Reduction of harbour porpoise bycatch in protected areas

The overall purpose of the project was to investigate how the behavior of harbor porpoises is affected in relation to acoustic alarms, so called pingers. In the project, the main focus was on studies of the effective range of pingers, their long-term effects, how porpoises reacts to the pinger signals and models of pingers effect on porpoises. Furthermore, monitoring of porpoise bycatch of porpoises have been conducted in the North Sea.

The project has been divided into 5 work packages:
Work package 1: Observations of porpoises in relation to different pinger-types.
Work package 2. Distance trials with new types of pingers in the field
Work package 3: Bycatch of porpoises and long-term effects of pinger use in the North Sea
Work package 4. Modeling the pinger effect.
Work package 5: Outreach of the project.

The main task of Work package 1 was to observe how porpoises were affected by different pingers. The first part conducted acoustic measurements of the pingers in order to have accurate data for which soundscapes the porpoises are exposed to when they swim into areas where pingers are used. In total, four pingers have been examined. These show large variations in both in source level and frequencies. Work package 1 furthermore conducted direct observations of porpoises using drones. Here pingers were deployed in the vicinity of wild porpoises and the behavior of the porpoises was then followed when the pinger was activated. The results showed that pingers can cause very strong avoidance responses, which explains the effectiveness of the pingers in reducing bycatch when used in fisheries. However, the experiment also showed that 25% of the animals did not respond to the pingers, which suggests that there is great diversity in porpoises' behavioral reactions to pingers. However, the results point to the importance of understanding the effect of the pingers, for example if they are implemented in areas that are particularly important for porpoises.

To investigate the long-term effectiveness of pingers, acoustic recorders were deployed at different distances from a specially designed pinger. The pinger emitted sounds in on and off cycles. Each trial lasted 2-6 months and was repeated at two locations. In this way, it was possible to investigate, how effective pingers are in keeping porpoises at a distance. Two types of 'Banana pingers' with sound signals at 40-70 kHz were tested. The results were used under WP 4.

The main purpose of Work package 2 was to test the effectivity of the pingers when the spacing between the pingers was increased. The reason is that an increased distance between pingers can reduce fishing expenses for the purchase of pingers and reduce the emission of noise pollution in the sea. A total of 3 experiments were conducted. The first trial was carried out in 2020, but due to Covid19 it was not possible to make a proper setup. The results from this trial were therefore invalid. In 2021, the second attempt was carried out. Here, pingers were set with 200 and 500 meter spacings. The result showed, that when the pingers were set with 200m distance, they reduced the bycatch of porpoises by 90%, while when the pingers were set with 500m spacing, they reduced the bycatch by 51%. The 90% reduction, however, showed large uncertainties. Thus, a third trial was conducted in 2022. Again, the pingers were set with 200 and 500 meter spacings. In, however, collaboration with the manufacturer, a new type of pinger with a higher sound level was made. The idea was to increase the efficiency when the pingers were set with 500m distance. Unexpectedly, the results from the last trial showed that the pingers were less effective both when the pingers were set with 200 and 500 meter spacings (51 and 36% reduction respectively). However, the new louder pinger with the higher source level showed a reduction of 77%. The reason for the low effect in 2022 is unknown. However, the fishers observed large numbers of porpoises and juvenile mackerel during the period. This could suggest that the porpoises had their sonar locked on prey items and thereby only played attention when the source level was loud enough.

Work package 3 focused on monitoring bycatch in the North Sea. In 2019, DTU Aqua and a commercial gillnet fishing vessel began a collaboration to monitor bycatch of protected species including porpoises. The vessel had a camera surveillance system installed and the vessel has been collected video footage during the full trial of the project. A total of 584, 756 and 628 days have been monitored in 2019, 2020 and 2021 respectively. The data processing from 2022 is still ongoing as the fisher has decided to continue the data collection despites the project has ended. All data has been reported to ICES and are used in modeling of the total bycatch of porpoise for the North Sea.

Work package 4 has investigated how the porpoise population is affected when/if pingers are implemented in the fishery. Here, data from work package 2 has been used, but the work has also focused on developing models and carrying out various simulations to answer this. In the modeling work, it was found that the pingers can be an effective management intervention to reduce bycatch. However, it was also shown that when designing such a management intervention, it is important to consider the implementation plan, as the effectiveness of the plan depends on it. If, for example, there is a low use of pingers on the net and they therefore do not work optimally, this can counteract the gains from reduced bycatch. This latter effect is probably mediated by an influence on the reproduction rate. In order to detect this effect, it will therefore be necessary to ensure that monitoring programs include observations that make it possible to estimate this demographic parameter in addition to estimating the bycatch rate. Such a measure will ensure that fisheries are not the only carrier of management interventions to help achieve conservation objectives for these species.

Work package 5 has applied for the results from the project to be further disseminated. This has resulted in the following: Scientific articles (5), of which 2 have already been published, conference contributions (4), Meeting presentations (4), Presentations in working groups (3).