Report prepared by Elgin Associates for the Department of the Environment (on behalf of the Expert Panel) 12 May 2014
A technical review to evaluate existing work relating to the application, development and trial of marine mammal mitigation devices was undertaken to assess the availability and effectiveness of mitigation measures that could be used in mid-water trawl fisheries for small pelagic species. Where appropriate the review drew on lessons learnt from trawl fisheries, particularly those using mid-water trawl gear, targeting species other than small pelagic fish. The intent of the review was to highlight global examples of interactions between marine mammals and mid-water trawl gear, the mitigation measures developed, and their efficacy.
The review was undertaken by reviewing and compiling relevant information from both worldwide and Australian fisheries. Management and mitigation measures to reduce marine mammal bycatch typically include the introduction of codes of practice (including offal management), the imposition of fisheries-related mortality limits, temporal and spatial closures of the fishery and the deployment of mitigation devices. This review focussed on interactions between marine mammals and large mid-water trawl vessels, and the technical details of mitigation devices employed in each.
Fisheries that operate mid-water trawl vessels were identified and relevant professionals contacted to obtain information. Countries or locations where most work on bycatch mitigation has been undertaken include Australia, New Zealand, Northwest Africa, Antarctica, the United Kingdom and France. Information and data from global fisheries that utilise mid-water trawl gear was compiled into fishery case studies, from which conclusions on mitigation efficacy were drawn.
The review of the available literature on management of marine mammal interactions with pelagic trawl gear shows that only two technical solutions have been developed and tested to reduce bycatch at this stage, with another showing potential. These are:
- Excluder devices (physical barriers to prevent marine mammals entering and becoming entrapped in the cod-end and directing them out of the net through an escape hatch);
- Pingers (acoustic deterrent devices that emit acoustic signals to alert or deter marine mammals from the immediate vicinity of fishing gear); and
- Auto-trawling systems (net-monitoring systems designed to ensure the entrance to the net remains open at all times)
Most work has focussed on excluder devices and pingers. The development of auto-trawling systems has been done primarily to improve fishing efficiency, but may also have benefits for reducing incidental mortality of marine mammals.
Excluder devices comprises an additional section of netting inserted between the entrance and the codend of the trawl net with an angled grid that directs sea lions and other large animals to an escape hole in the top or bottom of the net and prevents them from entering the trawl codend. Excluders have used both hard and soft grids to exclude marine mammals from the codend of trawls. The escape hole may be covered to reduce the loss of target species, or open. Some exclusion devices are fitted with a ‘hood’ and ‘kite’. These function to both minimise the possible loss of commercial catch and to minimise the possible loss of dead or incapacitated marine mammals so that mortalities or injuries can be detected. Exclusion devices are widely accepted internationally as being effective in mitigating the incidental mortalities of many large non-target species in many fishing operations.
Development of an excluder device for a fishery requires careful design to ensure problems of fish-loss via the escape hole and net blockage via the excluder grid are avoided. These devices need to be designed specifically for each fishery, taking into account the particular characteristics of each gear type, fishing operation, the size and operation of gear, towing speed, the hydrodynamics of trawl set up in relation to scaling (trawl size/ grid and escape hole ratios), how trawl nets are stored on the vessel, and the size of target and non-target species. An exclusion device that is effective in reducing mortality of marine mammals in one fishery while maintaining catch per unit effort (CPUE) of target species may not be effective in another fishery that is targeting different fish species and encountering different marine mammal species. Effective design and use of excluders also requires detailed knowledge of the spatial and temporal behaviour of the species of mammal that are to be excluded from fishing gear.
While properly designed excluder devices have been shown to be effective in reducing bycatch of pinnipeds, there are no studies that indicate excluder designs tested to date are fully effective in reducing cetacean mortality in trawls. This is probably because fur seals and sea lions are more manoeuvrable within the confines of a trawl net than are dolphins. Use of underwater cameras monitoring effectiveness of excluder devices typically show pinnipeds entering and leaving a trawl through excluder escape holes, whereas dolphins appear distressed when near excluder grids and reluctant or unable to find an escape hole. Fur seals and sea lions will approach an excluder head-on and readily turn around and swim back. In one study dolphins were mostly observed to back down into the net to a position near the grid and later swim upstream out of the net. Very few dolphins were seen swimming head-first towards the grid, and those that did turned around before reaching the grid and swam out the mouth of the net).
Further information is required on the escape behaviour of dolphin species that are known to interact with trawl nets. At present there exists no solution to filter or deter cetaceans from the net opening. Although individual dolphins can potentially escape from a trawl using an escape hole or escape tunnel, cetaceans appear to be less likely to enter a narrow (3–4 m) and confined release route. The most practical way to reduce cetacean bycatch is to have an exit in the net’s top panel because dolphins have been observed to seek an exit in the upper part of the trawl. However some studies report that bottlenose dolphins (Tursiops spp.) prefer to exit at the bottom of the net.
For one fishery concern has been raised about the efficacy of excluder devices and post-escape survival of pinnipeds. It has been proposed the observed mortalities could be underestimated due to “cryptic” mortality, because some animals may suffer head trauma from impacting the excluder’s hard grid that may compromise their post-escape survival, or may drown outside the net after escaping through the SLED, because they run out of breath before they reach the surface. These assertions have been extensively investigated and are not supported by scientific evidence.
Pingers were originally developed to warn dolphins about the presence of gillnets used in fishing operations, and the technology has now been extended to pelagic trawl gear. Pingers are commercially available and are marketed under various trade names. These differ in the level of sound emitted, the direction of the pulse emitted and the pulse duration of the sound emitted. The trade name Dolphin Dissuasive Devices or DDD is also used in a generic sense to refer to loud pingers.
The effectiveness of commercially available and protype pinger devices has been trialled in the UK bass pair trawl fishery and the adjacent European fishery. Tests have evaluated parameters such as sound source level, pulse durations, immersion depths, and distance from dolphin groups whilst assessing the behavioural response of animals. The placement of pingers within trawl gear has also been evaluated and is considered a critical factor in their effectiveness in deterring small cetaceans from interacting with fishing gear. Experimental work has produced mixed results, with significant reductions in bycatch rates being observed when pingers have been used, but the absence of a sufficient number of control tows prevents confidence in the results.
The results of monitoring pinger deployment over three years showed three potential problems with implementing these devices as a mitigation measure. Devices may not always be properly charged or working when deployed; they may be placed too close to the surface; and, they may degrade after three years and are unable to hold adequate charge
As a result, the effectiveness of pingers in reducing bycatch of dolphins in pelagic trawls is unclear. A decline in observed bycatch in UK pair-trawl fisheries was reported following the introduction of pingers, and trials with pingers in French trawlers indicated a 70% reduction in common dolphin bycatch. However, at-sea trials off Ireland indicated that pingers may not provide a consistently effective deterrent signal for common dolphins.
Of the commercial gillnet pingers, only the DDD has shown some effect in pelagic fisheries. Pingers (DDDs) should be fully charged and deployed on the lower wing ends or bridles of the trawl to ensure they continue to function correctly. Although DDDs appear to be effective in reducing dolphin bycatch, there are still challenges to address including determining the most effective configuration for mid-water trawls.
Deployment of pingers may interfere with normal fishing operations, and French fishermen prefer to use a softer pinger set on the rear part of the trawl rather than use a DDD set on the wings of the trawls because there is less interference with the netsonder. Concerns also exist that cetaceans will become habituated to pinger use over time, and their effectiveness will, therefore, decline. Currently, habituation is not thought to pose a problem, although it may be hard to test for this and to link any changes in pinger effectiveness to habituation. Concerns have also been expressed that the wide use of pingers in certain fisheries may result in the exclusion of cetaceans from habitat that may be significant to their survival. There is currently no evidence that this occurs where pingers have been used.
Some trawlers employ auto-trawl systems that are designed to maintain the shape of the trawl gear when turning, thus ensuring the entrance to the net remains open at all times. Auto-trawl systems are able to do this by monitoring and controlling the trawl doors via telemetry and sensors. It has been proposed that the use of auto-trawling systems has a potential mitigation effect for pinnipeds and cetaceans
Auto-trawl systems have never been evaluated as a marine mammal bycatch mitigation approach. It is intuitive that ensuring the net entrance does not collapse during trawling operations will be effective in reducing marine mammal entrapment in trawl nets and maintaining the effective operation of excluder devices. However, while this may reduce the likelihood of marine mammal entrapment and ensuing mortality, there is currently no evidence that the use of auto-trawl equipment will be effective in minimising the capture of marine mammals. Use of auto-trawl systems as a marine mammal mitigation device should therefore be treated with caution until the efficacy of these systems has been demonstrated.