species, and may attract seals when used on fishing gear. Louder ADDs such as the seal scaring devices used at fish farms, are likely to be more effective for a wider range of species (see section 184.108.40.206), however, there are several contextual considerations that may constrain deployments. For example, it may be desirable in some cases to deter all marine mammals, whereas in other cases it may be preferable to deter only one group (such as seals). In other cases the use of a specific ADD or signal type, which may be associated with food in other circumstances, might attract rather than deter particular species. The species specificity of different devices remains poorly understood.
- What is the relative effectiveness of existing ADDs on marine mammals (considering seals and cetaceans separately)? For example, at what range do they exclude mammals? Do certain devices exclude seals and not cetaceans, and vice versa?
Given our current state of knowledge of the efficacy of the different devices it is not possible to make such a comparison.
Our understanding of the relative effectiveness of ADDs on seals is particularly limited. There are better data on deterrent/exclusion effects on porpoises and other odontocetes. It is known for example that Airmar and Lofitech ADDs are effective in deterring porpoises to several km from source (see section 220.127.116.11) though in many applications this is not a desired effect. Some studies have also tried to quantify the degree of exclusion of porpoises for various types of pingers. In part the range of effectiveness will be contextually driven, but will also likely relate to the amplitude and frequency characteristics of the devices. It is likely that devices could be targeted towards particular marine mammal groups by designing them to emit frequencies to which that group was more sensitive. Thus, devices which emit higher frequency sounds are more likely to be aversive to odontocete cetaceans whose high frequency hearing sensitivity is greater than that of seals. Baleen whales are more likely to be sensitive to low frequency sounds, but once again our understanding is limited by the paucity of relevant studies. Recent work at the University of St Andrews has used this approach in an attempt to develop an ADD that will be effective against seals but not disturb odontocetes. Preliminary results suggest that one of the ADD types used routinely at Scottish fish farm sites is much less disturbing to porpoises compared to others that have been tested.
- Are there efficiency improvements which could be made by best practice in using existing ADDs? For example, targeted activation of devices when marine mammals are located in the vicinity of the devices (as opposed to continuous use).
Targeted activation could make deterrent devices more efficient, in that power consumption could be reduced and acoustic output reduced. Whether or not such devices would be more effective in minimising seal depredation or in deterring marine mammals more generally remains to be determined. Less acoustic energy will be released into the environment and unwanted exclusion effects should be reduced. However, some argue that some hearing damage risks could be increased. Certain fish farms already employ different tactics in this respect (targeted or continuous activation). There is no clear evidence to suggest either tactic is preferable but insights might be derived by analysing data collected at fish farms.
- What are the ecological consequences of ADD's in terms of underwater noise?
The introduction of loud sounds into the environment is considered a form of pollution by some authorities and it is generally recognised that it should be avoided where possible. ADD signals have been shown to alter the distribution of some species, especially the more neophobic animals with greatest hearing sensitivity ( e.g. porpoises). The extent to which such distributional changes are of ecological relevance remains unknown. The output characteristics of some ADDs are fairly well described see ( section 2.2.1), but for others there is very little reliable information available. The extent to which different types of ADDs displace marine mammals can be measured relatively easily and is likely to be influenced by to source levels and spectral composition of ADD signals and how these relate to animal's hearing sensitivity. Displacement studies have only been reported for two of the devices routinely used. A measurement of displacement provides straight-forward metric, allowing regulators to judge one device against another.
- Beyond ADDs are there any other current or developing technologies for deterring marine mammals? When answering this question, consideration should be given to the reasons for deterrence ( e.g., aquaculture, fisheries, mitigation for renewable development).
There are several other methods that are currently being used to limit seal depredation at fish farms including net tensioning, seal blinds and mort removal. Other methods of deterrence such as conditioned taste aversion and electric fields also show some promise, but require development and testing before they can be applied. Lethal removal to limit depredation at fish farms and nets is widely used but has not been adequately assessed as an effective management strategy, and may not be considered acceptable to the public or other stake holders ( see 2.3.3). It also raises specific conservation concerns for harbour seals in some areas. For renewable energy developments and fisheries interactions, acoustic deterrents appear to be the only viable means of deterring or alerting marine mammals.
- Can baseline information be improved which would benefit developing marine industries?
There is still a dearth of information on several key issues. For fish farms, more standardised recording of anti-predator techniques and associated levels of damage, coupled with a commitment to analyse such data, should help in isolating and refining the most effective management measures. Lethal removal of seals yields sources of data (carcasses) that could be extremely insightful, but are currently only very rarely collected or analysed. For marine renewables, key areas of uncertainty include the density and distribution of marine mammals around development sites, the behavioural responses of different species to different acoustic stimuli, and how these may vary in different contexts and in different motivational states.