Interactions between protected species and fisheries

The overall objective of the IMBAF project was to propose and test solutions to mitigate the effects of existing conflicts between protected species and coastal commercial fisheries in Danish waters. Specifically, the project aimed to reduce bycatch of marine mammals and seabirds in Danish net fisheries and to minimize the impact of seals on fisheries through the development and optimization of seal-safe gear.

The IMBAF project was divided in seven work packages (WP):
1. Test LED lights as a preventive seabird bycatch method in the lumpsucker gillnet fishery;
2. Measure the scaring effect of looming-eye buoys on seabirds around set pound nets;
3. Assess the potential of thinner twine gillnets to prevent bycatches of porpoises;
4. Compare the attractiveness of different bait types in cod pots;
5. Develop and test novel fisher’s ideas for seal-proof gear;
6. Evaluate the effectiveness of acoustically reflective nets at reducing harbour porpoise bycatch in gillnet fisheries;
7. Disseminate the findings and conclusions of the IMBAF project.

WP.1 investigated the seabird bycatch reduction effect of flashing LED lights fixed on lumpsucker. The experimental fishing trials were conducted in 2021 and 2022 during the late winter and early spring. The trials were monitored using electronic monitoring (EM) systems and DTU Aqua onboard observers. During the experiment, a total of 167 bird bycatches belonging to 6 different species was recorded. The results showed a bycatch rate of 0.29 seabird per km.day and 0.4 seabird per km.day for light-nets and for control-nets, respectively. However, these bycatch rates were not significantly different, and therefore, these flashing LED lights cannot be recommended as a mitigation solution for reducing seabird bycatch in lumpsucker gillnets, based on the collected data.

The aim of the trials in WP.2 was to assess the long-term effectiveness of a device designed to scare away seabirds from an area of interest. Here, looming-eye buoys were experimented around pound nets in a fishery in the Western Baltic in 2021. One of two pound nets in the trials was equipped with two looming-eye buoys and the number of seabirds before and after implementation was statistically compared (paired-BACI design). Both pound nets were monitored by DTU Aqua for several weeks before and after the implementation of the mitigation devices. In total, 722 birds belonging to 8 different species were observed. Before the installation of the looming-eye buoys, there were similar numbers of seabirds in the two pound nets. However, after the installation of the buoys, it appeared that there were more seabirds in the control area. The difference between the two areas, however, decreased during the observation period. Based on the data collected in this experiment, no conclusive effect of the scaring effect of looming-eye buoys on seabirds could be detected.

In WP.3, experiments were conducted using thin-twined nets to prevent porpoise bycatch. Three fishers were provided with two types of nets, one set of thin twine and the other with standard gillnet twine. The fishing activities were monitored using video systems and the catch rates and bycatch rates of the standard and of the modified nets were compared to evaluate the effectiveness of thin twine nets at reducing bycatch with affecting catches. In total, 8 months of usable data were collected for the year 2022. During the course of the experiment, 5 porpoise bycatches were recorded in standard nets and 4 porpoises were captured in thin-twined nets. The result of the statistical analyses indicated that the reduction in twine thickness was not enough to significantly reduce porpoise bycatch. Moreover, the fishers involved in the trials believed that further reduction in twine thickness would lead to substantial reductions in fish catches due to the fragility of the gear, while reducing its durability.

WP.4 tested different types of baits in Atlantic cod (Gadus morhua) pot fishery in the Baltic Sea. The trials were conducted differently than outlined in the initially submitted application because, over the timeframe of the project, substantial changes in the cod quota were implemented (TAC was reduced to zero), which made it impossible to operate a targeted cod fishery. Therefore, we decided to conduct the baiting experiments without pots to avoid catching cod and, instead, attach the bait bags to sunk anchors that were monitored with underwater video cameras. Five different types of bait (herring, sprat, sandeel, squid, and an artificial bait) were tested, both near Bornholm and in the Belt Sea, Denmark. The results showed that herring, sandeel, and sprat attracted cod, but there was no significant difference between these three types of bait. The artificial bait and squid had a very low attractiveness and therefore, these two types of bait are not recommended for use in cod pots.

WP.5 aimed to develop and test fisher's ideas for seal-proof fishing gear. In total, the project received 4 ideas from the fishing industry: jigging-machines, lumpsucker-fykes, Dyneema fykes, and a pound net trap with a large chamber. The idea of testing jigging machines was approved, but the fisher who was supposed to carry out the experiment chose to withdraw from fishing shortly after for reasons independent of the IMBAF project. The second accepted fisher’s idea was a large lumpsucker trap. Unfortunately, the landings of lumpsucker in Denmark have hit a record low in 2022 and 2023, following several years of decline. In turn, the partnering fisher could not use the trap, although he plans to test it in 2024. Thirdly, it was proposed to use a new fyke design where parts of the fyke are made in Dyneema twine, a particularly strong material that seals cannot tear apart. These traps were built, but due to a significant reduction in eel fishing in the recent past, the fisher could only conduct very little fishing in the time allocated to the trials. He will however continue to test this design in 2024. The last idea we received was to build a large chamber in a pound net to make it possible for seals to swim out of it and thus not destroy the entire catch and the gear. The idea was approved, but for unknown reasons, the fisher decided not to participate in the project in the last minute, so the idea was not tested.

In WP.6, we conducted experiments using reflective nets aiming to reduce porpoise bycatch in gillnets. Specifically, we tested acrylic pearls, known to be acoustically reflective in the frequencies used by porpoises, and measured if bycatch rates of protected species and catch rates of fish were affected. In this experiment, pearls were attached to the netting material to increase the acoustic reflectivity of the net. The so-called pearl nets were tested multiple times because the initial results suggested that the pearls increased fish catches. However, after repeated experiments, the results showed that the pearls did not seem to change fish catch rates significantly. No porpoises were bycaught in control or treatment nets, so the potential of pearl nets to reduce porpoise bycatch remains unknown. Nevertheless, it is an important step in the development of a pearl net fishery to demonstrate that fish catches are not negatively affected by pearls.

The last work package (WP.7) dealt with disseminating the project’s results. As promised, the IMBAF project was presented to the seal group from the Danish Ministry of Environment and the Ministry of Food, as well as dedicated workshops and working groups within ICES, HELCOM, ASCOBANS, and Birdlife Denmark. Furthermore, a Bachelor student wrote a thesis using data collected in the project, and three scientific articles are currently in the process of being published. The IMBAF project has not yet been presented to the Ministry of Food's porpoise group as initially planned, because the Ministry did not hold such meetings during the project period.

The IMBAF project contributed to scientifically assess the potential of mitigation methods and alternatives to reduce the impact of fishing on sensitive species of marine mammals and seabirds in Denmark and maintain acceptable levels of catches for fishers. Even though the results in several of the work packages proved that some of the tested mitigation devices did not work as well as intended, this work is an important part of the process of developing and testing new tools. It is equally important for the fishing industry to know what does and what does not work, so that unnecessary efforts are not spent on implementing new gear and devices when these have not proven unambiguously effective at achieving their target – i.e., reducing bycatch of protected species and maintain catch rates of target species. That said, the pearl nets tested in WP.6 to replace traditional monofilament gillnets showed promises, as they do not affect fish catches, while potential limiting interactions with cetaceans, thus helping to solve the problem of porpoise bycatch in Danish gillnets, and possibly elsewhere.