Innovation of seal-safe fishing gear

The overall goal of the project was to develop innovative fishing gears which can constitute alternatives to gillnet and hook fisheries in areas where seal depredation is causing significant catch losses. Specifically, the project has worked on optimization of a mini-seine targeting cod and flatfish, trialling the Pontoon-trap at Bornholm and collecting data on potential catch rates over the year for the fish pots.

As a first step the seine had to be optimized in order to obtain better catches, as technical and gear failures had been identified in earlier tests of the mini-seine. First the drums were adjusted to reach a better ratio between force and speed of the system. Second, by adding a remote control of the drums which made the system more adaptable to the working conditions on-board a small vessel. The results showed that several of parameters were improved but the hydraulic engines could not be rebuilt to achieve the optimal power.

The second step was to investigate how seine rope length, seine rope diameter and seine net affected the catch efficiency. Here seine rope length was found to have a positive significant effect on the catch efficiency even though setting and retrieving longer ropes takes more time. Seine rope diameter did not have any effect on the catches. Catch efficiency did not differ significantly between the tested seine nets as long as each net was rigged properly.

To optimize catch rates, different layout patterns of the seine ropes were tested. The results indicated that when seine ropes had been laid out in an open circle the highest catches were obtained, which was most likely due to the increased size of the fishing area covered. It is, however, important not to cover the biggest possible area, as this changes the angle of the seine rope towards the towing directions, which becomes problematic as this might allow fish to escape during the herding process.

In order to make sure that no fish left the seine-net once they were caught a so-called stop-net was tested. In the first test of the stop-net no significant effect in total catches was found when fishing with or without the stop-net. Only catches of dab were significantly higher when fishing with stop-net. These observations could further be supported by video observations done inside the seine net. Here the recordings showed also that the stop-net stayed quite open even though the seine net did not move at all. Therefore, similar trials had to be repeated, but instead of higher retrieval speeds, slower retrieval speeds should be tested in combination with the stopnet.

Additionally to the test of slower retrieval speeds the second test of the stop-net included an optimisation process of it, which could finally allow for a comparison between catches of the miniseine and the gill nets.

In general, the catches during the experimental phase were low. Thus it was difficult to compare the catches form gillnets with the mini-seine net. In general, the gillnet catches were highest for the coastal and lowest for gillnet set close to the seine fishing locations. When trying to compare the maximal gillnet catches by upscaling the catch from both the coastal positions and the positions close to the seine net positions, the catches from the mini-seine were somewhat in between. This outcome bases on the catches of flounder, a species dominating the catches, but with very low commercial value. The raised catches of turbot and plaice were highest for seine, while raised catches of cod were very low for all. The differences between the coastal gillnet and gillnet not used under optimal conditions shows how important the right location is for proper gillnetting.

In summary, the seine seems to be more suitable for catching the more valuable species turbot and plaice but catches of flounder are smaller. However, such trials should be repeated when there is more fish in the area.

The Pontoon-trap was modified to fish at Bornholm at a greater depth than in earlier trials by adding an extra pontoon, a longer connector and ballast to adjust the levelling at the surface. The trap fished for 54 days, but daily catches of cod above the minimum landing size were only a little more than 1 kg. The trap frame was found not to be sufficiently strong to cope with the impacts of current and wind. These issues combined with a lack of interest from Danish fishermen led to abandoning further trials with the Pontoon-trap.

The last part if the project focussed on pot fishery. Sets of ten collapsible fish pots were given to eight fishers around Denmark, who were asked to fish with them in all conscience for a longer periods. The fishers could use their own preferred bait, soak times and fishing areas. All catches were registered for each pot set and the results showed that there was large variation in catches both in terms of regions but also over the year. However, the catches were the highest in those months which are most affected by depredation from seals.

During all trials a cooperation with the commercial fishery was essential. All participating fishers was somehow affected by the seal conflict and had a wish to solve this. The fisher’s ideas and views, from modifications of the seine to design of the pots were in all work packages taken into account, and the project really benefited by a close collaboration.